**5. Physiological, biochemical and molecular responses of plant to abiotic stresses**

#### **5.1 Photosynthetic responses toward oxidative stress**

In higher plants, photosynthesis takes place in chloroplasts, which contain a highly organized thylakoid membrane system that harbours all components of the light-capturing photosynthetic apparatus and provides all structural properties for optimal light harvesting. Oxygen generated in the chloroplasts during photosynthesis can accept electrons passing through the photosystems, thus forming O2 <sup>−</sup>. Through a variety of reactions, O2 <sup>−</sup> leads to the formation of H2O2, OH and other ROS. The ROS comprising O2 <sup>−</sup>, H2O2, 1O2, HO2 <sup>−</sup>, OH, ROOH, ROO, and RO are highly reactive and toxic and causes damage to proteins, lipids, carbohydrates, DNA which ultimately results in cell death (Bryan , 1996; Downs et al., 1999). In chloroplast activated oxygen species can be generated by direct transfer of excitation energy from chlorophyll to produce singlet oxygen, or by univalent oxygen reduction at PSI, in the Mehler reaction (Asada et al.,1998). The latter process results in the formation of the superoxide anion radical (O2.-), singlet oxygen (1O2) and eventually H2O2 and the highly toxic hydroxyl radical (. OH). It is well known that Cu2+ catalyze the formation of OH. from the non-enzymatic chemical reaction between superoxide and H2O2.

Fig. 9. Involvement of H2O2 and NO in cellular responses to various stresses and

Protein oxidation is defined as covalent modification of a protein induced by ROS or byproducts of oxidative stress. Most types of protein oxidations are essentially irreversible, whereas, a few involving sulfur-containing amino acids are reversible (Ghezzi &Bonetto, 2003). Protein carbonylation is widely used marker of protein oxidation (Moller et al., 2007) and (Job et al., 2005). The oxidation of a number of protein amino acids particularly Arg, His, Lys, Pro, Thr and Trp give free carbonyl groups which may inhibit or alter their activities and increase susceptibility towards proteolytic attack (Moller et al., 2007). Protein carbonylation may occur due to direct oxidation of amino acid side chains (e.g. proline and arginine to γ-glutamyl semialdehyde, lysine to amino adipic semialdehyde, and threonine to

**5. Physiological, biochemical and molecular responses of plant to abiotic** 

In higher plants, photosynthesis takes place in chloroplasts, which contain a highly organized thylakoid membrane system that harbours all components of the light-capturing photosynthetic apparatus and provides all structural properties for optimal light harvesting. Oxygen generated in the chloroplasts during photosynthesis can accept electrons passing

ROO, and RO are highly reactive and toxic and causes damage to proteins, lipids, carbohydrates, DNA which ultimately results in cell death (Bryan , 1996; Downs et al., 1999). In chloroplast activated oxygen species can be generated by direct transfer of excitation energy from chlorophyll to produce singlet oxygen, or by univalent oxygen reduction at PSI, in the Mehler reaction (Asada et al.,1998). The latter process results in the formation of the superoxide anion radical (O2.-), singlet oxygen (1O2) and eventually H2O2 and the highly

<sup>−</sup>. Through a variety of reactions, O2

OH). It is well known that Cu2+ catalyze the formation of OH. from

<sup>−</sup>, H2O2, 1O2, HO2

<sup>−</sup> leads to the

<sup>−</sup>, OH, ROOH,

stimuli(Desikan et al., 2004).

aminoketobutyrate) (Shringarpure& Davies, 2002).

through the photosystems, thus forming O2

toxic hydroxyl radical (.

**5.1 Photosynthetic responses toward oxidative stress**

formation of H2O2, OH and other ROS. The ROS comprising O2

the non-enzymatic chemical reaction between superoxide and H2O2.

**4.3 Protein oxidation**

**stresses** 

Thylakoids are considered to be one of the major sites of superoxide production because of the simultaneous presence in chloroplasts of a high oxygen level and an electron transport system. Most of the superoxide is produced by photosystem I via the univalent reduction of oxygen through the ferredoxin / ferredoxin NADP+ oxidoreductase system (Mehler reaction). The use of DCMU, the known inhibitor of photosynthetic electron transport, and the use of the new spin trap DEPMPO have demonstrated that photosystem II also contributes to superoxide production (Navari-Izzo et al.,1998).

The modifications of the chloroplast in response to various environmental stresses have been widely studied in different laboratories and, thus the literature in the area is vast. The stress is sensed at the levels of pigment composition, structural organization, primary photochemistry and the CO2 fixation(Biswal et al., 2003; Biswal, 2005).

Spatial and temporal complexity of photosynthesis makes photostasis prone to stress. The sequence of photosynthesis is known to cover a wide time-span and begins with photophysical and photochemical events, i.e. light absorption, excitation energy transfer and charge separation in the timescale of femtoseconds (10–15 s) to nanoseconds (10–9 s). This is followed by electron transport in the microseconds (10–6 s) to milliseconds (10–3 s) range, and finally by enzyme mediated reactions in the milliseconds to seconds range. Relatively slow reactions are rate-limiting and thus, incompatible with the fast reactions. Further, the fast primary photochemical reactions are relatively stress-resistant compared to temperaturedependent, slow, enzyme-mediated reactions associated with the electron transport system and carbon dioxide fixation in the Calvin–Benson cycle (Krause & Jahns, 2004). This results in the development of excitation pressure at the source. Since plants are photoautotrophs, light at any intensity in combination with other environmental stresses can bring a change in photostasis in terms of accumulation of excess unutilized quanta because of weakened sink demand induced by stress. In addition, high light always accumulates excess energy at the 'source'. NPQ of excess quanta at the source is one of the major processes for restoration of the balance and maintenance of photostasis(Biswal et al., 2011).

Fig. 10. Electron transport system in the thylakoid membrane showing three possible sites of activated oxygen production (Elstner, 1991; Bryan, 1996).

Instinctive Plant Tolerance Towards Abiotic Stresses in Arid Regions 231

(Leipner et al., 1999; Fryer et al., 1998), an increased pool size of xanthophyll cycle pigments,

Leaf antioxidant systems can prevent or alleviate the damage caused by reactive oxygen species (ROS) under stress conditions, and include enzymes such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), and metabolites including ascorbate acid (AsA) and glutathione (GSH) (Asada, 1999; Xu *et al*., 2008). Phenolics are ubiquitous secondary metabolites in plants including large group of biologically active components, from simple phenol molecules to polymeric structures with

*Artemisia monosperma* showed the lowest activities for Guaiacol peroxidase(GuPx) and polyphenol oxidase (PPO) at 38°C and at 47°C in comparison with activities on plants collected at 9 and 15 ºC (Table 1). Moreover, the relationship between GuPx and PPO activities and soluble phenolics concentration in A. monosperma plants appear to indicate that 47ºC and 9°C caused heat and cold stress, by subjecting the plants to a super-optimal

The metabolism of phenolic compounds includes the action of oxidative enzymes such as GuPx and PPO, which catalyze the oxidation of phenols to quinones (Thypyapong et al., 1995; Vaughn and Duke, 1984). Some studies have reported that these enzyme activities increase in response to different types of stress, both biotic and abiotic (Ruiz et al., 1998, 1999). More specifically, both enzymes have been related to the appearance of physiological

Phenylalanine ammonia-lyase (PAL) is considered to be the principal enzyme of the phenylpropanoid pathway (Kacperska, 1993) catalyzing the transformation, by deamination, of L-Phenyalanine into *trans*-cinnamic acid, which is the prime intermediary in the biosynthesis of phenolics (Levine et al., 1994). This enzyme increases in activity in response to thermal stress and is considered by most authors to be one of the main lines of cell acclimation against stress in plants (Leyva et al., 1995). Phenols are oxidized by peroxidase (POD) and primarily by polyphenol oxidase (PPO), this latter enzyme catalyzing the oxidation of the *o*-diphenols to *o*-diquinones, as well as hydroxylation of monophenols(Thypyapong et al., 1995). These activities of enzymes increase in response to different types of stress, both biotic and abiotic (Ruiz et al., 1998, 1999). More specifically, both enzymes have been related to the appearance of physiological injuries caused in plants

U mg protein min -1 PPO

µmol caffeic acid mg-1 protein min-1

POD µmol guaiacol mg-1 protein min-1

by different stress (Grace et al., 1998; Ruiz et al., 1998; Ibrahim et al., 2011).

SOD APX CAT

15 June 38ºC 14.7±1.12 4.32±0.66 3.27±0.06 11.4±0.83 12.7±0.99 15 Aug 47ºC 16.6±1.22 5.31±0.71 4.87±0.07 22.3±2.30 16.9±1.23 15 Dec 9ºC 22.9±2.08 20.8±2.14 18.74±1.32 38.8±3.86 37.3±2.98 15 Feb 15ºC 19.0±2.03 17.5±1.65 13.58±0.98 28.2±2.19 25.3±2.07 Table 1. Variation of antioxidant enzymes activities( superoxide dismutase, SOD; ascorbate peroxidase, APX; catalase, CAT; phenol peroxidase, PPO and guaiacol peroxidase, POD in *Artemisia monosperma* plant in response to temperature divergence in Riyadh (Saudi Arabia)(

reduced photosynthetic capacity (Baker et al., 1994; Fryer et al., 1998).

molecular mass above 30 kDa (Dreosti, 2000, Ibrahim *et al*., 2011).

and suboptimal temperatures respectively(Ibrahim et al., 2011).

injuries caused by thermal stress (Grace et al., 1998).

Sampling

date Temp.

Ibrahim et al., 2011).
