Regulation of photosynthesis, fluorescence, stomatal conductance and water-use efficiency of cowpea (Vigna unguiculata [L.] Walp.) under drought.

Drought is the major abiotic stress factor that causes extensive losses to agriculture production worldwide. The objective of this study was to evaluate the dynamics of photosynthesis and water-use efficiency parameters in 15 cowpea genotypes under well-watered and drought condition. Photosynthesis (A) and chlorophyll fluorescence (Fv'/Fm') declined linearly with decreasing soil water content whereas intrinsic water-use efficiency (WUE) increased under drought stress, suggesting stomatal regulation was a major limitation to photosynthesis. However, under increasing drought conditions, increase in ratio of intercellular CO(2) to ambient CO(2) concentrations along with reduced WUE showed the role of non-stomatal limitation of photosynthesis. The resistant nature of Fv'/Fm' and electron transport rate under drought appeared to be important mechanisms for photoinhibition protection under drought stress. Oxidative stress was apparent due to drought-induced reduction in total chlorophyll and carotenoid which was accompanied with increased leaf wax contents. The accumulation of proline appeared to be in response of drought injury rather than a drought tolerance mechanism. A clear separation based on the genotypes site of origin among the genotypes for drought tolerance could not be established when analyzed using principal component analysis. The identified genotypes and physiological traits from this study may be useful for genetic engineering and breeding programs integrating drought adaptation in cowpea.