Adaptive responses of soybean and cotton to water stress II.: Changes in CO2 assimilation rate, chlorophyll fluorescence and photochemical reflectance index in relation to leaf temperature

Adaptive changes were studied comparatively in soybean and cotton grown in pots under four irrigation conditions i.e. normal irrigation (equal to the evapotranspiration of the crop), and 50%, 25% and 10% of the normal irrigation. In soybean, the maximum quantum yield of PSII (Fv/Fm) was generally higher while the actual quantum yield of PSII (Delta F/Fm') and CO2 assimilation rate (A(N)) were lower than in cotton. The intensity of the decrease in Fv/Fm, Delta F/Fm' and A(N) by water-stress treatments was larger in soybean than in cotton. The decrease in Delta F/Fm' in soybean under water stress was accompanied by a significant increase in non-photochemical quenching (NPQ) and significant decrease in photochemical reflectance index (PRI). Chlorophyll content decreased significantly tinder severe water stress only in soybean. The increase in leaf temperature (T-L) in response to water stress was significantly larger in soybean than in cotton. T-L was highly and negatively correlated with Fv/Fm, A(N), PRI and Delta F/Fm' while it was highly and positively correlated with NPQ of both crops. Especially in soybean, the correlations of T-L with A(N), Fv/Fm and PRI were significant. It was concluded that soybean adapted to water stress by dissipating the excess excitation energy thermally with the clown-regulation of PSII activity to protect its photosynthetic apparatus from the photodamaging effect of water stress and high T-L. This photoprotective mechanism might be supported by the paraheliotropic leaf movement of the crop. Cotton adapted to water stress by keeping T-L lower to protect the photosynthetic apparatus from photodamage. Probably higher transpiration kept T-L of the crop lower under drought stress.