Photosynthesis (Pn)
Drought affects the rate of photosynthesis in water stressed leaves mainly due to stomatal closure [84]. [68] stated that a decrease in photosynthesis under deficit soil moisture may not necessarily be related to stomatal opening rather than non-stomatal control of photosynthesis. Chloroplast metabolism can be inhibited in plants subjected to water deficits [78]. This could substantially contribute to the overall inhibition of photosynthesis in leaves of droughted plants [68]. Spence et al. (1986) reported that plant stomata adapted to drought stress maintained stomatal opening at lower plant water potentials than non-adopted plants. In plants, chloroplast obtain high energy by means of photosystem II and I (PS II and PS I), which capture the electrons that become excited when sunlight is observed by chlorophyll molecules. The PS I and PS II are composed of an antenna complex attached to a photochemical reaction centre. This is a precisely ordered complex of proteins and pigments, where the photochemistry of photosynthesis occurs [39]. The two photosystem are normally linked in series and transfer electron from water to NADP+ to form NADPH, with the concominant production of a transmembrane electrochemical proton gradient generating O2 as a byproduct. All electron transport processes occur in the thylakoid membrane to make ATP. In this process, H+ is first pumped into the thylakoid space and then a backflow of H+ occurs through an ATP synthase to produce the ATP in the chloroplast stroma [4].