**4.3 Gas exchange traits**

One of the more immediate responses of the drought stress is a reduction in the water potential of plant tissues leading to diminished stomatal aperture [42, 43] and consequent reduction in transpiration rate and photosynthesis, as well as longerterm responses such as growth inhibition, and accumulation of osmolytes [44]. In C4 plants some evidences demonstrate that photosynthesis is highly sensitive to water deficit [45]. Moreover, these plants present low recovery capacity mainly when water deficit exceeds the plant recovery capacity limiting the photosynthesis metabolic pathways [46]. Sugarcane plants when subjected to decreased soil water content under moderate (42%) and severe stress (22%) caused changes in all photosynthetic apparatus, such as stomatal closure, reduction of transpiration and photosynthetic rate, as well as in RWC, photochemical efficiency of photosystem II (PS II), and increase in leaf temperature [47, 48]. The photosynthetic rate and stomatal conductance decreased significantly in drought-tolerant (SP 83–2847 and CTC 15) and sensitive sugarcane cultivars (SP 86–155), when submitted to water deficit however higher reduction percent, was recorded in sensitive cultivar [49]. Medeiros et al. [29] also reported that when young sugarcane plants of two varieties RB 867515 and RB 962962 were subjected to irrigation suspension until total stomata closure, and then rewatered, a significant reduction on stomatal conductance, transpiration rate, and net photosynthesis were observed. RB 867515 showed a faster stomatal closure while RB 962962 slowed the effects of drought on the gas exchanges parameters with a faster recovery after rewatering. Farooq et al. [38] that maximum water use efficiency was observed under 60% irrigation coefficient as compared to 80% and 100% irrigation coefficient and under 60% irrigation coefficient maximum water use efficiency was recorded in variety NSG followed by HSF-240. Water

*Physiological and Molecular Adaptation of Sugarcane under Drought vis-a-vis Root System Traits DOI: http://dx.doi.org/10.5772/intechopen.103795*

stress also caused a reduction in gas exchange traits and the associated pigments by 56.57% in stomatal conductance (gS), 56.55% in photosynthetic rate (pN), 38.21% in transpiration rate (E), 28.01% in internal CO2 (Ci) and 16.86% in the chlorophyll content [2]. Maximum water use efficiency (pN/E) under drought stress was recorded in Co 0238 (4.12) and Co 98,014 (3.93).
