## 4 Drought Stress and Photosynthesis  
Major consequence of water deficit in plants is the decrease or suppression of photosynthesis [54] (Figure 2). Reduced leaf area, increased stomata closure and consequent reduced leaf cooling by evapotranspiration increases osmotic stress leading to damages to the photosynthetic apparatus are among the major constraints for photosynthesis [55; 56]. Among these, the decrease in photosynthetic process in plants under drought is mainly attributable to the decline in CO\({}_{2}\) conductance via stomata and mesophyll limitations [57]. Decrease in photosynthetic activity due to drought may also be due to reduced ability of stomatal movement [58; 59]. Declined activity of photosynthesis is triggered by the loss of CO\({}_{2}\)[60] uptake, whose drop has been shown to affect Rubisco activity and decrease the function of nitrate reductase and sucrose phosphate synthase and the ability for ribulose bisphosphate (RuBP) production. Supportively, CO\({}_{2}\) enrichment eliminated many early responses of maize metabolites and transcripts attributable to drought stress [61].  
Water deficit also resulted in decreased leaf area per shoot, and, thus, modification in canopy architecture, and this feature can alter gas exchange, water relations, vegetative growth and sink development (e.g., fruits or grains) [62], altering, for example, berry sugar concentration in grape [63] and biomass partition in maize (i.e., kernel number and 100-kernel dry weight decreased with increasing water stress duration) [64].  
Chlorophyll content, which is of outmost importance for photosynthesis [65], is another photosynthetic attribute strongly influenced by water deficit that has been recognized as a distinctive indication of photo oxidation and degradation of chlorophylls [66]. For example, leaf chlorophyll synthesis and chlorophyll _a/b_ proportion in soybean is altered by drought stress [67]. Decline in photosynthetic activity, amount of chlorophylls, loss of photosystem II photochemical efficiency, alteration in stomatal movement and disturbance in water status of plants resulted in declined plant productivity [68]. Among others, a major cause for decline in amount of chlorophyll due to drought stress is the drought-promoted O\({}_{2}\)- and H\({}_{2}\)O\({}_{2}\), which results in lipid peroxidation and ultimately chlorophyll degradation [69]. The decrease of plant development and yield in several plant species under water deficit is often associated with decline in photosynthetic action and chlorophyll content impairment [70]. Water deficit alters the action of photosynthetic moieties and chlorophyll pigments, which ultimately resulted in reduced photosynthetic activities in _Vigna mungo_[71].  
Drought stress induces a decreased net photosynthesis and also changes the plant carbon allocation and metabolism, which ultimately results in energy dissipation and declined yield [72]. For example, drought stress decreased the physiological metabolic disorders by suppressing the photosynthetic products production and disrupting the carbon balance in soybean [16]. Drought stress also caused a reduction in the abundance of several Calvin cycle proteins, including Rubisco downregulation in olive [73]. Acute drought stress conditions also cause the damage to Rubisco enzyme and other enzymes associated with photosynthesis and are responsible for the loss of photosynthetic pigment content [74].