Drought stress causes accumulation of reactive oxygen species by disturbing the balance between productions of reactive oxygen species and the antioxidant defense which induces oxidative stress. During drought stress plants close stomata which decrease the CO2 influx. Besides reducing the carboxylation reduction in CO2 also directs more electrons to form reactive oxygen species. Under severe drought condition photosynthesis is limited by decrease in the activities of ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPCase), NADP-malic enzyme (NADP-ME), fructose-1, 6-bisphosphatase (FBPase) and pyruvate orthophosphate dikinase (PPDK). To match the reduced requirements of NADPH production, non-cyclic electron transport is down regulated which reduce ATP synthesis (Farooq et al., 2008).
Photosynthesis is highly sensitive to water stress. Plant photosynthesis decrease with reduction in relative water content and leaf water potential. Mechanism of photosynthesis involves various components such as photosynthetic pigments and photosystem, the electron transport system, and CO2 reduction pathway. Any damage of this photosynthetic component at any stage due to stress may reduce the photosynthetic capacity of green plant. Change in the pool of photosynthesizing pigments, low CO2 uptake due to stomatal closure and resistance, poor assimilation rates in photosynthetic leaves are prominent due to water stress. Several studies have showed drought stress changes photosynthetic pigment and components (Anjum et al., 2003), damage photosynthetic apparatus (Fu J. and Huang, 2001), and diminished activities of Calvin cycle enzymes.
Decreased in photosynthetic rate is associated with stomatal and non-stomatal limitations (Wise et al., 1992; Yordanov et al., 2003). Some studies reported stomatal closure as the main factor for inhibition of photosynthesis while others conclude that non-stomatal factor play major role (Lawlor and Fock, 1978; Becker and Fock, 1986; and Lal and Edwards, 1996). Stomatal factors refers the CO2 limitation on photosynthetic activity whereas non-stomatal factors includes direct effect of reduced leaf and cellular water status, electron transport, enzymes involved in the CO2 fixation, induction of early senescence and changes of leaf anatomy and ultrastructure (Ghannoum, 2009). Non stomatal limitation of photosynthesis is associated with reduced carboxylation efficiency (Jia and gray, 2004), reduced ribulose -1, 5-bisphospate (RuBP) regeneration (Tezara and Lawlor, 1995), reduced amount of functional Rubisco (Kanechi et al., 1995), or inhibited functional activity of PSII.
Drought stress causes substantial damage to photosynthetic pigment and thylakoid membrane (Huseynova et al. 2009; Anjum et al., 2011; Kannan and Kulandaivelu 2011). Photosynthetic pigments are important for harvesting light. Chlorophyll is of the major component of chloroplast and has positive relation with photosynthesis. Depending on the duration and severity of drought decreased or unchanged chlorophyll level has been reported in many species (Farooq et al., 2009). In plant exposed to drought stress decreased in chlorophyll is common phenomenon (Bijanzadeh and Enam 2010, Mafakheri et al. 2010, Din et al. 2011). In contrast, no significant effect of drought on Chlorophyll content in wheat has been reported by Kulshrestha et al. (1987). Some reports shows increased in Chlorophyll accumulation during drought stress (Estill et al. 1991, Hamada and Al-Hakimi 2001, Pirzad et al. 2011). Ashraf and Karim (1991) have reported increase in Chlorophyll content of some cultivars of blackgram (Vigna mungo) and decrease in others during drought conditions. Such variation in Chlorophyll content among different cultivars of same species might be due to variation in the activities of specific enzymes involved in the biosynthesis of Chlorophyll. Carotenoids are less sensitive to water stress than chlorophyll while xanthophyll pigments increases under water stress. Xanthophyll pigments are involved in xanthophyll cycle and they have protective role on plants. Low concentrations of photosynthetic pigment directly limit photosynthesis which reduces production (Lisar, 2012).
The first response of plant to water stress is closure of stomata (Mansfield and Atkinson, 1990). Plant closes stomata to avoid further water loss in water deficit condition which leads to restriction of CO2 diffusion into the leaves (Cornic 1994; Chaves, 1991; Flexas et al., 2006). There has been a debate for a long time whether drought mainly limits photosynthesis through stomatal closure or metabolic impairment (Sharkey, 1990; Tezara et al., 1999). During last decade under mild to moderate drought conditions stomatal closure was generally considered to be the main detrimental factors for decrease in photosynthesis (Cornic and Massacci, 1996; Yokota et al., 2002). Stomatal closure decreases the inflow of CO2 into the leaves and spares more electrons for the formation of active oxygen species. Stomatal responses are more often closely related to soil moisture content than to leaf water status (Farooq et al., 2009). Stomata response to chemical signal generated by abcissic acid produced by dehydrating roots when leaf water status is kept constant (Morgan, 1990; Turner et al., 2001; Taylor, 1991).
Stomata close gradually as drought progress followed by decline in net photosynthesis. Stomata conductance is controlled by soil moisture availability as well as by complex interaction of intrinsic and extrinsic factors. Stomatal limitation of photosynthesis is a primary effect of drought and is followed by the adequate change of photosynthetic reactions (Chaves, 1991; Zlatev and Yordanov, 2004).
Non stomatal inhibition of photosynthesis reduces the activity of photosynthetic enzymes, decrease ATP concentration, inhibition of nitrate assimilation, induction of early senescence, and changes to the leaf anatomy and ultra-structures. Several studies have reported significant change in the activity of photosynthetic enzymes in plant subjects to water stress. In severe drought condition photosynthesis is limited due to decline in Rubisco activity (Bota et al., 2004). In leaves Rubisco level is controlled by the rate of synthesis and degradation. According to Hoekstra et al., (2001) Rubisco holoenzyme is relatively stable with half-life of several days even under drought stress. Rubisco activity is modulated either by binding inhibitors within the catalytic site or by reaction with CO2 and Mg2+ to carbamylate a lysine residue in the catalytic site. Such a binding either block activity or carbamylation of the lysine residue which is essential for activity (Farooq et al., 2009). Tight-binding inhibitors can decrease Rubisco activity in the light (Farooq et al., 2009). In tobacco (Nicotiana tabacum), under drought stress decrease in Rubisco activity is due to presence of tight-binding inhibitors rather than the change in activation by CO2 and Mg2+ (Parry et al., 2002). Under drought stress decreased maximum velocity of ribulose-1, 5-bisphosphate carboxylation by Rubisco, speed of ribulose-1, 5-bisphosphate regeneration, Rubisco and stromal fructose bis-phosphatase activities, and the quantum efficiency of photosystem II in higher plants causes rapid decline in photosynthesis (Reddy et al., 2004; Zhou et al., 2007).
Rubisco the key enzyme for carbon metabolism in leaves acts as both carboxylase and oxygenase. Rubisco acts as carboxylase in Calvin cycle where as it acts as oxygenase in photorespiration. Under drought conditions amount of Rubisco decrease resulting lower activity of enzymes. Closure of stomatal during drought reduces the supply of CO2 which increases the photorespiration. Increase in photorespiration ensure partial substrate replenishment and maintain the carboxylating function of Rubisco. Drought stress conditions acidify the chloroplast stroma causing inhibition to Rubisco activity. Besides this Rubisco activity is also decline by lack of substrate for carboxylation, reduction in the amount of ATPase, structure alternations of chloroplast and Rubisco and reduced activity of other photosynthetic enzymes to different extent. In addition to dark reaction drought stress also disrupts the cyclic and non-cyclic electron transfer during the light reaction of photosynthesis. Photophosphorylation is negatively affected by lower electron transport and decrease ATP synthesis as well as NADP+ reduction. Reduction of ATP in chloroplast is also affected by ATPase inhibition under drought. Water stress affects both Photo System (PS) in chloroplast. PSI of some plants is more severely damaged compared to PSII, though there are opposite conclusion as well (Lisar, 2012)
In bundle sheath CO2 concentration will fall, if carboxylation activity decreases more than the decarboxylation activity which result an increase in CO2 at bundle sheath. This leads to greater CO2 gradient across the bundle sheath cell wall, hence a greater leakage of CO2. Increased leakiness was related to decrease in Rubisco/PEPC activity (Ghannoum, 2009).Nitrate assimilation and nitrate uptakes are strongly reduced under water stress (Foyer et al., 1998). Water stress causes decrease in chlorophyll and protein content (Foyer et al., 1998). Decrease in chlorophyll and protein content is due to protein degradation as a result of senescence by increased content of amino acid (Becker and Fock, 1986). Lal and Edwards (1996) observed change in chloroplast position, distortion of intercellular spaces in leaves under water stress. Such changes in leaf affects CO2 diffusion inside the leaf as well as light penetration (Flexas et al., 2004).
Tezara et al., reported even under mild drought reduced photophosphorylation and adenosine triphosphate synthesis are main factors for limiting photosynthesis. Under drought stress, production of limited nicotinamide adenine dinucleotide phosphate maintains the continuation of electron transport. Non-cyclic electron transport is down regulated to match the requirements of decreased nicotinamide adenine dinucleotide phosphate production and cyclic electron transport is activated under drought stress. A proton gradient is generated which includes the protective process of high energy state quenching (Golding and Johnson, 2003). Under drought condition the support for cyclic electron transport come from non-steady state measurements (Cornic et al., 2000). Depending upon the extent of available moisture the activities of carbon assimilation enzymes and the enzymes involved in adenosine triphosphate synthesis are retarded and sometimes inhibited. Rubisco acts as oxygenase under water limiting conditions and therefore limit CO2 fixation (Farooq, 2009).
Drought is a worldwide problem constraining global food production. Drought stress affects plant growth and overall yield. Following drought stomata close which results decline in net photosynthesis. Stomata conductance is controlled by soil moisture availability as well as by complex interaction of intrinsic and extrinsic factors. Depending upon the severity of drought activities of enzymes involved in carbon assimilation and adenosine triphosphate synthesis decrease and sometimes even inhibited. The main factors for reduction in plant growth and productivity under drought is the production of reactive oxygen species in organelles including chloroplast, mitochondria, and peroxisomes. The reactive oxygen species target the peroxidation of cellular membrane lipids and degradation of enzyme proteins and nucleic acids.
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