*Drought Stress: Manifestation and Mechanisms of Alleviation in Plants DOI: http://dx.doi.org/10.5772/intechopen.102780*

Physiological reactions to moisture stress provides some escape mechanisms to the water stress comprise physiological and morphological adaptations [18]. Decreased leaf area (**Figure 1**), reduced stomatal number and conductance, enlargement of root system, increased leaf thickness, and leaf folding to lessen evapotranspiration are strictly associated with an adaptive response [17, 19–21]. Plant growth and productivity decreased under moisture stress, which are caused by alterations in plantwater relations, CO2 assimilation reduction, membrane damage of affected tissues, cellular oxidative stress, and inhibition of enzymes activity.

Plants can alter water relations to continue cellular mechanisms under drought stress conditions. Plants show osmotic adjustment by accumulating and integrating compatible solutes likely, proline, sugars and free amino acids [22]. Maintenance of turgor pressure as well as cell volume at low water potential is facilitated by osmotic adjustment and is vital for metabolic functions. Osmotic adjustment also plays role in recovery of metabolic activities post drought stress [23]. Previously, there are lot of studies investigated which showed the recovery of photosynthesis from moisture stress in various crop species and also recovered from drought stress in terms of oxidative stress, membrane stability index and antioxidative mechanisms [16, 24]. Osmolytes also have a significant role in drought stress recovery.

Drought stress at higher intensity decreases the activities of photosynthetic enzymes as well as leaf chlorophyll content which ultimately hampers the process of photosynthesis [20, 25]. Chlorophyll *a*/*b* proportion and synthesis of leaf chlorophyll altered during drought stress. A lower content of chlorophyll (**Figure 2**), inactivation of key proteins linked to the photosynthesis process, and alteration of thylakoid membranes happen as a result of drought stress. The decline in chlorophyll content is due to over production of O2 − and H2O2 production, which ultimately results significant chlorophyll degradation and lipid peroxidation. During drought stress, in stomata and mesophyll cell the CO2 conductance declined as the decrease in the photosynthetic process. The decrease in photosynthetic activity also may be because

**Figure 1.** *Effects of different levels of drought stress on ricebean seedlings.*

## **Figure 2.**

*Visual effects of drought stress in rice. Source: [26].*

of the reduction of stomatal movement [27]. Rubisco activity greatly affected by the loss of CO2 uptake, similarly it decreases the activity of sucrose phosphate synthase, nitrate reductase and RuBP production [20, 28]. Decline in photosynthetic activity, loss of photosystem II photochemical efficiency, reduction in chlorophyll content and alteration in stomatal movement results the reduction in plant productivity. As a consequence of reduction in photosynthetic activity in drought stress, it dismantles the production of carbohydrate in various way likely prevents the transport of sucrose into sink organs and reduces the level of sucrose in leaves, which in turn limits reproductive development. The free sugars and different metabolites thereof support plant growth under drought, and take up osmolytic role and compatible solutes to mitigate the drastic effect of the stress [29, 30].

The relative leaf water content (RLWC) is an estimate of leaf's hydration status relative to its maximal water holding capacity at full turgid state. The relative leaf water content (RLWC) is one of the reliable parameters to know the water status in plants and it decreases gradually with increases in the severity of drought stress conditions. The decline of RLWC as a response to osmotic stress was earlier reported by several investigators under different stress conditions [31–34]. The physiological traits considered for evaluating drought stress tolerance include root trait characteristics (root length, root density, root biomass, root length density, delayed canopy wilting (DCW) and leaf pubescence density (LPD) [35], delayed leaf senescence (DLS) [36], and recovery ability after wilting (RAW) [37]. Drought stress drastically affects seed germination and decreases the speed of germination (**Figure 3**). Apart from these, stomatal conductance, chlorophyll content and use of carbon isotope discrimination are also effective screening methods for drought stress tolerance and has been used for some food legumes.

*Drought Stress: Manifestation and Mechanisms of Alleviation in Plants DOI: http://dx.doi.org/10.5772/intechopen.102780*

**Figure 3.** *Ricebean response to varying levels of PEG as drought induction agent.*
