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Variation in the response of six diverse rice genotypes to imposed water stress was investigated in terms of leaf photosynthetic traits and leaf water potential measurements (W leaf ) during and following a period of water stress. The W leaf reduced from 0 days up to 6th day of irrigation deprivation and the reduction in W leaf was more pronounced in Swarna and minimum in PA-6201. A gradual recovery in W leaf was observed in all the tested varieties after re-irrigation. Similarly, water stress significantly reduced rate of photosynthesis (P N ), stomatal conductance (g s ), transpiration (E) and internal CO 2 concentration (C i ). Significant differences were noticed amongst the varieties in their response to water stress. The P N partially recovered in all tested varieties after re-irrigation. Vandana recorded greater recovery (65 %), where as the recovery was only 32 % in Sugandha Samba. Water stress (6 days of irrigation deprivation) significantly decreased the mean Chl a, Chlb, total chlorophyll and total carotenoid content. However, a marginal increase in total chlorophyll was observed under water stress in Vandana and N-22. Water stress had no significant effect on maximum photochemical efficiency (F v /F m ) for all the tested varieties. However, in Sugandha Samba and Swarna a significant reduction was noticed. Water stress reduced the efficiency of excitation capture (U e ), in vivo quantum yield of PSII photochemistry (U PSI ), coefficient of photochemical quenching (qP) and increased the coefficient of nonphotochemical quenching (qN). The data on photosynthetic traits and chlorophyll fluorescence parameters indicate that Vandana, N-22, DRR-Dhan-38 and PA-6201 which maintained relatively higher, W leaf , P N and g s under water stress and could also maintain high values of photochemical efficiency (Fv/Fm, U e , U PSI and ETR) than Sugandha Samba and Swarna and the genotypic differences in the response of PSII activity could be exploited as traits for the selection of drought tolerant rice genotypes.

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Influence of different phosphorus concentrations was studied in four rice varieties (Akhanphou, MTU1010, RP BIO 226, and Swarna) differing in their tolerance to low phosphorus. There was an increase in shoot and root dry mass with the increase in phosphorus concentration. At the low phosphorus concentration at both tillering and reproductive stages, Swarna, followed by Akhanphou, recorded maximum biomass for both roots and shoots, while the minimum was observed in RP BIO 226. Reduction in photosynthetic rate, stomatal conductance, transpiration rate, and internal CO 2 concentration at low phosphorus concentrations were observed at both tillering and reproductive stages in all the genotypes. In low phosphorus, maximum photosynthetic rate was found in Swarna followed by Akhanphou. Phosphorus deficiency did not alter the maximum efficiency of PSII photochemistry, however, there was a reduction in effective PSII quantum yield, electron transport rate, and coefficient of photochemical quenching, while the coefficient of nonphotochemical quenching was higher in the low phosphorus-treated plants. Prolonged exposure to excessive energy and failure to utilize the energy in carbon-reduction cycle induced the generation of reactive oxygen species, which affected PSII as indicated by the fluorescence traits. The reduction was less severe in case of Swarna and Akhanphou. The activities of superoxide dismutase, peroxidase, and catalase increased in roots under low phosphorus concentration indicating that photoprotective mechanisms have been initiated in rice plants in response to phosphorus deficiency. Comparatively, Swarna and Akhanphou exhibited a higher biomass, higher photosynthetic rate, and better reactive oxygen species-scavenging ability which conferred tolerance under low phosphorus conditions.