**4.1 Plant water relations**

Plant–water relations under stress conditions explain how plants control or maintained the hydration of their cells up to an optimal level because it has important implications in the physiological and metabolic processes. It controls almost all metabolic activities within the cell which are dependent on the availability of sufficient amount of water present. Relative water content signifies as an indicator of plant water balance [25] and it indicates the level of cellular and tissues hydration which is imperative for the physio biochemical processes [26]. Plants under water deficit condition tends to have lesser RWC which triggers stomatal closure resulting in decreased CO2 uptake [27]. Generally, the tolerant sugarcane genotype displays a higher RWC than the susceptible clones and Silva et al. [26] have described that the tolerant clones maintain better RWC (~87%) than the susceptible genotypes (~80%). Almeida et al.

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

[28] also reported a 50% decline in RWC particularly in RB 943365 (drought-sensitive sugarcane genotype) while the tolerant clone RB 72910 (drought-tolerant sugarcane genotype) remained constant at 86% RWC upon exposure to the water stress for 30 days. Medeiros et al. [29] witnessed significant decline in RWC with average values of 88.7 and 90.7% in the varieties RB 867515 and RB 962962 varieties owing to decrease of water in the soil, correspondingly.

Water potential (ψw) and Osmotic potential (ψs) are the physiological parameter used for recording the extent of stress level in plant. Water deficit induced decrease in leaf water potential led to interfere with plants' ability to extract water from the soil and maintain turgor [30]. Water potential controls stomatal conductance, which affects transpiration and photosynthesis, and affects root water uptake driven by the potential difference between leaf and soil water. Leaf water potential (ψw) was significantly reduced by 34.50% due to drought stress. However, the reduction in water potential was comparatively less in resistant genotypes *viz*., Co 99,004 and Co 99,012 (26.96 and 30.15%) while the reduction was more than 50% in sensitive genotypes *viz*., CoVc 93,136 and Co 99,014 [31]. The RB 72910 variety was able to maintain higher values Ψw when compared to RB 943365 variety under water deficit [28]. The water potential (ψw) was significantly reduced during stress treatment and values around 11-fold times smaller than those found in the control treatment, on average − 1.19 and − 0.78 MPa in the RB 867515 and the RB 962962 varieties, respectively. On rewatering treatment, a recovery of the plant water status was observed, as expected, with no difference among treatments, the average values of that treatment were − 0.16 MPa in the RB 867515 variety and −0.14 MPa in the RB 962962 variety [29]. Osmotic potential (ψs) is another physiological parameter used for recording the extent of stress level in plants. Reduction in osmotic potential of leaves has been observed under drought stress in sugarcane cultivar [32]. Basra et al. [33] studied the water relations in drought sensitive (BL-4) and drought tolerant (CP 43/33) sugarcane varieties by exposing to 200 mol m−3 mannitol solution and found that decline in turgor was faster in BL-4 than in CP 43/33 with time. Bulk leaf osmotic pressures and cell wall solutes were higher in BL-4 than in CP 43/33. Biancos et al. [34] observed the maximum difference in osmotic potential, between well-watered and stressed plants (i.e., the maximum osmotic adjustment) of about −0.5 and − 0.6 MPa for mature leaves and shoot tips in sugarcane, respectively. Pooja et al. [35] noted a significant decline in leaf RWC, leaf osmotic potential and leaf water potential in four sugarcane varieties exposed to drought stress. In another study, Pooja et al. [2] found that sugarcane clones Co 05011 (78.77%) and Co 0238 (76.88%) showed/maintained better RWC under stress conditions with a mean reduction of 16.70% over Co-canes RWC.
