**Drought and Its Consequences to Plants – From Individual to Ecosystem**

Elizamar Ciríaco da Silva, Manoel Bandeira de Albuquerque, André Dias de Azevedo Neto and Carlos Dias da Silva Junior

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/53833

**1. Introduction**

[21] Itsubo N., Inaba A., LIME2: Life-cycle Impact assessment Method based on Endpoint modeling. Tokyo: Life-Cycle Assessment Society of Japan; 2012. http://lca-forum.org/

[22] Steen B., A systematic approach to environmental strategies in product development (EPS), Version 2000 – Models and data of the default methods. Gothenburg : Chalm‐

english/pdf/No14\_C1\_Outline.pdf (accessed 19 August 2012)

ers University of Technology; 1999.

16 Responses of Organisms to Water Stress

Climate-change scenarios around the world indicate that many areas of the globe will in‐ crease in aridity. Thus, all living organisms will suffer from a water scarcity, especially plants, which do not have locomotive structures that allow them to move elsewhere when water and food becomes scarce. As a result, different terrestrial ecosystems (natural and ag‐ ricultural) will be severely affected and some may even collapse due to the extinction of plant species.

It is therefore important to gain a better understanding regarding the effect of frequent drought stress on biochemical and physiological processes in plants as well as on the plant population and/or community in a particular ecosystem. Despite the negative aspects of such changes, severe environmental conditions can induce interesting adaptations in plants that allow them to survive and reproduce. These adaptations can lead to the emergence of new functional groups in a given ecosystem or serve as an important tool for improving ag‐ ricultural practices and plant breeding programs.

In recent decades, a large number of investigations have addressed strategies used by plants to control water status, avoid oxidative stress and maintain vital functions in an attempt to understand the morphological and physiological changes plants undergo to ensure their survival under different environmental conditions. Special attention has been given to mo‐ lecular processes involved in drought tolerance and resistance. While some advances have

© 2013 da Silva et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 da Silva et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

been made, we still do not fully understand the underlying survival mechanisms in plants due the complex interaction of different forms of stress in natural habitats.

stock farming and the indiscriminate extraction of firewood deplete the nutritional content of the soil, thereby contributing toward the process of desertification. These activities lead to

Drought and Its Consequences to Plants – From Individual to Ecosystem

http://dx.doi.org/10.5772/53833

19

Plants need a large amount of water and nutrients throughout their life cycle and all aspects of plant development are affected by a reduction in water content in the soil. This reduction in soil moisture leads to changes in the physical environment, which subsequently affect physiological and biochemical processes in plants [8-10]. Drought can cause nutrient defi‐ ciencies, even in fertilised soils, due the reduced mobility and absorbance of individual nu‐ trients, leading to a lower rate of mineral diffusion from the soil matrix to the roots [3].

Water is required for processes such as germination, cell division and elongation for the pro‐ motion of plant growth in height and width and metabolic activities, such as the synthesis of organic compounds, photosynthesis, respiration and a number of other physiological and biochemical processes [11]. Thus, when water availability decreases, changes occur in all

Drought triggers a wide variety of plant responses [12]. Plant growth is altered, with changes in the architecture of individuals, which are translated into lower height, reduced leaf size, a smaller number of leaves, less fruit production and changes in the reproductive phase. Osmoregulatory processes generally occurs to protect membrane integrity and main‐ tain the inflow of water to the cell as well as the accumulation of organic solutes as sugars, quaternary ammonium compounds (glycine betaine and alanine betaine) [13, 14], hydro‐ philic proteins (late embryogenesis abundant proteins) [15], soluble proteins and amino acids (proline) [10, 14]. Water is the most important substance in the initial phase of plant development from germination and seedling formation to establishment in the field [16] and

a reduction in the water supply in this stage can lead to dehydration and even death.

**2.1. Water relations and influence on plant growth and development**

In agricultural ecosystems, drought has a detrimental effect on crop production, affecting the growth rate and development of the economically important portions of the plant, such as fruits, grains and leaves. Without irrigation, production in crops such as coffee can be re‐ duced by as much as 80% in dry years [17]. In Mexico, 80% of the problems caused by drought are related to losses in agricultural systems [18]. During a 45-day drought in the state of Paraná, Brazil, the 2008/2009 soybean harvest was reduced by 80% in areas without dry cover [19]. The same can be estimated for important crops such as sugarcane, corn, wheat and a number of others. The tragic effect on productivity is explained by the vital im‐ portance of water in living cells, which affects all biochemical and metabolic processes.

Water is attracted to soil pores predominantly due to its attraction to other surfaces (ad‐ hesion) and capillarity. Its movement in the soil occurs mainly through mass flow: water fills micropores in the soil, which are interconnected and allow water movement. Contact between the surface of the roots (mainly in the root hair zone) and soil provide the sur‐

Thus, drought is doubtlessly the most important stress factor limiting plant life.

molecular, biochemical, physiological and morphological aspects of plants.

progressive degradation that results in the loss of soil cover [6, 7].

On the ecosystem level, drought induces changes in different processes and frequently de‐ mands functional plant responses. Some ecosystems, such as savannas, steppes and scrub‐ lands, have intermittent low annual precipitation. In these water-limited ecosystems, drought can seasonally modify carbon and nitrogen cycles, resulting in poor water and min‐ eral uptake by roots, lesser plant growth, a reduction in litter decomposition and the biogen‐ ic emission of CO2 from the soil. Severe drought can also induce a higher vegetation mortality rate due to cavitation and/or carbon starvation (reduced photosynthesis and en‐ hanced autotrophic respiration). Thus, more frequent and intense drought periods (and the consequent death of plant species) can alter the phytosociology of entire plant communities over time.

Reductions in aboveground net primary productivity and alterations in functional plant groups are observed in places subjected to prolonged, severe drought. This chapter offers an overview of the effect of drought on individual plants and ecosystems, emphasising aspects of growth, water relations and photosynthesis, especially the electron transport chain, as well as radical oxygen species (ROS) scavenging and its role in avoiding oxidative stress. On the ecosystem level, functional traits commonly associated to water stress tolerance and changes in ecological processes and functional responses in plants will be also discussed.

## **2. Drought as a stress factor to the plants**

In recent decades, a large number of models have been developed to estimate climate changes around the world. Climate change is defined as a significant difference between two mean climatic states, with substantial impact on the ecosystem [1]. Extreme climatic events, such higher temperatures, more intense precipitation, increased drought risk and duration as well as cyclones and flooding in coastal areas, are expected to increase in both frequency and intensity [2, 3]. In some countries, large arid and semiarid areas are expected to increase in size, leading to desertification. Currently, the consequences of global warming are widely discussed, especially regarding plant productivity and the increase in areas sub‐ ject to desertification.

According to Assad et al. [4], the average temperature of the planet will increase by 1.4 to 5.8 °C by the end of the century, with drought being one of the consequences of this warming. Thus, one may deduce that the planet is heading toward a serious water crisis. Desertifica‐ tion corresponds to a reduction in the productive capacity of arid, semiarid and sub-humid lands as a result of climatic and edaphic factors. This growing, worldwide phenomenon has been causing both social and environmental problems, including the disappearance of ani‐ mal and plant species [5].

In semiarid regions of Brazil, for example, inappropriate cultivation techniques resulting in soil erosion and a loss of water retention capacity in the soil as well as the expansion of live‐ stock farming and the indiscriminate extraction of firewood deplete the nutritional content of the soil, thereby contributing toward the process of desertification. These activities lead to progressive degradation that results in the loss of soil cover [6, 7].

been made, we still do not fully understand the underlying survival mechanisms in plants

On the ecosystem level, drought induces changes in different processes and frequently de‐ mands functional plant responses. Some ecosystems, such as savannas, steppes and scrub‐ lands, have intermittent low annual precipitation. In these water-limited ecosystems, drought can seasonally modify carbon and nitrogen cycles, resulting in poor water and min‐ eral uptake by roots, lesser plant growth, a reduction in litter decomposition and the biogen‐ ic emission of CO2 from the soil. Severe drought can also induce a higher vegetation mortality rate due to cavitation and/or carbon starvation (reduced photosynthesis and en‐ hanced autotrophic respiration). Thus, more frequent and intense drought periods (and the consequent death of plant species) can alter the phytosociology of entire plant communities

Reductions in aboveground net primary productivity and alterations in functional plant groups are observed in places subjected to prolonged, severe drought. This chapter offers an overview of the effect of drought on individual plants and ecosystems, emphasising aspects of growth, water relations and photosynthesis, especially the electron transport chain, as well as radical oxygen species (ROS) scavenging and its role in avoiding oxidative stress. On the ecosystem level, functional traits commonly associated to water stress tolerance and changes in ecological processes and functional responses in plants will be also discussed.

In recent decades, a large number of models have been developed to estimate climate changes around the world. Climate change is defined as a significant difference between two mean climatic states, with substantial impact on the ecosystem [1]. Extreme climatic events, such higher temperatures, more intense precipitation, increased drought risk and duration as well as cyclones and flooding in coastal areas, are expected to increase in both frequency and intensity [2, 3]. In some countries, large arid and semiarid areas are expected to increase in size, leading to desertification. Currently, the consequences of global warming are widely discussed, especially regarding plant productivity and the increase in areas sub‐

According to Assad et al. [4], the average temperature of the planet will increase by 1.4 to 5.8 °C by the end of the century, with drought being one of the consequences of this warming. Thus, one may deduce that the planet is heading toward a serious water crisis. Desertifica‐ tion corresponds to a reduction in the productive capacity of arid, semiarid and sub-humid lands as a result of climatic and edaphic factors. This growing, worldwide phenomenon has been causing both social and environmental problems, including the disappearance of ani‐

In semiarid regions of Brazil, for example, inappropriate cultivation techniques resulting in soil erosion and a loss of water retention capacity in the soil as well as the expansion of live‐

due the complex interaction of different forms of stress in natural habitats.

**2. Drought as a stress factor to the plants**

over time.

18 Responses of Organisms to Water Stress

ject to desertification.

mal and plant species [5].

Plants need a large amount of water and nutrients throughout their life cycle and all aspects of plant development are affected by a reduction in water content in the soil. This reduction in soil moisture leads to changes in the physical environment, which subsequently affect physiological and biochemical processes in plants [8-10]. Drought can cause nutrient defi‐ ciencies, even in fertilised soils, due the reduced mobility and absorbance of individual nu‐ trients, leading to a lower rate of mineral diffusion from the soil matrix to the roots [3]. Thus, drought is doubtlessly the most important stress factor limiting plant life.

Water is required for processes such as germination, cell division and elongation for the pro‐ motion of plant growth in height and width and metabolic activities, such as the synthesis of organic compounds, photosynthesis, respiration and a number of other physiological and biochemical processes [11]. Thus, when water availability decreases, changes occur in all molecular, biochemical, physiological and morphological aspects of plants.

Drought triggers a wide variety of plant responses [12]. Plant growth is altered, with changes in the architecture of individuals, which are translated into lower height, reduced leaf size, a smaller number of leaves, less fruit production and changes in the reproductive phase. Osmoregulatory processes generally occurs to protect membrane integrity and main‐ tain the inflow of water to the cell as well as the accumulation of organic solutes as sugars, quaternary ammonium compounds (glycine betaine and alanine betaine) [13, 14], hydro‐ philic proteins (late embryogenesis abundant proteins) [15], soluble proteins and amino acids (proline) [10, 14]. Water is the most important substance in the initial phase of plant development from germination and seedling formation to establishment in the field [16] and a reduction in the water supply in this stage can lead to dehydration and even death.

In agricultural ecosystems, drought has a detrimental effect on crop production, affecting the growth rate and development of the economically important portions of the plant, such as fruits, grains and leaves. Without irrigation, production in crops such as coffee can be re‐ duced by as much as 80% in dry years [17]. In Mexico, 80% of the problems caused by drought are related to losses in agricultural systems [18]. During a 45-day drought in the state of Paraná, Brazil, the 2008/2009 soybean harvest was reduced by 80% in areas without dry cover [19]. The same can be estimated for important crops such as sugarcane, corn, wheat and a number of others. The tragic effect on productivity is explained by the vital im‐ portance of water in living cells, which affects all biochemical and metabolic processes.

#### **2.1. Water relations and influence on plant growth and development**

Water is attracted to soil pores predominantly due to its attraction to other surfaces (ad‐ hesion) and capillarity. Its movement in the soil occurs mainly through mass flow: water fills micropores in the soil, which are interconnected and allow water movement. Contact between the surface of the roots (mainly in the root hair zone) and soil provide the sur‐ face area necessary for water uptake. The growth of the roots into the soil maximises wa‐ ter absorption [11].

Dry soil and the loss of water through a high transpiration rate makes the plant experience drought stress [12], which leads to the loss of turgor. As a result, the development of some structures is compromised and the growth rate slows. Thus, plants are generally shorter in dry environments. Although the formation of the organs is genetically defined, environmen‐ tal conditions exert an influence on this process. Once formed, the cells of the leaves and fruit rarely undergo cell division and their growth relies on cell expansion. If the water pres‐ sure is insufficient to promote elongation, these organs will be small in relation with the

Drought and Its Consequences to Plants – From Individual to Ecosystem

http://dx.doi.org/10.5772/53833

21

Plants also need carbon dioxide and light to produce organic matter throughout the process of photosynthesis. Carbon dioxide enters the leaves through the stomata and the turgor of the guard cells is the regulatory mechanism for maintaining the stomata opened [11]. Plants differ morphologically and/or physiologically under drought conditions. Different mecha‐ nisms allow plants to survive and even produce with a limited water supply, such as the maximisation of water uptake by deep, dense root systems, the minimisation of water loss by stomatal closure and a reduction in leaf area, osmotic adjustment or changes in cell wall elasticity as well as other essential processes for maintaining physiological activities

Deciduous species have an efficient mechanism for coping with drought, which involves stomatal closure, changes in the orientation of the leaf and the reduction in leaf area by shedding leaves to minimise water loss through transpiration [24]. In the dry season, the leaves that remain on the plant can strongly influence the water balance by adjusting tran‐ spiration as a function of hydraulic limitation due to an increase in atmospheric vapor pres‐

Cell turgor is maintained by the accumulation of organic substances and inorganic ions in a stress response mechanism denominated osmotic adjustment [26, 27]. Organic solutes, also referred to as compatible solutes, are highly soluble compounds of low molecular weight that have no toxicity at high concentrations within the cells [14]. When plants are exposed to water deficit, changes occur in biochemical substances, such as the conversion of starch to soluble sugars (sucrose, glucose, fructose, etc.) [9, 27,28]. Nitrogenous compounds, such as proteins, amino acids (arginine, proline, lysine, histidine, glycine, etc.) and polyamines, are another group of compounds affected by water deficit that participate in osmotic adjustment [29]. In response to drought, there is an increase in the levels of free amino acids [9] and a reduction in the rate of synthesis or a decrease in proteins [29]. The increase in proline con‐ tent is of considerable importance to plant adaptation during stress [8] and its accumulation usually occurs in large amounts in higher plants in response to environmental stress [14]. Proline is an amino acid resulting from the hydrolysis of proteins and plays an important role as an osmoprotectant in many cultivated species [27, 28, 30]. The increase of proline has also been linked to the reduction in leaf water potential [30]. In addition to its role as an os‐ moregulator, proline stabilises membranes and proteins and contributes to the removal of

those formed in a well-hydrated environment [22].

throughout extended periods of drought [23].

sure deficit and surface soil desiccation [25].

free radicals [14].

Water flow from the soil to the roots depends on the water potential gradient between the soil and plant, which is affected by the water needs of the plant, the hydraulic conductivity of the soil, soil type, moisture content in the soil [20] and the atmospheric demand, which directly affects water loss through transpiration, generating considerable tension in the xy‐ lem and contributing to the creation of this potential gradient. Water potential (*Ψw*) is an ex‐ pression of the energy status of water in any system, such as soil, tissues, the whole plant or the atmosphere, and its energy is influenced by four components: surface force or matrix po‐ tential (*Ψm*), gravitational potential (*Ψg*), hydrostatic pressure or pressure potential (*Ψp*) and solutes or osmotic potential (*Ψs*), which, in most cases, exert a negative effect on total water potential (*Ψw*), reducing water energy and consequently water capacity for moving into a system. Thus, water flow in the soil-plant-atmosphere system always follows a downhill di‐ rection from higher to lower, which is the driving force of water transport [11, 20]. Water potential is always represented by negative values. The reference is pure water under nor‐ mal conditions of temperature and pressure assumed to be equal to zero (*Ψw* = 0), which denotes maximum energy status.

In wet soil, the hydrostatic pressure is closer to zero and *Ψw* is about -0.03 MPa [11]. A re‐ duction in the water supply when the soil becomes dry leads to a decrease in hydrostatic pressure (*Ψp*), which becomes quite negative. Thus, due to the high surface tension that tends to minimise the air-water interface, water becomes strongly adsorbed by the electrical charges of the soil particles (adhesion) [11, 20]. Under this condition, the plant absorption process requires a reduction in *Ψw* in the roots cells in relation to the rhizosphere. Moreover, the constant absorption of water by the plant leads to a reduction in the moisture content of the neighbouring soil.

The coordination of water flow from the soil to the roots, xylem, leaf apoplast and bulk air follows a decreasing status of water energy. This water gradient established between the rhizosphere through the plant and atmosphere favours the inflow of water in well-watered plants. In dry soil, however, the flow is interrupted due to barriers in the soil, such as in‐ creased surface forces, as well as in the plant, such as resistance offered by stomatal closure [20, 21]. When moisture availability in the soil decreases and there is continuity in the loss of water through transpiration, cavitation can occur, causing the interruption of water flow through the xylem due to the formation of air bubbles.

The continued inflow of water contributes to growth processes, as turgor pressure is respon‐ sible for cell elongation. Plant growth is the result of daughter-cell production by meriste‐ matic cell divisions in the shoot and root and the subsequent massive expansion of the young cells [12]. The constant inflow of water exerts pressure within the cell, causing the cell wall to stretch and inducing the elongation or growth of the cell in both size and volume. This physical process is repeated until the cell becomes mature, at which point cell size is no longer significantly altered [11]. These two processes (cell division and expansion) are im‐ portant to the growth and development of tissues and organs.

Dry soil and the loss of water through a high transpiration rate makes the plant experience drought stress [12], which leads to the loss of turgor. As a result, the development of some structures is compromised and the growth rate slows. Thus, plants are generally shorter in dry environments. Although the formation of the organs is genetically defined, environmen‐ tal conditions exert an influence on this process. Once formed, the cells of the leaves and fruit rarely undergo cell division and their growth relies on cell expansion. If the water pres‐ sure is insufficient to promote elongation, these organs will be small in relation with the those formed in a well-hydrated environment [22].

face area necessary for water uptake. The growth of the roots into the soil maximises wa‐

Water flow from the soil to the roots depends on the water potential gradient between the soil and plant, which is affected by the water needs of the plant, the hydraulic conductivity of the soil, soil type, moisture content in the soil [20] and the atmospheric demand, which directly affects water loss through transpiration, generating considerable tension in the xy‐ lem and contributing to the creation of this potential gradient. Water potential (*Ψw*) is an ex‐ pression of the energy status of water in any system, such as soil, tissues, the whole plant or the atmosphere, and its energy is influenced by four components: surface force or matrix po‐ tential (*Ψm*), gravitational potential (*Ψg*), hydrostatic pressure or pressure potential (*Ψp*) and solutes or osmotic potential (*Ψs*), which, in most cases, exert a negative effect on total water potential (*Ψw*), reducing water energy and consequently water capacity for moving into a system. Thus, water flow in the soil-plant-atmosphere system always follows a downhill di‐ rection from higher to lower, which is the driving force of water transport [11, 20]. Water potential is always represented by negative values. The reference is pure water under nor‐ mal conditions of temperature and pressure assumed to be equal to zero (*Ψw* = 0), which

In wet soil, the hydrostatic pressure is closer to zero and *Ψw* is about -0.03 MPa [11]. A re‐ duction in the water supply when the soil becomes dry leads to a decrease in hydrostatic pressure (*Ψp*), which becomes quite negative. Thus, due to the high surface tension that tends to minimise the air-water interface, water becomes strongly adsorbed by the electrical charges of the soil particles (adhesion) [11, 20]. Under this condition, the plant absorption process requires a reduction in *Ψw* in the roots cells in relation to the rhizosphere. Moreover, the constant absorption of water by the plant leads to a reduction in the moisture content of

The coordination of water flow from the soil to the roots, xylem, leaf apoplast and bulk air follows a decreasing status of water energy. This water gradient established between the rhizosphere through the plant and atmosphere favours the inflow of water in well-watered plants. In dry soil, however, the flow is interrupted due to barriers in the soil, such as in‐ creased surface forces, as well as in the plant, such as resistance offered by stomatal closure [20, 21]. When moisture availability in the soil decreases and there is continuity in the loss of water through transpiration, cavitation can occur, causing the interruption of water flow

The continued inflow of water contributes to growth processes, as turgor pressure is respon‐ sible for cell elongation. Plant growth is the result of daughter-cell production by meriste‐ matic cell divisions in the shoot and root and the subsequent massive expansion of the young cells [12]. The constant inflow of water exerts pressure within the cell, causing the cell wall to stretch and inducing the elongation or growth of the cell in both size and volume. This physical process is repeated until the cell becomes mature, at which point cell size is no longer significantly altered [11]. These two processes (cell division and expansion) are im‐

ter absorption [11].

20 Responses of Organisms to Water Stress

denotes maximum energy status.

through the xylem due to the formation of air bubbles.

portant to the growth and development of tissues and organs.

the neighbouring soil.

Plants also need carbon dioxide and light to produce organic matter throughout the process of photosynthesis. Carbon dioxide enters the leaves through the stomata and the turgor of the guard cells is the regulatory mechanism for maintaining the stomata opened [11]. Plants differ morphologically and/or physiologically under drought conditions. Different mecha‐ nisms allow plants to survive and even produce with a limited water supply, such as the maximisation of water uptake by deep, dense root systems, the minimisation of water loss by stomatal closure and a reduction in leaf area, osmotic adjustment or changes in cell wall elasticity as well as other essential processes for maintaining physiological activities throughout extended periods of drought [23].

Deciduous species have an efficient mechanism for coping with drought, which involves stomatal closure, changes in the orientation of the leaf and the reduction in leaf area by shedding leaves to minimise water loss through transpiration [24]. In the dry season, the leaves that remain on the plant can strongly influence the water balance by adjusting tran‐ spiration as a function of hydraulic limitation due to an increase in atmospheric vapor pres‐ sure deficit and surface soil desiccation [25].

Cell turgor is maintained by the accumulation of organic substances and inorganic ions in a stress response mechanism denominated osmotic adjustment [26, 27]. Organic solutes, also referred to as compatible solutes, are highly soluble compounds of low molecular weight that have no toxicity at high concentrations within the cells [14]. When plants are exposed to water deficit, changes occur in biochemical substances, such as the conversion of starch to soluble sugars (sucrose, glucose, fructose, etc.) [9, 27,28]. Nitrogenous compounds, such as proteins, amino acids (arginine, proline, lysine, histidine, glycine, etc.) and polyamines, are another group of compounds affected by water deficit that participate in osmotic adjustment [29]. In response to drought, there is an increase in the levels of free amino acids [9] and a reduction in the rate of synthesis or a decrease in proteins [29]. The increase in proline con‐ tent is of considerable importance to plant adaptation during stress [8] and its accumulation usually occurs in large amounts in higher plants in response to environmental stress [14]. Proline is an amino acid resulting from the hydrolysis of proteins and plays an important role as an osmoprotectant in many cultivated species [27, 28, 30]. The increase of proline has also been linked to the reduction in leaf water potential [30]. In addition to its role as an os‐ moregulator, proline stabilises membranes and proteins and contributes to the removal of free radicals [14].

## **3. Drought and phothosynthesis**

Drought is arguably the most important factor limiting plant yields throughout the world. Climate change and global warming in the tropical zone is expected to affect the photosyn‐ thesis, development and biomass production of plants in many regions as a result of the sig‐ nificant rise in temperature and concentration of atmospheric CO2, which will also lead to a reduction in water availability in the soil, with a consequent effect on carbon assimilation and plant growth [31]. Semiarid regions are subject to water shortages and soil degradation in such places is likely to increase with climate change. The response of photosynthesis to drought merits special attention, as water is an electron donor that allows the maintenance of this process and biomass productivity [32, 33].

[44]. In some situations, F0 can be used as an indicator of irreversible damage to PSII [45] associated with LHCII dissociation [43, 46] and the blocking of the electron transference on the reductant side of PSII. In wheat and barley plants, high temperature tolerance is posi‐ tively correlated with maximum F0 [47]. However, Yamane et al. [48] suggest that the inacti‐ vation of the PSII reaction centre caused by the denaturation of chlorophyll-protein complexes in response to high temperature correlates with a decay in Fm values. Changes in these fluorescence variables cause alterations in the Fv/Fm ratio, indicating a disturbance in the photochemical activity of photosynthesis. The Fv/Fm ratio has been inferred as an indi‐ cator of environmental stress, such as high temperature, drought and excess light, as it is

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23

**3.1. Aspects of chlorophyll** *a* **florescence transient:** *Kielmeyera rugosa* **Choisy as case**

sis, leishmaniasis and malaria, as well as fungal and bacterial infections [51].

perature in the rainy and dry seasons was 26.8 and 39 ºC, respectively.

was -0.34 MPa in the wet season and -0.75 MPa in dry season.

The genus Kielmeyera belongs to the family Clusiaceae (Guttiferae), subfamily Kielmeyeroi‐ deae, and is endemic to South America. The vast majority of these species occur exclusively in Brazil, where nearly 50 species are found chiefly in the *restinga* (sand dune), rocky savan‐ nah and the savannah-like *cerrado* vegetation south of the Amazon [50]. Some species are traditionally used in Brazilian folk medicine to treat tropical diseases, such as schistosomia‐

A case study was performed with a population of 10 adult plants of *Kielmeyera rugosa* Choisy (Clusiaceae) in a *restinga* ecosystem in the municipality of Pirambu, state of Sergipe (north‐ eastern Brazil), where the climate is characterised by irregular rainfall, with a wet season from April to September. Leaf water potential (*Ψw*) was determined between 9:00 and 11:00 am and the chlorophyll and chlorophyll *a* fluorescence indexes were determined between 12:00 and 1:00 pm in March 2011 (dry season) and July 2011 (wet season). The mean air tem‐

Chlorophyll transient florescence (JIP-test): Polyphasic Chl *a* florescence transient (OJIP) was measured in healthy, completely expanded leaves using a hand-held fluorometer (Handy-PEA, Hansatech, King Lynn, UK). The selected leaves were subjected to a 30-min dark adap‐ tation period, which is enough time for all reaction PSII centres to open [52]. The leaves were then immediately exposed to a pulse of saturating light at an intensity of 3000 µmol.m-2s-1 provided by an array of three high-intensity light-emitting diodes. The JIP-test [53] was used to analyse each Chl *a* fluorescence transient. This test is based on the energy flux from bio-membranes [54]. The performance index (PIABS) [55] was employed as a pa‐ rameter to quantify the effects of environmental factors on photosynthesis in several studies.

Figure (1A) shows that *K. rugosa* underwent a significant decrease of 120 and 38% in leaf wa‐ ter potential and the chlorophyll index (1B), respectively, in the dry season. Mean leaf *Ψw*

An analysis of florescence transients in *K. rugosa* under the two distinct water availability conditions (wet and dry season) may provide information on changes taking place in the structure, conformation and function of the photosynthetic apparatus, especially in PSII. Ini‐

easy and fast to measure [49].

**study**

Under conditions of low water availability, a reduction in stomatal conductance constitutes one of the first strategies used by plants to diminish the transpiration rate and maintain tur‐ gescence [34]. Accordingly, stomatal behaviour in response to situations of drought stress may be indicative of water use efficiency for the production of photosynthates. Exposure to stress may induce alterations in photobiological processes, resulting in stomatal restrictions regarding the supply of carbon dioxide, the loss of water vapour and limitations to non-sto‐ matal components, with harm to the reaction centres of photosystems I and II (PSI and PSII), thereby compromising photosynthesis efficiency [32]. According to Bolhàr-Nordenkampf et al. [35], Bolhàr-Nordenkampf and Öquist [36] and Baker [37], changes in the photochemical efficiency of plants under drought conditions may be assessed through an analysis of chlor‐ ophyll *a* fluorescence efficiency associated with PSII.

The chlorophyll fluorescence of water-stressed barley plants is characterised by a mild de‐ crease in Fv/Fm (Fv is the variable part of Chl fluorescence and Fm is Chl fluorescence inten‐ sity at the peak of the continuous fluorescence inductive curve) and significant increase in F0 (Chl fluorescence with all PSII reaction centres open), together with a slight decrease in Fm [38]. The optimal temperature for most species ranges from 25 to 35 o C, above which a decline in the rate of photosynthesis is observed [39, 40]. Under natural conditions, momen‐ tary water deficit is observed during warm hours of the day, which promotes stomatal clo‐ sure. Consequently, the temperature of leaves exposed to direct sunlight can be equal to or higher than the air temperature. This rise in leaf temperature results in biochemical and bio‐ physical disturbances in the mesophyll, which may or may not be reversible [39].

The main effects of high temperature on photosynthesis result from alterations in thylakoid physical-chemical properties [41], besides inducing an increase in lipid matrix fluidity [42], with the consequent formation of a single-layer structure. High temperature causes the fol‐ lowing disturbances to the organisation of the photosynthetic apparatus: a) destruction of the oxygen evolution complex; b) dissociation of the light harvesting complex of PSII accom‐ panied by variations in energy distribution between PSII and PSI; and c) inactivation of the PSII reaction centre (P680), which disturbs grana stacking [43]. All these events result in the loss of photochemical and carboxylation efficiency as well as serious metabolic restrictions in the Calvin cycle, such as the inactivation of ribulose-1,5-bisphosphate carboxylase/ oxygenase and variations in the metabolic pool, especially ATP and NADPH availability [44]. In some situations, F0 can be used as an indicator of irreversible damage to PSII [45] associated with LHCII dissociation [43, 46] and the blocking of the electron transference on the reductant side of PSII. In wheat and barley plants, high temperature tolerance is posi‐ tively correlated with maximum F0 [47]. However, Yamane et al. [48] suggest that the inacti‐ vation of the PSII reaction centre caused by the denaturation of chlorophyll-protein complexes in response to high temperature correlates with a decay in Fm values. Changes in these fluorescence variables cause alterations in the Fv/Fm ratio, indicating a disturbance in the photochemical activity of photosynthesis. The Fv/Fm ratio has been inferred as an indi‐ cator of environmental stress, such as high temperature, drought and excess light, as it is easy and fast to measure [49].

**3. Drought and phothosynthesis**

22 Responses of Organisms to Water Stress

of this process and biomass productivity [32, 33].

ophyll *a* fluorescence efficiency associated with PSII.

Drought is arguably the most important factor limiting plant yields throughout the world. Climate change and global warming in the tropical zone is expected to affect the photosyn‐ thesis, development and biomass production of plants in many regions as a result of the sig‐ nificant rise in temperature and concentration of atmospheric CO2, which will also lead to a reduction in water availability in the soil, with a consequent effect on carbon assimilation and plant growth [31]. Semiarid regions are subject to water shortages and soil degradation in such places is likely to increase with climate change. The response of photosynthesis to drought merits special attention, as water is an electron donor that allows the maintenance

Under conditions of low water availability, a reduction in stomatal conductance constitutes one of the first strategies used by plants to diminish the transpiration rate and maintain tur‐ gescence [34]. Accordingly, stomatal behaviour in response to situations of drought stress may be indicative of water use efficiency for the production of photosynthates. Exposure to stress may induce alterations in photobiological processes, resulting in stomatal restrictions regarding the supply of carbon dioxide, the loss of water vapour and limitations to non-sto‐ matal components, with harm to the reaction centres of photosystems I and II (PSI and PSII), thereby compromising photosynthesis efficiency [32]. According to Bolhàr-Nordenkampf et al. [35], Bolhàr-Nordenkampf and Öquist [36] and Baker [37], changes in the photochemical efficiency of plants under drought conditions may be assessed through an analysis of chlor‐

The chlorophyll fluorescence of water-stressed barley plants is characterised by a mild de‐ crease in Fv/Fm (Fv is the variable part of Chl fluorescence and Fm is Chl fluorescence inten‐ sity at the peak of the continuous fluorescence inductive curve) and significant increase in F0 (Chl fluorescence with all PSII reaction centres open), together with a slight decrease in

decline in the rate of photosynthesis is observed [39, 40]. Under natural conditions, momen‐ tary water deficit is observed during warm hours of the day, which promotes stomatal clo‐ sure. Consequently, the temperature of leaves exposed to direct sunlight can be equal to or higher than the air temperature. This rise in leaf temperature results in biochemical and bio‐

The main effects of high temperature on photosynthesis result from alterations in thylakoid physical-chemical properties [41], besides inducing an increase in lipid matrix fluidity [42], with the consequent formation of a single-layer structure. High temperature causes the fol‐ lowing disturbances to the organisation of the photosynthetic apparatus: a) destruction of the oxygen evolution complex; b) dissociation of the light harvesting complex of PSII accom‐ panied by variations in energy distribution between PSII and PSI; and c) inactivation of the PSII reaction centre (P680), which disturbs grana stacking [43]. All these events result in the loss of photochemical and carboxylation efficiency as well as serious metabolic restrictions in the Calvin cycle, such as the inactivation of ribulose-1,5-bisphosphate carboxylase/ oxygenase and variations in the metabolic pool, especially ATP and NADPH availability

C, above which a

Fm [38]. The optimal temperature for most species ranges from 25 to 35 o

physical disturbances in the mesophyll, which may or may not be reversible [39].

#### **3.1. Aspects of chlorophyll** *a* **florescence transient:** *Kielmeyera rugosa* **Choisy as case study**

The genus Kielmeyera belongs to the family Clusiaceae (Guttiferae), subfamily Kielmeyeroi‐ deae, and is endemic to South America. The vast majority of these species occur exclusively in Brazil, where nearly 50 species are found chiefly in the *restinga* (sand dune), rocky savan‐ nah and the savannah-like *cerrado* vegetation south of the Amazon [50]. Some species are traditionally used in Brazilian folk medicine to treat tropical diseases, such as schistosomia‐ sis, leishmaniasis and malaria, as well as fungal and bacterial infections [51].

A case study was performed with a population of 10 adult plants of *Kielmeyera rugosa* Choisy (Clusiaceae) in a *restinga* ecosystem in the municipality of Pirambu, state of Sergipe (north‐ eastern Brazil), where the climate is characterised by irregular rainfall, with a wet season from April to September. Leaf water potential (*Ψw*) was determined between 9:00 and 11:00 am and the chlorophyll and chlorophyll *a* fluorescence indexes were determined between 12:00 and 1:00 pm in March 2011 (dry season) and July 2011 (wet season). The mean air tem‐ perature in the rainy and dry seasons was 26.8 and 39 ºC, respectively.

Chlorophyll transient florescence (JIP-test): Polyphasic Chl *a* florescence transient (OJIP) was measured in healthy, completely expanded leaves using a hand-held fluorometer (Handy-PEA, Hansatech, King Lynn, UK). The selected leaves were subjected to a 30-min dark adap‐ tation period, which is enough time for all reaction PSII centres to open [52]. The leaves were then immediately exposed to a pulse of saturating light at an intensity of 3000 µmol.m-2s-1 provided by an array of three high-intensity light-emitting diodes. The JIP-test [53] was used to analyse each Chl *a* fluorescence transient. This test is based on the energy flux from bio-membranes [54]. The performance index (PIABS) [55] was employed as a pa‐ rameter to quantify the effects of environmental factors on photosynthesis in several studies.

Figure (1A) shows that *K. rugosa* underwent a significant decrease of 120 and 38% in leaf wa‐ ter potential and the chlorophyll index (1B), respectively, in the dry season. Mean leaf *Ψw* was -0.34 MPa in the wet season and -0.75 MPa in dry season.

An analysis of florescence transients in *K. rugosa* under the two distinct water availability conditions (wet and dry season) may provide information on changes taking place in the structure, conformation and function of the photosynthetic apparatus, especially in PSII. Ini‐ tial florescence (F0) represents the basal emission of Chl florescence when redox compo‐ nents of photosystems are fully oxidised. This requires appropriate dark adaptation. The results reveal an increase in F0 in the dry season, which may be explained by the initial damage occurring in PSII, likely due to the high temperatures and low water availability (Table 1). This increase in F0 is dependent on structural conditions affecting the probability of the energy transference within the pigments of the light harvesting complex to the PSII reaction centre [56]. According to Bolhàr-Noderkampf et al. [35], the increase in F0 increase in the dry season may indicate a reduction in energy transference to the PSII reaction centre or a partially-reversible inactivation [48].

**Seasons Variable Wet Dry**

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F0 513 ± 7b 627 ± 26a F50µs (O) 570.5 ± 9b 692.8 ± 27a 100 µs 622.3 ± 12b 760.5 ± 30a F300µs 840.4 ± 19b 966.2 ± 41a F2ms (J) 1434 ± 32a 1299 ± 53b F30ms (I) 2394 ± 67a 1575 ± 88b Fv 2425 ± 59a 1352 ± 123b Fm (P) 2938 ± 65a 1979 ± 110b tFm 370.0 ± 26a 248,0 ± 12b **Area 67636 ± 2308a 35236 ± 2657b**

**Table 1.** Initial florescence (F0), florescence intensity at 50 µs (O=F50µs), 100 µs (F 100 µs), 300 µs (F300µs), 2 ms ("J"F2ms), and 30ms ("I"F30ms), variable florescence (Fv) maximum florescence (Fm=P) time to reach Fm (tFm) and area beneath the florescence in *Kielmeyera rugosa* Choisy on wet and dry season. Mean values (*n*=10) ±SE are show. Mean values followed by the same small letters for the seasons are not significantly different (*P>0.05*; t test). (Silva Junior CD,

The area over the fluorescence curve between F0 and Fm was lower in dry season than in the wet season, suggesting a decrease in the electron pool size of PSII, including QA, QB and PQ (Table 1) [60]. If the electron transfer from the reaction centre (RC) to the quinone pool is blocked, this area is dramatically reduced [61]. In comparison to the wet season, the area over the florescence curve was significantly decreased with the increase in drought and tem‐ perature. This inhibition is more accentuated by the interaction between high temperatures and light intensity, which leads to the blockage of electron transfers from the RC to the qui‐ none pool. These results are in agreement with those described by Metha et al. [62], who found an inhibition in the electron transfer rates on the donor side of PSII in *Triticum aesti‐*

The results of flux ratio (yields) in *K. rugosa* revealed a decrease in TRO/ABS (φPo), ETO/TRO (ψo) and, consequently, ETO/ABS (φEo) in the dry season (Figure 2 A, E and B). The decrease in φPo (18%) under water stress indicates a loss of the maximum quantum efficiency of pri‐ mary photochemistry due to photoinhibition caused by excess energy. Moreover, this excess induced the inactivation of 31% of active RCs per cross-section in the dry season, causing increased energy dissipation as well as lower φPo values (Figure 2C). Under water stress, *K.*

The performance index (PIABS) combines three independent functional steps of photosyn‐ thesis (the density of RCs in the chlorophyll bed, excitation energy trapping and conversion of excitation energy to electron transport) in a single multi-parametric expression [55], which is a function of ψ0, φPo and RC/ABS [63, 64]. The results revealed much higher PIABS

*rugosa* also exhibited a 35% decrease in φEo in comparison to the wet season.

unpublished data).

*vum* leaves treated with 0.5 M NaCl.

**Figure 1.** Mean values of leaf water potential (A) and Chlorophyll index (B) on wet and dry season in Kielmeyera rugo‐ *sa* Choisy growing under field conditions at 'restinga' in the Municipality of Pirambu, Sergipe State, Brazil. Each value represents a means of 10 replicates and bars indicate standard deviations. Mean values followed by the same small letters for the seasons are not significantly different (*P>0.05*; t-test). (Silva Junior CD, unpublished data).

The strong decrease in Fm in the dry season was likely associated with the higher tempera‐ tures (Table 1). This decrease in *K. rugosa* may be related to the loss of PSII activity due to conformational changes in the D1 protein [57], causing alterations in the properties of PSII electron acceptors [58]. Other factors may be associated with the heat-related decrease in Fm, such as the migration of damaged PSII reaction centres to non-stacked thylacoid regions and accelerated energy transference to non-fluorescent PSI [48]. The decrease in Fm may al‐ so be due to the disruption of electron donation from water to PSII due to the loss of the manganese atom and extrinsic proteins from the oxygen evolution complex [59]. Such events may be related to susceptibility to high temperatures.


tial florescence (F0) represents the basal emission of Chl florescence when redox compo‐ nents of photosystems are fully oxidised. This requires appropriate dark adaptation. The results reveal an increase in F0 in the dry season, which may be explained by the initial damage occurring in PSII, likely due to the high temperatures and low water availability (Table 1). This increase in F0 is dependent on structural conditions affecting the probability of the energy transference within the pigments of the light harvesting complex to the PSII reaction centre [56]. According to Bolhàr-Noderkampf et al. [35], the increase in F0 increase in the dry season may indicate a reduction in energy transference to the PSII reaction centre

**Figure 1.** Mean values of leaf water potential (A) and Chlorophyll index (B) on wet and dry season in Kielmeyera rugo‐ *sa* Choisy growing under field conditions at 'restinga' in the Municipality of Pirambu, Sergipe State, Brazil. Each value represents a means of 10 replicates and bars indicate standard deviations. Mean values followed by the same small

The strong decrease in Fm in the dry season was likely associated with the higher tempera‐ tures (Table 1). This decrease in *K. rugosa* may be related to the loss of PSII activity due to conformational changes in the D1 protein [57], causing alterations in the properties of PSII electron acceptors [58]. Other factors may be associated with the heat-related decrease in Fm, such as the migration of damaged PSII reaction centres to non-stacked thylacoid regions and accelerated energy transference to non-fluorescent PSI [48]. The decrease in Fm may al‐ so be due to the disruption of electron donation from water to PSII due to the loss of the manganese atom and extrinsic proteins from the oxygen evolution complex [59]. Such

letters for the seasons are not significantly different (*P>0.05*; t-test). (Silva Junior CD, unpublished data).

events may be related to susceptibility to high temperatures.

or a partially-reversible inactivation [48].

24 Responses of Organisms to Water Stress

**Table 1.** Initial florescence (F0), florescence intensity at 50 µs (O=F50µs), 100 µs (F 100 µs), 300 µs (F300µs), 2 ms ("J"F2ms), and 30ms ("I"F30ms), variable florescence (Fv) maximum florescence (Fm=P) time to reach Fm (tFm) and area beneath the florescence in *Kielmeyera rugosa* Choisy on wet and dry season. Mean values (*n*=10) ±SE are show. Mean values followed by the same small letters for the seasons are not significantly different (*P>0.05*; t test). (Silva Junior CD, unpublished data).

The area over the fluorescence curve between F0 and Fm was lower in dry season than in the wet season, suggesting a decrease in the electron pool size of PSII, including QA, QB and PQ (Table 1) [60]. If the electron transfer from the reaction centre (RC) to the quinone pool is blocked, this area is dramatically reduced [61]. In comparison to the wet season, the area over the florescence curve was significantly decreased with the increase in drought and tem‐ perature. This inhibition is more accentuated by the interaction between high temperatures and light intensity, which leads to the blockage of electron transfers from the RC to the qui‐ none pool. These results are in agreement with those described by Metha et al. [62], who found an inhibition in the electron transfer rates on the donor side of PSII in *Triticum aesti‐ vum* leaves treated with 0.5 M NaCl.

The results of flux ratio (yields) in *K. rugosa* revealed a decrease in TRO/ABS (φPo), ETO/TRO (ψo) and, consequently, ETO/ABS (φEo) in the dry season (Figure 2 A, E and B). The decrease in φPo (18%) under water stress indicates a loss of the maximum quantum efficiency of pri‐ mary photochemistry due to photoinhibition caused by excess energy. Moreover, this excess induced the inactivation of 31% of active RCs per cross-section in the dry season, causing increased energy dissipation as well as lower φPo values (Figure 2C). Under water stress, *K. rugosa* also exhibited a 35% decrease in φEo in comparison to the wet season.

The performance index (PIABS) combines three independent functional steps of photosyn‐ thesis (the density of RCs in the chlorophyll bed, excitation energy trapping and conversion of excitation energy to electron transport) in a single multi-parametric expression [55], which is a function of ψ0, φPo and RC/ABS [63, 64]. The results revealed much higher PIABS values in the wet season than in the dry season, possibly due to the photoinhibition caused by excess of light energy and lower water potential (Figure 1).

[66]. Moreover, due to excess irradiance, the transfer of energy to other systems could also

Drought and Its Consequences to Plants – From Individual to Ecosystem

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27

Analysing Ψ0, the lowest φPo values in *K. rugosa* were found under drought conditions. Ψ0 val‐ ues decreased to a remarkably greater extent in the dry season in comparison to wet season. This result reflects a reduction in the pool of plastoquinone (PQ) in an oxidised state and the reoxida‐ tion inhibition of QA- and demonstrates that, besides the loss of energy to QA, the loss of excita‐ tion energy further from QA was significant [67]. The φPo, Ψ0 and φEo results allow one to deduce that *K. rugosa* may use light energy more efficiently in the wet season due to the greater

The performance index (PIABS) is a consistent parameter for the evaluation of plant perform‐ ance regarding light energy absorption, excitation energy trapping and the conversion of exci‐ tation energy to electron transport by photosynthesis under different stress conditions [55, 68]. The PIABS expresses both a function of the fluorescence extreme F0 and Fm as well as the inter‐ mediate J-step and the slope at the origin of the fluorescence rise, whereas φPo expresses a func‐ tion of only F0 and Fm, independently of how the trajectory of the fluorescence intensity reaches its maximal value [68]. Furthermore, the PIABS allows a broader analysis of photosynthetic per‐ formance, such as the relationship between photon absorption efficiency and the capture of ex‐ cited energy in PSII, as well as an analysis of the density of active RCs and the probability that excited energy moves an electron further than QA-. Therefore, the PIABS is a better parameter for evaluating the responses of PSII to stressful conditions than φPo alone. In the present case

The production of reactive oxygen species (ROS) is an unavoidable consequence of life with oxygen. The introduction of molecular oxygen (O2) in the atmosphere during the Paleoprotero‐ zoic era (between 2.7 billion and 1.6 billion years ago) by the emergence of photosynthetic bluegreen algae and later by higher plants led to the accumulation of O2 in the atmosphere and oceans, inducing substantial changes in the living conditions of the earth. The atmosphere grad‐ ually changed from a reducing to an oxidising environment, thereby altering the pace and di‐ rection of evolution [69]. Ever since, ROS have been the unwelcome companions of aerobic life. Unlike of O2, these partially reduced or activated derivatives of oxygen [singlet oxygen (1

active and toxic and can cause oxidative damage to carbohydrates, lipids, amino acids, proteins and nucleic acids [70]. Consequently, the evolution of all aerobic organisms has been dependent

Under normal plant growth conditions, ROS are continuously produced and scavenged in organelles, such as chloroplasts, mitochondria and peroxisomes. However, the balance be‐ tween ROS-producing pathways and ROS-scavenging mechanisms may be disrupted when plants experience environmental stress, such as drought, flooding, salt, heat, chill, heavy

), hydrogen peroxide (H2O2) and hydroxyl radical (•OH)] are highly re‐

O2),

take place, such as the energy-dependent formation of ROS [61].

amount of chlorophyll and higher leaf water potential (Figure 1A,B).

study, the PIABS in *K. rugosa* was much lower in the dry season.

on the development of efficient ROS-scavenging mechanisms.

**4. Oxidative stress and its effect to plants**

**4.1. Living with oxygen**

superoxide radical (•O2ˉ

**Figure 2.** Maximum efficiency of PSII (φPo= TRO/ABS), maximum efficiency of non-photochemical de-excitation (φDo= DIO/ABS), probability that a trapped exciton (ψo= ETO/TRO) or that an absorbed photon (φEo= ETO/ABS) can move an electron further from QA, density of active reaction centers per cross section (RC/CS), and performance index (PIABS) in *Kielmeyera rugosa* Choisy under wet and dry season. Mean values followed by the same small letters for the seasons are not significantly different (*P>0.05*; t test). Mean values (*n*=10) ±SE. (Silva Junior CD, unpublished data).

φPo (Fv/Fm = TRO/ABS) is a parameter that expresses maximal PSII efficiency, which is con‐ trolled by the primary photochemistry of PSII (charge separation, recombination and stabili‐ sation), the non-radiative loss of excited states in light-harvesting antennae and excited states quenched by oxidised PQ molecules from the PQ pool [65]. The low φPo values in *K. rugosa* under drought conditions could have resulted from the inactivity of the RCs, which may favour greater energy dissipation in the form of heat and fluorescence, as deduced from the high φDovalues. This may be associated with increased heat sinks (heat-sink centres or silent centres), which may absorb light in a similar manner as that of active RCs, but are unable to store the excitation energy as redox energy and dissipate their total energy as heat [66]. Moreover, due to excess irradiance, the transfer of energy to other systems could also take place, such as the energy-dependent formation of ROS [61].

Analysing Ψ0, the lowest φPo values in *K. rugosa* were found under drought conditions. Ψ0 val‐ ues decreased to a remarkably greater extent in the dry season in comparison to wet season. This result reflects a reduction in the pool of plastoquinone (PQ) in an oxidised state and the reoxida‐ tion inhibition of QA- and demonstrates that, besides the loss of energy to QA, the loss of excita‐ tion energy further from QA was significant [67]. The φPo, Ψ0 and φEo results allow one to deduce that *K. rugosa* may use light energy more efficiently in the wet season due to the greater amount of chlorophyll and higher leaf water potential (Figure 1A,B).

The performance index (PIABS) is a consistent parameter for the evaluation of plant perform‐ ance regarding light energy absorption, excitation energy trapping and the conversion of exci‐ tation energy to electron transport by photosynthesis under different stress conditions [55, 68]. The PIABS expresses both a function of the fluorescence extreme F0 and Fm as well as the inter‐ mediate J-step and the slope at the origin of the fluorescence rise, whereas φPo expresses a func‐ tion of only F0 and Fm, independently of how the trajectory of the fluorescence intensity reaches its maximal value [68]. Furthermore, the PIABS allows a broader analysis of photosynthetic per‐ formance, such as the relationship between photon absorption efficiency and the capture of ex‐ cited energy in PSII, as well as an analysis of the density of active RCs and the probability that excited energy moves an electron further than QA-. Therefore, the PIABS is a better parameter for evaluating the responses of PSII to stressful conditions than φPo alone. In the present case study, the PIABS in *K. rugosa* was much lower in the dry season.

## **4. Oxidative stress and its effect to plants**

## **4.1. Living with oxygen**

values in the wet season than in the dry season, possibly due to the photoinhibition caused

**Figure 2.** Maximum efficiency of PSII (φPo= TRO/ABS), maximum efficiency of non-photochemical de-excitation (φDo= DIO/ABS), probability that a trapped exciton (ψo= ETO/TRO) or that an absorbed photon (φEo= ETO/ABS) can move an electron further from QA, density of active reaction centers per cross section (RC/CS), and performance index (PIABS) in *Kielmeyera rugosa* Choisy under wet and dry season. Mean values followed by the same small letters for the seasons

φPo (Fv/Fm = TRO/ABS) is a parameter that expresses maximal PSII efficiency, which is con‐ trolled by the primary photochemistry of PSII (charge separation, recombination and stabili‐ sation), the non-radiative loss of excited states in light-harvesting antennae and excited states quenched by oxidised PQ molecules from the PQ pool [65]. The low φPo values in *K. rugosa* under drought conditions could have resulted from the inactivity of the RCs, which may favour greater energy dissipation in the form of heat and fluorescence, as deduced from the high φDovalues. This may be associated with increased heat sinks (heat-sink centres or silent centres), which may absorb light in a similar manner as that of active RCs, but are unable to store the excitation energy as redox energy and dissipate their total energy as heat

are not significantly different (*P>0.05*; t test). Mean values (*n*=10) ±SE. (Silva Junior CD, unpublished data).

by excess of light energy and lower water potential (Figure 1).

26 Responses of Organisms to Water Stress

The production of reactive oxygen species (ROS) is an unavoidable consequence of life with oxygen. The introduction of molecular oxygen (O2) in the atmosphere during the Paleoprotero‐ zoic era (between 2.7 billion and 1.6 billion years ago) by the emergence of photosynthetic bluegreen algae and later by higher plants led to the accumulation of O2 in the atmosphere and oceans, inducing substantial changes in the living conditions of the earth. The atmosphere grad‐ ually changed from a reducing to an oxidising environment, thereby altering the pace and di‐ rection of evolution [69]. Ever since, ROS have been the unwelcome companions of aerobic life. Unlike of O2, these partially reduced or activated derivatives of oxygen [singlet oxygen (1 O2), superoxide radical (•O2ˉ ), hydrogen peroxide (H2O2) and hydroxyl radical (•OH)] are highly re‐ active and toxic and can cause oxidative damage to carbohydrates, lipids, amino acids, proteins and nucleic acids [70]. Consequently, the evolution of all aerobic organisms has been dependent on the development of efficient ROS-scavenging mechanisms.

Under normal plant growth conditions, ROS are continuously produced and scavenged in organelles, such as chloroplasts, mitochondria and peroxisomes. However, the balance be‐ tween ROS-producing pathways and ROS-scavenging mechanisms may be disrupted when plants experience environmental stress, such as drought, flooding, salt, heat, chill, heavy metals, nutrient deficiencies, UV radiation, intense light, air pollutants, herbicides, mechani‐ cal stress and attacks from pathogens [71].

The superoxide (•O2ˉ) produced during the first reaction is a short-lived ROS (approximate‐ ly 2 to 4 µs) and not readily diffusible [72]. In the cellular environment, •O2ˉ may cause lipid peroxidation, thereby weakening cell membranes. The second reduction is an exergonic re‐ action that generates hydrogen peroxide (H2O2), a relatively long-lived (1 ms) and stable form of ROS that can diffuse through membranes and therefore reach cellular components distant from its site of synthesis [73]. The last ROS generated by this series of reductions is also exergonic and produces the highly reactive hydroxyl radical (•OH), which is the most harmful form of ROS in plant tissues, has a half-life of 1 ρs and has a very high affinity for biological molecules [74]. The hydroxyl radical is generated from the reaction between •O2ˉ and H2O2 either spontaneously through the Haber-Weiss reaction or in the presence of re‐

Under normal cell conditions, the Haber-Weiss reaction (1) occurs very slowly and very low

The hydroxyl radical is also formed in very low amounts in the Fenton reaction (2), which is

The availability of Fe2+ limits the reaction rate, but Fe3+ can be efficiently reduced by super‐ oxide, thereby maintaining the Fenton reaction ongoing and leading to the generation of

The prevention of the Haber-Weiss and Fenton reactions is achieved when H2O2 and •O2ˉ

Due to the high reactivity of •OH radicals, which is the main cause of cell damage under oxidative stress, it is difficult to control their concentration enzymatically. Therefore, plants reduce the presence of this radical by controlling the upstream reactions of •OH formation via Haber-Weiss/Fenton reactions through the elimination of H2O2 and •O2ˉ prior to their contact with each other. The efficient destruction of •O2ˉ and H2O2 requires the coordinated action of several antioxidative enzymes and a network of low molecular mass antioxidants.

are eliminated prior to these molecules entering into contact with each other.

common in biological systems, with its transition metals Fe2+ and Cu+

H2O2+•O2 <sup>ˉ</sup> <sup>→</sup>• OH + OH2 (1)

Drought and Its Consequences to Plants – From Individual to Ecosystem

2+ 3+ • H O + Fe Fe + OH + OH 2 2 ® (2)

( ) ( ) 2+ + 3+ 2+ • H O + Fe Cu Fe Cu + OH + OH 2 2 ® (3)

( ) ( ) • 3+ 2+ 2+ + O + Fe Cu Fe Cu + O 2 2 <sup>ˉ</sup> ® (4)

in a chelated form:

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29

duced transition metals through the Fenton reaction.

•OH, as shown in the two half reactions (3) and (4):

amounts of •OH are formed:

The excessive production of ROS is responsible for secondary stress known as oxidative stress. Therefore, plant tolerance to drought and other forms of abiotic stress that induce an increase in the generation of ROS depends on the development of efficient ROS-scavenging mechanisms.

#### **4.2. Chemistry of ROS**

Much of the behaviour of molecular oxygen (or dioxygen) and its partially reduced species de‐ rive from their reduction potentials and molecular orbital structures. The dioxygen molecule is a highly unusual, stable diradical with a pair of electrons with parallel spins. To oxidise a nonradical atom or molecule, dioxygen would need to react with a chemical species that provides a pair of electrons with parallel spins that fit into its free electron orbitals. Fortunately, pairs of electrons typically have opposite spins, which imposes a restriction on the reaction of molecular oxygen with most organic molecules, such as amino acids and nucleic acids [70].

However, dioxygen may be converted into ROS either by energy transfer or monovalent re‐ duction. If oxygen absorbs enough energy to reverse the spin of one of its unpaired elec‐ trons, it forms singlet oxygen (1 O2), in which the two electrons have opposite spins. Since paired electrons are common in organic molecules, singlet oxygen is much more reactive to‐ ward organic molecules than dioxygen in its ground state. The second mechanism of oxygen activation is stepwise monovalent reduction through electron transfer reactions with the un‐ paired electrons of transition metals and organic radicals, resulting in the sequential forma‐ tion of superoxide anion (•O2ˉ ), hydrogen peroxide (H2O2), hydroxyl radical (•OH) and, finally, water (Figure 3). The first reduction step is free energy dependent (endergonic) and requires electron donation, but the following one-electron reduction steps are exergonic and can occur spontaneously, using transition metal ions (Fe2+ and Cu+ ) and semiquinones as electron donors [70].

**Figure 3.** Pathways in the univalent reduction of oxygen to water leading for the formation of various intermediate reactive oxygen species (ROS). Numbers give approximate redox potentials (in volts) or the standard free energy of the reaction (in kJ mol-1).

The superoxide (•O2ˉ) produced during the first reaction is a short-lived ROS (approximate‐ ly 2 to 4 µs) and not readily diffusible [72]. In the cellular environment, •O2ˉ may cause lipid peroxidation, thereby weakening cell membranes. The second reduction is an exergonic re‐ action that generates hydrogen peroxide (H2O2), a relatively long-lived (1 ms) and stable form of ROS that can diffuse through membranes and therefore reach cellular components distant from its site of synthesis [73]. The last ROS generated by this series of reductions is also exergonic and produces the highly reactive hydroxyl radical (•OH), which is the most harmful form of ROS in plant tissues, has a half-life of 1 ρs and has a very high affinity for biological molecules [74]. The hydroxyl radical is generated from the reaction between •O2ˉ and H2O2 either spontaneously through the Haber-Weiss reaction or in the presence of re‐ duced transition metals through the Fenton reaction.

metals, nutrient deficiencies, UV radiation, intense light, air pollutants, herbicides, mechani‐

The excessive production of ROS is responsible for secondary stress known as oxidative stress. Therefore, plant tolerance to drought and other forms of abiotic stress that induce an increase in the generation of ROS depends on the development of efficient ROS-scavenging

Much of the behaviour of molecular oxygen (or dioxygen) and its partially reduced species de‐ rive from their reduction potentials and molecular orbital structures. The dioxygen molecule is a highly unusual, stable diradical with a pair of electrons with parallel spins. To oxidise a nonradical atom or molecule, dioxygen would need to react with a chemical species that provides a pair of electrons with parallel spins that fit into its free electron orbitals. Fortunately, pairs of electrons typically have opposite spins, which imposes a restriction on the reaction of molecular

However, dioxygen may be converted into ROS either by energy transfer or monovalent re‐ duction. If oxygen absorbs enough energy to reverse the spin of one of its unpaired elec‐

paired electrons are common in organic molecules, singlet oxygen is much more reactive to‐ ward organic molecules than dioxygen in its ground state. The second mechanism of oxygen activation is stepwise monovalent reduction through electron transfer reactions with the un‐ paired electrons of transition metals and organic radicals, resulting in the sequential forma‐

finally, water (Figure 3). The first reduction step is free energy dependent (endergonic) and requires electron donation, but the following one-electron reduction steps are exergonic and

**Figure 3.** Pathways in the univalent reduction of oxygen to water leading for the formation of various intermediate reactive oxygen species (ROS). Numbers give approximate redox potentials (in volts) or the standard free energy of the

O2), in which the two electrons have opposite spins. Since

), hydrogen peroxide (H2O2), hydroxyl radical (•OH) and,

) and semiquinones as

oxygen with most organic molecules, such as amino acids and nucleic acids [70].

can occur spontaneously, using transition metal ions (Fe2+ and Cu+

cal stress and attacks from pathogens [71].

mechanisms.

**4.2. Chemistry of ROS**

28 Responses of Organisms to Water Stress

trons, it forms singlet oxygen (1

tion of superoxide anion (•O2ˉ

electron donors [70].

reaction (in kJ mol-1).

Under normal cell conditions, the Haber-Weiss reaction (1) occurs very slowly and very low amounts of •OH are formed:

H2O2+•O2 <sup>ˉ</sup> <sup>→</sup>• OH + OH2 (1)

The hydroxyl radical is also formed in very low amounts in the Fenton reaction (2), which is common in biological systems, with its transition metals Fe2+ and Cu+ in a chelated form:

$$\text{H}\_2\text{O}\_2 + \text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{OH} + \text{OH} \tag{2}$$

The availability of Fe2+ limits the reaction rate, but Fe3+ can be efficiently reduced by super‐ oxide, thereby maintaining the Fenton reaction ongoing and leading to the generation of •OH, as shown in the two half reactions (3) and (4):

$$\text{^1H}\_2\text{O}\_2 \star \text{Fe}^{2+} \text{(Cu}^{\ast}) \rightarrow \text{Fe}^{3+} \text{(Cu}^{2+} \text{)} \star \text{^7OH} \star \text{OH} \tag{3}$$

$$\text{"{O}}\_2\text{"{}+}\text{Fe}^{3+}\left(\text{Cu}^{2+}\right)\rightarrow\text{Fe}^{2+}\left(\text{Cu}^{+}\right)\star\text{O}\_2\tag{4}$$

The prevention of the Haber-Weiss and Fenton reactions is achieved when H2O2 and •O2ˉ are eliminated prior to these molecules entering into contact with each other.

Due to the high reactivity of •OH radicals, which is the main cause of cell damage under oxidative stress, it is difficult to control their concentration enzymatically. Therefore, plants reduce the presence of this radical by controlling the upstream reactions of •OH formation via Haber-Weiss/Fenton reactions through the elimination of H2O2 and •O2ˉ prior to their contact with each other. The efficient destruction of •O2ˉ and H2O2 requires the coordinated action of several antioxidative enzymes and a network of low molecular mass antioxidants.

#### **4.3. Antioxidative system**

To mitigate oxidative harm from ROS, plants possess a complex antioxidative system that involves both non-enzymatic and enzymatic antioxidant defences. Non-enzymatic defences include hydrophilic compounds, such as ascorbate and reduced glutathione, and lipophilic compounds, such as tocopherols and carotenoids, which are capable of quenching ROS. Enzymatic defences include superoxide dismutase, catalase and peroxi‐ dase. Moreover, an entire array of enzymes is needed for the regeneration of the active forms of antioxidants (glutathione reductase, monodehydroascorbate reductase and dehy‐ droascorbate reductase) [70, 75].

peroxisomes and glyoxysomes [78]. APX seems to play a key role as a scavenger of H2O2

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31

APX uses two ASC molecules to reduce H2O2 to water and produce two monodehydroascor‐ bate (MDHA) molecules (Figure 2). MDHA is a short-lived radical that can either spontane‐ ously dismutate to ascorbate and dehydroascorbate (DHA) (Figure 2) or be reduced to ascorbate by NAD(P)H via monodehydroascorbate reductase (MDHAR; EC 1.6.5.4) (Figure 2), which is found in different cell compartments [16] (Asada, 1997). DHA is reduced to as‐ corbate by the action of dehydroascorbate reductase (DHAR; EC 1.8.5.1), using reduced glu‐ tathione (GSH) as the reducing substrate. This reaction generates reduced glutathione (GSSG), which is, in turn, re-reduced to GSH by NADPH, a reaction catalysed by gluta‐ thione reductase (GR; EC 1.6.4.2). The removal of H2O2 through this series of reactions is known as the ascorbate-glutathione cycle or the Halliwell-Asada pathway (Figure 2) [75]. Ascorbate and glutathione are not consumed in this pathway, but participate in the cyclic transfer of reducing equivalents, which allows the reduction of H2O2 to H2O, with NADPH

Class III plant peroxidase (EC 1.11.1.7) is a plant-specific oxidoreductase, the activity of which was described as early as 1855. This enzyme is a heme-containing glycoprotein en‐ coded by a large multigene family in plants. POX, which is found in the cytosol, vacuole and cell wall, is less specific to the electron donor substrate than APX and decomposes H2O2 through the oxidation of co-substrates, such as phenolic compounds and/or ascorbate [79]. This enzyme is relatively stable at high temperatures and its activity is easily measured us‐

The different types of GPX (EC 1.11.1.9) form a large family of diverse isozymes that reduce H2O2 and organic and lipid hydroperoxides using GSH as a reducing agent. In plants, how‐ ever, it has been suggested that GPX preferably uses thioredoxin as a reductant [80, 81]. Most cellular GPXs are tetrameric enzymes with four identical 22 kDa subunits, each con‐ taining a selenocysteine residue in the active site [82]. Selenocysteine participates directly in electron donation to the peroxide substrate and becomes oxidised in the process. The en‐ zyme then uses reduced glutathione as a hydrogen donor to regenerate selenocysteine. GPX uses two GSH molecules to reduce H2O2 to water and produce a GSSG molecule (Figure 4).

Taken together, the major ROS-scavenging pathways in plants include SOD, found in al‐ most all cell compartments, CAT in peroxisomes, POX in the cytosol, vacuole and cell wall and the ascorbate-glutathione cycle in the chloroplasts, cytosol, mitochondria, apoplast and peroxisomes. As mentioned above, CAT has extremely high maximal catalytic rates, but low substrate affinities, while APX has a much higher affinity for H2O2 than CAT. The high affin‐ ity of APX for H2O2, in conjunction with the finding of the ascorbate-glutathione cycle in nearly all cell compartments, suggests that this cycle plays a crucial role in controlling the level of ROS in these compartments. Moreover, APX might also be responsible for the fine modulation of H2O2 for signalling. In contrast, CAT, which is only present in peroxisomes, is indispensable to H2O2 detoxification during stress, when high levels of ROS are produced.

that could leak from these cell organelles.

as the reducing equivalent donor.

ing simple chromogenic reactions.

#### *4.3.1. Superoxide Dismutases (SOD)*

Superoxide dismutases (EC 1.15.1.1) catalyse the dismutation of superoxide into hydrogen peroxide and water. SOD activity modulates the relative amounts of •O2ˉ and H2O2 (the two Haber-Weiss reaction substrates) and decreases the risk of the formation of the •OH radical. Since SOD is one of the ubiquitous enzymes in aerobic organisms and is present in most subcellular compartments that generate ROS, this enzyme is considered to play a key role in cell defence mechanisms against ROS [76, 77]. The product of SOD activity is H2O2, which is toxic and must be eliminated by conversion into H2O in subsequent reactions. Although a number of enzymes regulate the intracellular levels of H2O2 in plants, catalases and peroxi‐ dases are considered to be the most important.

## *4.3.2. Catalases (CAT)*

Catalases (EC 1.11.1.6) are tetrameric heme-containing enzymes that catalyse the dismuta‐ tion of hydrogen peroxide into water and molecular oxygen, thereby protecting the cell from the harmful effects of H2O2 accumulation. CAT is found in all aerobic eukaryotes and is associated with the removal of H2O2 generated in biochemical processes, such as the β-oxidation of fatty acids, the glyoxylate cycle (photorespiration) and purine catabo‐ lism. CAT activity may decrease under salt stress, heat shock or cold stress, which may be related to plant tolerance to the secondary oxidative stress induced by these forms of environmental stress.

#### *4.3.3. Peroxidases and enzymes regenerating active forms of ascorbate and glutathione*

Peroxidases constitute a class of enzymes in the tissues of animals, plants and microorgan‐ isms and catalyse the oxidoreduction between hydrogen peroxide and different reductants. There are three classes of plant peroxidases, but ascorbate peroxidase (APX), class III plant peroxidases [or non-specific peroxidases or guaiacol-type peroxidase (POX)] and gluta‐ thione peroxidase (GPX) are considered to be the most important plant peroxidases related to the antioxidative system.

Ascorbate peroxidase (EC 1.11.1.11) catalyses the reduction of H2O2 to H2O and has high specificity and affinity for ascorbate (ASC) as a reductant. Its sequence is distinct from other peroxidases and different forms of APX are found in the chloroplasts, cytosol, mitochondria, peroxisomes and glyoxysomes [78]. APX seems to play a key role as a scavenger of H2O2 that could leak from these cell organelles.

**4.3. Antioxidative system**

30 Responses of Organisms to Water Stress

droascorbate reductase) [70, 75].

*4.3.1. Superoxide Dismutases (SOD)*

*4.3.2. Catalases (CAT)*

environmental stress.

to the antioxidative system.

dases are considered to be the most important.

To mitigate oxidative harm from ROS, plants possess a complex antioxidative system that involves both non-enzymatic and enzymatic antioxidant defences. Non-enzymatic defences include hydrophilic compounds, such as ascorbate and reduced glutathione, and lipophilic compounds, such as tocopherols and carotenoids, which are capable of quenching ROS. Enzymatic defences include superoxide dismutase, catalase and peroxi‐ dase. Moreover, an entire array of enzymes is needed for the regeneration of the active forms of antioxidants (glutathione reductase, monodehydroascorbate reductase and dehy‐

Superoxide dismutases (EC 1.15.1.1) catalyse the dismutation of superoxide into hydrogen peroxide and water. SOD activity modulates the relative amounts of •O2ˉ and H2O2 (the two Haber-Weiss reaction substrates) and decreases the risk of the formation of the •OH radical. Since SOD is one of the ubiquitous enzymes in aerobic organisms and is present in most subcellular compartments that generate ROS, this enzyme is considered to play a key role in cell defence mechanisms against ROS [76, 77]. The product of SOD activity is H2O2, which is toxic and must be eliminated by conversion into H2O in subsequent reactions. Although a number of enzymes regulate the intracellular levels of H2O2 in plants, catalases and peroxi‐

Catalases (EC 1.11.1.6) are tetrameric heme-containing enzymes that catalyse the dismuta‐ tion of hydrogen peroxide into water and molecular oxygen, thereby protecting the cell from the harmful effects of H2O2 accumulation. CAT is found in all aerobic eukaryotes and is associated with the removal of H2O2 generated in biochemical processes, such as the β-oxidation of fatty acids, the glyoxylate cycle (photorespiration) and purine catabo‐ lism. CAT activity may decrease under salt stress, heat shock or cold stress, which may be related to plant tolerance to the secondary oxidative stress induced by these forms of

Peroxidases constitute a class of enzymes in the tissues of animals, plants and microorgan‐ isms and catalyse the oxidoreduction between hydrogen peroxide and different reductants. There are three classes of plant peroxidases, but ascorbate peroxidase (APX), class III plant peroxidases [or non-specific peroxidases or guaiacol-type peroxidase (POX)] and gluta‐ thione peroxidase (GPX) are considered to be the most important plant peroxidases related

Ascorbate peroxidase (EC 1.11.1.11) catalyses the reduction of H2O2 to H2O and has high specificity and affinity for ascorbate (ASC) as a reductant. Its sequence is distinct from other peroxidases and different forms of APX are found in the chloroplasts, cytosol, mitochondria,

*4.3.3. Peroxidases and enzymes regenerating active forms of ascorbate and glutathione*

APX uses two ASC molecules to reduce H2O2 to water and produce two monodehydroascor‐ bate (MDHA) molecules (Figure 2). MDHA is a short-lived radical that can either spontane‐ ously dismutate to ascorbate and dehydroascorbate (DHA) (Figure 2) or be reduced to ascorbate by NAD(P)H via monodehydroascorbate reductase (MDHAR; EC 1.6.5.4) (Figure 2), which is found in different cell compartments [16] (Asada, 1997). DHA is reduced to as‐ corbate by the action of dehydroascorbate reductase (DHAR; EC 1.8.5.1), using reduced glu‐ tathione (GSH) as the reducing substrate. This reaction generates reduced glutathione (GSSG), which is, in turn, re-reduced to GSH by NADPH, a reaction catalysed by gluta‐ thione reductase (GR; EC 1.6.4.2). The removal of H2O2 through this series of reactions is known as the ascorbate-glutathione cycle or the Halliwell-Asada pathway (Figure 2) [75]. Ascorbate and glutathione are not consumed in this pathway, but participate in the cyclic transfer of reducing equivalents, which allows the reduction of H2O2 to H2O, with NADPH as the reducing equivalent donor.

Class III plant peroxidase (EC 1.11.1.7) is a plant-specific oxidoreductase, the activity of which was described as early as 1855. This enzyme is a heme-containing glycoprotein en‐ coded by a large multigene family in plants. POX, which is found in the cytosol, vacuole and cell wall, is less specific to the electron donor substrate than APX and decomposes H2O2 through the oxidation of co-substrates, such as phenolic compounds and/or ascorbate [79]. This enzyme is relatively stable at high temperatures and its activity is easily measured us‐ ing simple chromogenic reactions.

The different types of GPX (EC 1.11.1.9) form a large family of diverse isozymes that reduce H2O2 and organic and lipid hydroperoxides using GSH as a reducing agent. In plants, how‐ ever, it has been suggested that GPX preferably uses thioredoxin as a reductant [80, 81]. Most cellular GPXs are tetrameric enzymes with four identical 22 kDa subunits, each con‐ taining a selenocysteine residue in the active site [82]. Selenocysteine participates directly in electron donation to the peroxide substrate and becomes oxidised in the process. The en‐ zyme then uses reduced glutathione as a hydrogen donor to regenerate selenocysteine. GPX uses two GSH molecules to reduce H2O2 to water and produce a GSSG molecule (Figure 4).

Taken together, the major ROS-scavenging pathways in plants include SOD, found in al‐ most all cell compartments, CAT in peroxisomes, POX in the cytosol, vacuole and cell wall and the ascorbate-glutathione cycle in the chloroplasts, cytosol, mitochondria, apoplast and peroxisomes. As mentioned above, CAT has extremely high maximal catalytic rates, but low substrate affinities, while APX has a much higher affinity for H2O2 than CAT. The high affin‐ ity of APX for H2O2, in conjunction with the finding of the ascorbate-glutathione cycle in nearly all cell compartments, suggests that this cycle plays a crucial role in controlling the level of ROS in these compartments. Moreover, APX might also be responsible for the fine modulation of H2O2 for signalling. In contrast, CAT, which is only present in peroxisomes, is indispensable to H2O2 detoxification during stress, when high levels of ROS are produced.

stricted CO2 supply or CO2-limited carbon fixation and reduced NADP+ regeneration through the Calvin cycle. Photosynthetic electron transport is, however, maintained at a rel‐ atively higher rate in the stressed leaves in comparison to the accentuated reduction in the CO2 fixation rate [87]. This imbalance between the electron transport and CO2 fixation rates results in an accentuated reduction of the electron transport chain and the transfer of elec‐ trons to O2 through the Mehler reaction [88]. One study estimated a 50% increase in the leak‐ age of photosynthetic electrons through the Mehler reaction in drought-stressed wheat

Drought and Its Consequences to Plants – From Individual to Ecosystem

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33

The photorespiratory pathway is also enhanced under drought stress, especially when the oxygenation of ribulose-1,5-bisphosphate is maximal due to limited CO2 fixation [90]. Thus, O2-dependent electron flow and photorespiration can be considered common mechanisms that plants employ to protect the photosynthetic electron transport chain components from photodamage during water deficit. Although it is very difficult to dis‐ criminate the amount of ROS generated by the Mehler reaction from that generated by photorespiration, it has been estimated that photorespiration is likely to account for over 70% of total H2O2 production under drought stress conditions [90]. In such a scenario, there is considerable potential for the increased accumulation of •O2ˉ and H2O2 in plants [91]. In a number of plant species, an increased formation of ROS, lipid peroxidation and protein modification have been observed under water deficit conditions [92-94]. The fol‐ lowing the sequence of events occurs in plant tissues subjected to such conditions: 1) in‐ creased production of ROS and oxidised target molecules; 2) increased expression of genes for antioxidant functions; and 3) increased the levels of non-enzymatic and enzy‐

Drought stress enhances the *de novo* synthesis of some antioxidative enzymes to over‐ come the increase in oxidative stress. In rice plants, the *de novo* synthesis of MDHAR, DHAR and GR increases the capacity for ASC and GSH regeneration, which is consid‐ ered to be one of the primary responses to water deficit so as to mitigate oxidative stress [92, 93]. An increase in the activity of antioxidative enzymes has been reported in a number of plant species submitted to drought stress, enhancing the capacity of the anti‐ oxidative system to scavenge ROS and thereby suppressing the level of lipid peroxida‐

Additionally, the increase in the activity of antioxidative enzymes and antioxidant content under water deficit conditions appears to be extremely variable among different plant spe‐ cies and even cultivars of the same species. Thus, comparative studies using drought-toler‐ ant and drought-sensitive genotypes demonstrate greater antioxidant capacity in tolerant genotypes. In one study, among five mulberry cultivars subjected to drought, two had effi‐ cient antioxidative characteristics that could provide better protection against oxidative stress in leaves under water-limited conditions [98]. Under water stress, a drought-tolerant maize genotype exhibited lower MDA and H2O2 contents and an increase in the SOD, CAT, and POX activities in comparison to a drought-sensitive maize genotype [99]. A drought-tol‐ erant wheat genotype exhibited greater APX and CAT activities, higher ASC content and lower H2O2 and MDA contents in comparison to a drought-susceptible wheat genotype

plants in comparison to non-stressed plants [89].

matic antioxidants, resulting in tolerance to drought stress [95].

tion under drought conditions [93, 96, 97].

**Figure 4.** Generation of • OH by Fenton reaction (in red); • O2ˉ in the mitochondria, peroxisomes and glyoxysomes and by Mehler reaction in chloroplast (in green), singlet oxygen in chloroplast (in dark green), and H2O2 by SOD, photores‐ piration, fatty acid oxidation or other reactions. SOD acts as the first line of defense converting • O2ˉ into H2O2 (in yel‐ low). CAT (in grey), POX (in pink), GPX (in dark blue), and APX (in orange) then detoxify H2O2. In contrast to CAT, APX requires ASC, POX requires phenolic compounds and/or ASC, and GPX requires GSH as electron donor substrate. In the removal of H2O2 through the ascorbate-glutathione cycle (in orange), ASC and GSH participate of the cyclic trans‐ fer of reducing equivalents. This cycle uses NADPH as reducing power. • OH may be removed by GSH (in blue), and the GSSG formed is regenerated via GR. Although the pathways of generation and scavenging in the different cell com‐ partments are separate, H2O2 can easily diffuse through membranes and antioxidants such as GSH and ASC can be transported between the different compartments. Non-enzymatic pathways are indicated by dotted lines. Abbrevia‐ tions: APX, ascorbate peroxidase; ASC, ascorbate; AH2, oxidizable substrate; DHA, dehydroascorbate; DHAR, dehy‐ droascorbate reductase; GPX, glutathione peroxidase; POX, non-specific peroxidase; GR, glutathione reductase; GSH, reduced glutathione; GSSG, oxidized glutathione; hydrogen peroxide (H2O2); hydroxyl radical (• OH); MDHA, monode‐ hydroascorbate; MDHAR, monodehydroascorbate reductase SOD, superoxide dismutase; superoxide radical (• O2ˉ).

#### **4.4. ROS production and scavenging in drought-stressed plants**

The root system is the first plant organ to detect a reduction in the water supply. Besides water and minerals, the roots send signals to the shoots through the xylem sap and the phy‐ tohormone abscisic acid is considered to be one of the major root-to-shoot stress signals [83]. In leaves, abscisic acid triggers stomatal closure and the plant shifts to a water-saving behav‐ iour. By controlling the stomatal opening, plants reduce water loss by decreasing the tran‐ spiration flux. However, the entrance of carbon dioxide (CO2) is also reduced simultaneously. This plant response has direct and indirect effects on the net photosynthesis and overall production of ROS under water deficit conditions [84]. A number of studies re‐ port increased ROS accumulation and oxidative stress in plants under drought stress [85, 86]. When stomata close in order to limit water loss, there is the occurrence of either a re‐ stricted CO2 supply or CO2-limited carbon fixation and reduced NADP+ regeneration through the Calvin cycle. Photosynthetic electron transport is, however, maintained at a rel‐ atively higher rate in the stressed leaves in comparison to the accentuated reduction in the CO2 fixation rate [87]. This imbalance between the electron transport and CO2 fixation rates results in an accentuated reduction of the electron transport chain and the transfer of elec‐ trons to O2 through the Mehler reaction [88]. One study estimated a 50% increase in the leak‐ age of photosynthetic electrons through the Mehler reaction in drought-stressed wheat plants in comparison to non-stressed plants [89].

**photorespiration, fatty acid oxidation, water-splitting (oxidizing side of PSII), oxidative burst**

**O2**

**2 H2O**

**2 H2O H2O2**

**APX**

**2 MDHA 2 ASC**

**NADP NADPH <sup>+</sup>**

O2ˉ in the mitochondria, peroxisomes and glyoxysomes and

OH may be removed by GSH (in blue), and the

**MDHAR**

**CAT**

**AH2**

**2 GSH GSSG**

**H2O + ½ O2**

**POX**

**GPX H2O**

O2ˉ into H2O2 (in yel‐

OH); MDHA, monode‐

O2ˉ).

**A + 2 H2O**

**photorespiration, fatty acid oxidation, water-splitting (oxidizing side of PSII), oxidative burst**

**DHAR**

OH by Fenton reaction (in red); •

fer of reducing equivalents. This cycle uses NADPH as reducing power. •

piration, fatty acid oxidation or other reactions. SOD acts as the first line of defense converting •

reduced glutathione; GSSG, oxidized glutathione; hydrogen peroxide (H2O2); hydroxyl radical (•

**4.4. ROS production and scavenging in drought-stressed plants**

hydroascorbate; MDHAR, monodehydroascorbate reductase SOD, superoxide dismutase; superoxide radical (•

**2 • O2 ˉ**

**mitochondria, peroxisomes, glyoxysomes**

**2 O2**

**DHA**

**Mehler reaction**

**chloroplast**

**β-carotene Tocopherol Plastoquinone**

**ascorbate-glutatione cycle (Halliwell-Asada pathway)**

by Mehler reaction in chloroplast (in green), singlet oxygen in chloroplast (in dark green), and H2O2 by SOD, photores‐

low). CAT (in grey), POX (in pink), GPX (in dark blue), and APX (in orange) then detoxify H2O2. In contrast to CAT, APX requires ASC, POX requires phenolic compounds and/or ASC, and GPX requires GSH as electron donor substrate. In the removal of H2O2 through the ascorbate-glutathione cycle (in orange), ASC and GSH participate of the cyclic trans‐

GSSG formed is regenerated via GR. Although the pathways of generation and scavenging in the different cell com‐ partments are separate, H2O2 can easily diffuse through membranes and antioxidants such as GSH and ASC can be transported between the different compartments. Non-enzymatic pathways are indicated by dotted lines. Abbrevia‐ tions: APX, ascorbate peroxidase; ASC, ascorbate; AH2, oxidizable substrate; DHA, dehydroascorbate; DHAR, dehy‐ droascorbate reductase; GPX, glutathione peroxidase; POX, non-specific peroxidase; GR, glutathione reductase; GSH,

The root system is the first plant organ to detect a reduction in the water supply. Besides water and minerals, the roots send signals to the shoots through the xylem sap and the phy‐ tohormone abscisic acid is considered to be one of the major root-to-shoot stress signals [83]. In leaves, abscisic acid triggers stomatal closure and the plant shifts to a water-saving behav‐ iour. By controlling the stomatal opening, plants reduce water loss by decreasing the tran‐ spiration flux. However, the entrance of carbon dioxide (CO2) is also reduced simultaneously. This plant response has direct and indirect effects on the net photosynthesis and overall production of ROS under water deficit conditions [84]. A number of studies re‐ port increased ROS accumulation and oxidative stress in plants under drought stress [85, 86]. When stomata close in order to limit water loss, there is the occurrence of either a re‐

**2 H+**

**SOD**

**1O2**

**2Fe3+**

**2Fe2+**

**2 GSH**

**GSSG**

**2 H2O**

**GSSG**

**2 OH- + 2 OH**

**2 H2O2**

**2 GSH Fenton**

32 Responses of Organisms to Water Stress

**NADPH**

**Figure 4.** Generation of •

**NADP+**

**GR**

**reaction**

The photorespiratory pathway is also enhanced under drought stress, especially when the oxygenation of ribulose-1,5-bisphosphate is maximal due to limited CO2 fixation [90]. Thus, O2-dependent electron flow and photorespiration can be considered common mechanisms that plants employ to protect the photosynthetic electron transport chain components from photodamage during water deficit. Although it is very difficult to dis‐ criminate the amount of ROS generated by the Mehler reaction from that generated by photorespiration, it has been estimated that photorespiration is likely to account for over 70% of total H2O2 production under drought stress conditions [90]. In such a scenario, there is considerable potential for the increased accumulation of •O2ˉ and H2O2 in plants [91]. In a number of plant species, an increased formation of ROS, lipid peroxidation and protein modification have been observed under water deficit conditions [92-94]. The fol‐ lowing the sequence of events occurs in plant tissues subjected to such conditions: 1) in‐ creased production of ROS and oxidised target molecules; 2) increased expression of genes for antioxidant functions; and 3) increased the levels of non-enzymatic and enzy‐ matic antioxidants, resulting in tolerance to drought stress [95].

Drought stress enhances the *de novo* synthesis of some antioxidative enzymes to over‐ come the increase in oxidative stress. In rice plants, the *de novo* synthesis of MDHAR, DHAR and GR increases the capacity for ASC and GSH regeneration, which is consid‐ ered to be one of the primary responses to water deficit so as to mitigate oxidative stress [92, 93]. An increase in the activity of antioxidative enzymes has been reported in a number of plant species submitted to drought stress, enhancing the capacity of the anti‐ oxidative system to scavenge ROS and thereby suppressing the level of lipid peroxida‐ tion under drought conditions [93, 96, 97].

Additionally, the increase in the activity of antioxidative enzymes and antioxidant content under water deficit conditions appears to be extremely variable among different plant spe‐ cies and even cultivars of the same species. Thus, comparative studies using drought-toler‐ ant and drought-sensitive genotypes demonstrate greater antioxidant capacity in tolerant genotypes. In one study, among five mulberry cultivars subjected to drought, two had effi‐ cient antioxidative characteristics that could provide better protection against oxidative stress in leaves under water-limited conditions [98]. Under water stress, a drought-tolerant maize genotype exhibited lower MDA and H2O2 contents and an increase in the SOD, CAT, and POX activities in comparison to a drought-sensitive maize genotype [99]. A drought-tol‐ erant wheat genotype exhibited greater APX and CAT activities, higher ASC content and lower H2O2 and MDA contents in comparison to a drought-susceptible wheat genotype [100]. In response to water deficit, the drought-sensitive apple rootstock *Malus hupehensis* exhibited greater increases in H2O2, •O2ˉ and MDA levels than the drought-tolerant *M. pruni‐ folia*. In contrast, SOD, POX, APX, GR and DHAR activities and ASC and GSH contents in‐ creased to a greater extent in *M. prunifolia* than *M. hupehensis* [101]. It has also been reported that the drought-acclimated leaves of wheat plants exhibited a systematic increase in the APX and CAT activities and the maintenance of an adequate ascorbate redox pool through the efficient functioning of the APX enzyme. As a result, lesser membrane damage was found in the drought-acclimated plants [94, 102].

environmental factor exerting an influence on the ecological processes that regulate its vege‐

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35

On the ecosystem level, a drought event can be (i) **permanent –** in regions where a desert climate predominates; (ii) **seasonal** – as observed in semi-arid regions; (iii) **irregular or vari‐ able** – as occurs in regions with humid or sub-humid climates (this normally takes place in limited areas and the return of drought is unpredictable); or (iv) **invisible or green drought** – as occurs when precipitation is not interrupted, but lesser than evapotranspiration, caus‐ ing a regional moisture imbalance. In the latter case, there is a drop in relative air humidity, leading to a reduction in moisture content in the soil. Moisture is evaporated into the atmos‐ phere and comes back as rainfall, but not enough to increase the moisture content in the soil. This is considered the worst kind of drought due to the fact that is difficult to perceive.

Excessive insolation, fire, shade, wind, herbivory, nutrient availability and water availability are factors that force plants to exhibit different kinds of adaptation to overcome the con‐ straints to their survival and establishment. In some cases, plant species from unrelated tax‐ onomical groups use very similar strategies, resulting in a phenomenon denominated

Cummins [107] proposed a plant classification system based on similar roles or analogous processes in the ecosystem. This classification allows us to simplify the biodiversity in a giv‐ en location and correlate it with that of another location, even without taxonomic related‐ ness among the species found [108]. A number of papers have since been published revealing the existence of vegetation patterns as responses to the influence of biotic and/or abiotic factors in different ecosystems. Consequently, knowledge on how an assemblage of plants organises itself to occupy all available niches under given environmental conditions has continually increased. The three general mechanisms used by plants to cope with drought [**avoidance** (dormancy in the dry season), **delay** (through increased water uptake and reduced water loss) and **physiological tolerance** (maintenance of plant functioning with low cell water content)] are closely linked to the functional traits of the species [109]

**Functional trait Role Some co-existing species Source**

*Gomphrena* aff. *leucocarpa* Mart (Amaranthaceae) *Taccarum peregrinum* L (Araceae) *Pithecoseris pacourinoides* Mart (Asteraceae) *Cleome guianensis* Aublet (Capparaceae) *Euphorbia comosa* Vell. (Euphorbiaceae) *Cuphea ericoides* Cham. & Schlech (Lythraceae) *Richardia scabra* L. (Rubiaceae) *Amasonia campestris* L. (Verbenaceae)

Mendes [110]

tation maintenance and distribution [106].

convergent evolution.

(Table 2).

Life form Species can avoid drought

remaining as seed during dry season (Therophytes)

**5.1. Formation of functional groups under natural cycles**

The drought response of a plant species also depends on the duration and severity of the drought period. SOD and CAT activities are reported to have increased in response to se‐ vere water deficit in mature leaves of two clones of *Populus deltoids* x *nigra* [103]. For both clones, Mn-SOD, Fe-SOD, and Cu/Zn-SOD isoforms were detected in varying amounts, de‐ pending on drought intensity.

Taken together, these findings provide additional evidence that the antioxidative system plays a key role in the process of plant acclimation to drought stress. Thus, greater protec‐ tion from drought-induced oxidative damage may, at least in part, be involved in tolerance to water deficit.

## **5. Drought and ecosystems: changes in natural cycles and functional groups**

According to Chapman [104], there are an estimated 390,800 plant species worldwide (Mag‐ noliophyta, gymnosperms, ferns, allies and Bryophyta). Despite their occurrence on all con‐ tinents, biodiversity and distribution is quite variable even within a few kilometres. From an ecologic standpoint, the occurrence of a specific plant species in an area depends on the combination of three factors:


Among these needs, water availability can be considered the most influential and even shapes the phytophysiognomy of some ecosystems. According to Puig [105], while drought has little influence in a tropical rain forest (where precipitation surpasses evapotranspiration more than ten months per year), water regime variability in a tropical dry forest is the major environmental factor exerting an influence on the ecological processes that regulate its vege‐ tation maintenance and distribution [106].

On the ecosystem level, a drought event can be (i) **permanent –** in regions where a desert climate predominates; (ii) **seasonal** – as observed in semi-arid regions; (iii) **irregular or vari‐ able** – as occurs in regions with humid or sub-humid climates (this normally takes place in limited areas and the return of drought is unpredictable); or (iv) **invisible or green drought** – as occurs when precipitation is not interrupted, but lesser than evapotranspiration, caus‐ ing a regional moisture imbalance. In the latter case, there is a drop in relative air humidity, leading to a reduction in moisture content in the soil. Moisture is evaporated into the atmos‐ phere and comes back as rainfall, but not enough to increase the moisture content in the soil. This is considered the worst kind of drought due to the fact that is difficult to perceive.

## **5.1. Formation of functional groups under natural cycles**

[100]. In response to water deficit, the drought-sensitive apple rootstock *Malus hupehensis* exhibited greater increases in H2O2, •O2ˉ and MDA levels than the drought-tolerant *M. pruni‐ folia*. In contrast, SOD, POX, APX, GR and DHAR activities and ASC and GSH contents in‐ creased to a greater extent in *M. prunifolia* than *M. hupehensis* [101]. It has also been reported that the drought-acclimated leaves of wheat plants exhibited a systematic increase in the APX and CAT activities and the maintenance of an adequate ascorbate redox pool through the efficient functioning of the APX enzyme. As a result, lesser membrane damage was

The drought response of a plant species also depends on the duration and severity of the drought period. SOD and CAT activities are reported to have increased in response to se‐ vere water deficit in mature leaves of two clones of *Populus deltoids* x *nigra* [103]. For both clones, Mn-SOD, Fe-SOD, and Cu/Zn-SOD isoforms were detected in varying amounts, de‐

Taken together, these findings provide additional evidence that the antioxidative system plays a key role in the process of plant acclimation to drought stress. Thus, greater protec‐ tion from drought-induced oxidative damage may, at least in part, be involved in tolerance

**5. Drought and ecosystems: changes in natural cycles and functional**

According to Chapman [104], there are an estimated 390,800 plant species worldwide (Mag‐ noliophyta, gymnosperms, ferns, allies and Bryophyta). Despite their occurrence on all con‐ tinents, biodiversity and distribution is quite variable even within a few kilometres. From an ecologic standpoint, the occurrence of a specific plant species in an area depends on the

**a. Chance** – the possibility of a propagule reaching and establishing itself in a certain loca‐

**b. History** – the current abundance of a species is probably correlated with its abundance

**c. Necessity** – demands for growth, competence for competition and interactions with other organisms; Coexistence with other plants depends on the complexity of the envi‐ ronment in terms of fertility, sunlight and water availability and on how strongly the

Among these needs, water availability can be considered the most influential and even shapes the phytophysiognomy of some ecosystems. According to Puig [105], while drought has little influence in a tropical rain forest (where precipitation surpasses evapotranspiration more than ten months per year), water regime variability in a tropical dry forest is the major

plant can withstand the action of competitors, herbivores, parasites, etc.

found in the drought-acclimated plants [94, 102].

pending on drought intensity.

34 Responses of Organisms to Water Stress

combination of three factors:

in the near past;

to water deficit.

**groups**

tion;

Excessive insolation, fire, shade, wind, herbivory, nutrient availability and water availability are factors that force plants to exhibit different kinds of adaptation to overcome the con‐ straints to their survival and establishment. In some cases, plant species from unrelated tax‐ onomical groups use very similar strategies, resulting in a phenomenon denominated convergent evolution.

Cummins [107] proposed a plant classification system based on similar roles or analogous processes in the ecosystem. This classification allows us to simplify the biodiversity in a giv‐ en location and correlate it with that of another location, even without taxonomic related‐ ness among the species found [108]. A number of papers have since been published revealing the existence of vegetation patterns as responses to the influence of biotic and/or abiotic factors in different ecosystems. Consequently, knowledge on how an assemblage of plants organises itself to occupy all available niches under given environmental conditions has continually increased. The three general mechanisms used by plants to cope with drought [**avoidance** (dormancy in the dry season), **delay** (through increased water uptake and reduced water loss) and **physiological tolerance** (maintenance of plant functioning with low cell water content)] are closely linked to the functional traits of the species [109] (Table 2).



**5.2. Climate change: New challenge for plants**

availability in a specific region.

Drought is a deviation from normal climatic conditions in which there is a lack of precipita‐ tion over an extended period and the resulting water shortage has negative implications [116]. Drought differs from aridity, which is a normal condition of a severe lack of water

Drought and Its Consequences to Plants – From Individual to Ecosystem

http://dx.doi.org/10.5772/53833

37

In recent decades, the planet has witnessed intense climate changes due to global warming. Extreme climatic events, such as tornados, hurricanes, floods, blizzards and drought, have become more frequent and intense. Some annual plant events, such as flowering, fruiting and re-sprouting, follow a specific timing, which is denominated phenology. Global warm‐ ing can affect this timing and its consequences can affect water supplies, pollination and the overall functioning of natural and agricultural ecosystems. This situation suggests a bleak future for mankind and nature, as all organisms will face substantial disturbances in their environment, possibly beyond their capacity for resistance and resilience. Resistance is the ability of a system to maintain its structure and functioning after a disturbance and resil‐

A given plant species can either escape from or acclimate to adverse environmental condi‐ tions, which can change in space and time. When a specific genotype exteriorises different phenotypes under different conditions, it is considered to have adequate phenotypic plasti‐ city. Changes in the partitioning of resources can be the result of different strategies under different selection pressures. However, this phenotypic plasticity is quite limited due both

The following are the most detectable features of global warming: 1) its influence on the per‐ ception of plants regarding the seasons (the advance of biological spring and the delay in biological winter have been observed and such changes have a direct effect on the reproduc‐ tive events of flowering and fructification, which can affect the dynamics of plant popula‐ tions and communities) [120-122]; 2) alterations in the floristic composition and phytosociology of plant communities due to changes in the seedling mortality rate; 3) the occurrence of a climate-induced shift in the range of species, which can force the interaction of plants with those from which they were formerly spatially separated [123]; and 4) in‐ creased biological plant invasions, as global warming can modify the dynamics and climate

Despite the volume of studies on plant responses to global warming, a great deal of uncer‐ tainty remains. After an extensive survey of plant phenology databases for long-term obser‐ vations and short-term warming experiments involving 1634 species, Wolkovich et al. [126] concluded that such experimental studies underpredict plant phenological responses to global warming. Thus, more in-depth studies are needed to help predict the effects of global warming on plant communities in the near future and develop strategies to mitigate these

ience is the ability to re-establish equilibrium after it has been disrupted [117].

the physiological costs and ontogenetic drift [118, 119].

of new environments, making them suitable for invasion [124, 125].

effects.

**Table 2.** Some functional traits associated to drought tolerance in plants under dry conditions.

## **5.2. Climate change: New challenge for plants**

**Functional trait Role Some co-existing species Source**

*Prunus ilicifolia* (Nutt. ex Hook. & Arn.) Walp. (Rosaceae) *Ceanothus oliganthus* var. *sorediatus* (Rhamnaceae ) *Mimulus aurantiacus* Curtis (Phrymaceae) *Baccharis pilularis* DC. (Asteraceae)

*Cercocarpus betuloides* Nutt. (Rosaceae) *Comarostaphylis diversifolia* (Parry) Greene (Ericaceae) *Quercus agrifolia* Née (Fagaceae)

EVERGREEN *Capparis flexuosa* L. (Capparaceae) *Maytenus rigida* Mart. (Celastraceae) *Licania rigida* Benth. (Chysobalanaceae) *Ximenia americana* L. (Olacaceae) DECIDUOUS *Amburana cearensis* (Allemão)AC Smith (Faboideae) *Jatropha mollissima*(Pohl) Baill.(Euphorbiaceae) *Combretum leprosum* Mart. (Combretaceae) Pseudobombax marginatum (A. St. –Hil.,Juss&Camb.) A. Robyns (Bombacaceae)

*Anogeissus latifolia* (Roxb. Ex DC) Wall. ex Bedd. (Combretaceae) *Soymida febrifuga* (Roxb.) A. Juss. (Meliaceae) *Acacia catechu* (L. f.) Willd. (Fabaceae) *Shorea robusta* Roth (Dipterocarpaceae) *Chloroxylon swietenia* DC. (Rutaceae)

SHALLOW *Schefflera macrocarpa* (Seem.) D. C. Frodin (Araliaceae) *Miconia ferruginata* DC. (Melastomataceae) *Roupala Montana* Aubl. (Proteaceae) *Ouratea hexasperma* (St. Hil.) Baill. (Ochnaceae) DEEP *Vochysia elliptica* Mart*.* (Vochysiaceae) *Dalbergia miscolobium* Benth. (Fabaceae) *Kielmeyera coriacea* Mart. (Clusiaceae)

Ackerly et al. [111]

Scoffoni et al. [112]

Barbosa et al. [113]

Kushwaha et al. [114]

Franco et al. [115]

Specific leaf area This is an index of

36 Responses of Organisms to Water Stress

Leaf size Influences leaf cooling

Leaf phenology Plays an important role in

Stem / Wood density (WD) sclerophylly.

and light capture efficiency (self-shading)

drought resistance, as deciduous trees are able to reduce water loss by dropping leaves, while evergreen trees must resist drought.

This is negatively correlated with cavitation resistance and negatively correlated with water storage.

the moister deeper soil layers

**Table 2.** Some functional traits associated to drought tolerance in plants under dry conditions.

Root deep Allows an exploration of

Drought is a deviation from normal climatic conditions in which there is a lack of precipita‐ tion over an extended period and the resulting water shortage has negative implications [116]. Drought differs from aridity, which is a normal condition of a severe lack of water availability in a specific region.

In recent decades, the planet has witnessed intense climate changes due to global warming. Extreme climatic events, such as tornados, hurricanes, floods, blizzards and drought, have become more frequent and intense. Some annual plant events, such as flowering, fruiting and re-sprouting, follow a specific timing, which is denominated phenology. Global warm‐ ing can affect this timing and its consequences can affect water supplies, pollination and the overall functioning of natural and agricultural ecosystems. This situation suggests a bleak future for mankind and nature, as all organisms will face substantial disturbances in their environment, possibly beyond their capacity for resistance and resilience. Resistance is the ability of a system to maintain its structure and functioning after a disturbance and resil‐ ience is the ability to re-establish equilibrium after it has been disrupted [117].

A given plant species can either escape from or acclimate to adverse environmental condi‐ tions, which can change in space and time. When a specific genotype exteriorises different phenotypes under different conditions, it is considered to have adequate phenotypic plasti‐ city. Changes in the partitioning of resources can be the result of different strategies under different selection pressures. However, this phenotypic plasticity is quite limited due both the physiological costs and ontogenetic drift [118, 119].

The following are the most detectable features of global warming: 1) its influence on the per‐ ception of plants regarding the seasons (the advance of biological spring and the delay in biological winter have been observed and such changes have a direct effect on the reproduc‐ tive events of flowering and fructification, which can affect the dynamics of plant popula‐ tions and communities) [120-122]; 2) alterations in the floristic composition and phytosociology of plant communities due to changes in the seedling mortality rate; 3) the occurrence of a climate-induced shift in the range of species, which can force the interaction of plants with those from which they were formerly spatially separated [123]; and 4) in‐ creased biological plant invasions, as global warming can modify the dynamics and climate of new environments, making them suitable for invasion [124, 125].

Despite the volume of studies on plant responses to global warming, a great deal of uncer‐ tainty remains. After an extensive survey of plant phenology databases for long-term obser‐ vations and short-term warming experiments involving 1634 species, Wolkovich et al. [126] concluded that such experimental studies underpredict plant phenological responses to global warming. Thus, more in-depth studies are needed to help predict the effects of global warming on plant communities in the near future and develop strategies to mitigate these effects.

## **Author details**

Elizamar Ciríaco da Silva1\*, Manoel Bandeira de Albuquerque2 , André Dias de Azevedo Neto3 and Carlos Dias da Silva Junior1

\*Address all correspondence to: elizaciriaco@gmail.com

1 Laboratory of Applied Botany, Department of Biology, Federal University of Sergipe, Bra‐ zil

[9] Sircelj H, Tausz M, Grill D, Batic F. Biochemical responses in leaves of two apple tree cultivars subjected to progressing drought. Journal of Plant Physiology 2005;

Drought and Its Consequences to Plants – From Individual to Ecosystem

http://dx.doi.org/10.5772/53833

39

[10] Silva EC, Nogueira RJMC, Vale FHA, Melo NF, Araujo FP. Water relations and or‐ ganic solutes production in four umbu tree (Spondias tuberosa) genotypes under in‐

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3 Laboratory of Biochemistry, Center of Exate and Technological Sciences, Federal Universi‐ ty of Recôncavo da Bahia, Brazil

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**Author details**

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André Dias de Azevedo Neto3

38 Responses of Organisms to Water Stress

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and Carlos Dias da Silva Junior1

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**Chapter 3**

**Tolerance to Drought in Leguminous Plants Mediated**

The water availability is considered the climatic factor with large effect on agricultural productivity, being responsible to determine species distribution in different climate zones around the globe [1]. Effects of drought depend of plant development stage, inten‐ sity, and duration of the water restriction. In other hand, plant adaptive strategies will determine the tolerance level, and consequently your survival on these conditions of in‐

Water deficit is an abiotic factor that affects the agricultural production with high frequency and intensity, influencing aspects related to plant development, such as decrease in photo‐ synthesis rate, reduction in leaf area [3], and stomata closing [4]. Crops normally present performance affected by water deficiency, which can cause lower growth and development (Figure 1), with progressive reduction in leaf dry matter [5] and consequent repercussion on

Root system presents complex strategy aiming to maintain water supply in conditions of water deficit, by increasing the root elongation rate and completely inhibiting the shoot [6]. On the other hand, plants growing in low water potentials normally present root thinner [7], and this morphological modification is an adaptation to increase water absorption efficien‐ cy. Therefore, a combination of changes in morphological, physiological and biochemical

> © 2013 da Silva Lobato et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2013 da Silva Lobato et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

production parameters, such as number of grains and pods per plant.

levels are necessary to plant survival in environments affected by drought.

**by** *Rhizobium and Bradyrhizobium*

Allan Klynger da Silva Lobato,

http://dx.doi.org/10.5772/54094

**1. Introduction**

adequate water supply [2].

Joaquim Albenísio Gomes da Silveira, Roberto Cezar Lobo da Costa and Cândido Ferreira de Oliveira Neto

Additional information is available at the end of the chapter
