**3.3 Photosynthesis**

Drought stress causes stomatal closure, reducing CO2 absorption and consequently affecting photosynthesis rate [29]. In certain situations, however, stomata behavior is not necessarily linked to the photosynthetic rate, although this has to be clarified [109, 110]. As the water-deficit steadily rises in cotton, photosynthesis is strongly influenced along with growth. For example, photosynthesis and transpiration were shown to be altered in cotton in dry conditions [53, 106]. Incidentally, immature cotton leaves were shown to be photo-synthetically more resistant than mature leaves to drought and heat. There was no reduction in net photosynthesis when immature leaves were exposed to high temperatures (37°C). By comparison, the photosynthesis of mature leaf net decreased 66% under equal conditions [27]. A reduced lint production was reported when net photosynthesis deteriorated under water shortage conditions in the first growing season in another field trial of cotton in two consecutive growing seasons. However, there were no changes in the production of drought-treated fields in the next growing season due to heavy rainfall [28]. These researches have shown that drought stress decreases cotton photosynthesis, which in turn impacts growth and return.

Photosynthesis is the principal source of agricultural productivity, which is adversely affected by situations of water scarcity. When the stomata closures respond to wetness, the leaf photosynthetic capacity is reduced and the dehydration of chloroplast and CO2 into the leaf is reduced (**Figure 3**). For example, moderate moisture stress induces stomata closure by controlling transpiration to prevent water loss. This decreases stomata behavior and limits the concentration of intercellular CO2 [78]. In serious drought, decreased stomata conductance and (nonstomatal) metabolic damage, such as restricted carboxylation, become important photosynthetic constraints [87]. Likewise, stomata behavior is not continuously linked to photosynthesis, but investigation is needed [110]. Drought can impair both photosynthesis and seriously

**Figure 3.** *Physiological role of potassium in plant.*

influence transpiration and depend on the degree of drought and plant growth [111]. Under mature cotton leaves up to 66 percent decrease has been reported in water shortage situations about smaller leaves [27]. Overall, using K-efficient cultivars under reduced irrigation condition along with 50 Kg ha−1 K2O improve the net photosynthetic rate by about 30% as compared to potassium nonefficient cotton cultivars [11].
