The effects of drought stress on photosynthesis, chlorophyll fluorescence and chlorophyll content in the leaves of two candidate resistant genotypes (MCC392, MCC877), and two candidate susceptible genotypes (MCC68, MCC448), were studied in podding stage under drought stress (25% field capacity) and control condition. In photosynthesis study, net CO2 assimilation rate and water use efficiency (WUE) were significantly higher in resistant genotypes than that of susceptible genotypes in drought stress. Water use efficiency was significantly increased under drought stress. MCC392 and MCC68 genotypes had the highest and the lowest water use efficiency respectively, under drought stress. Transpiration rate decreased in all genotypes in drought stress as compared with control condition. Chlorophyll contents increased in all genotypes under drought stress. Carotenoid contents decreased in MCC877 and increased in MCC448 and MCC68 genotypes. Chlorophyll a/b ratio in drought stress was higher than that of control condition in MCC392, MCC448 and MCC877 genotypes. Chlorophyll stability index (CSI) was higher in MCC68 than other genotypes. In chlorophyll fluorescence study, PSÉÉ photochemical efficiency (Fv/Fm) ratio was significantly decreased in all genotypes under drought stress as compared with control condition. Fv/Fm ratio in resistant genotype (MCC392) was significantly higher than that of susceptible genotype (MCC68) in drought stress. Thus, based on these results, higher Fv/Fm ratio that demonstrated effective efficiency of PSÉÉ and higher WUS can be useful markers to identify resistant genotypes under drought stress.

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