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Two contrasting guar varieties [Cyamopsis tetragonoloba (L.) Taub.], RGC-1002—a better drought tolerant than RGC-936—a comparatively sensitive variety, were subjected to 8 days of water stress. The physiological characteristics of gas exchange, pigments and the spectral properties of the leaves were monitored and assessed. The guar variety, RGC-1002 exhibited higher relative water content (RWC) and biomass accumulation in water stress condition as compared to RGC-936. The RGC-1002 variety also showed a higher rate of photosynthesis, stomatal conductance, and lesser reduction in chlorophyll content as compared to RGC-936 variety. On the 8th day of drought, the leaf anthocyanin levels were also higher in RGC-1002 than RGC-936. Increased levels of anthocyanin result in decrease in absorption of light, and an increase in reflectance, and transmittance. The scattering coefficient of leaf spectra was significantly increased in RGC-936 than in RGC-1002 leaves; this is due to an increase in the size of intercellular air spaces and shrinkage of cells as a result of water loss. The leaf reflectance was also observed to be significantly increased in RGC-936 than in RGC-1002 leaves. The physiological and leaf optical observations substantiate that RGC-1002 was better adapted to water stress than RGC-936 variety. [ABSTRACT FROM AUTHOR]