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

Inamullah and Akihiro Isoda

Abstract: Adaptive changes were studied in soybean and cotton grown in pots under four irrigation treatments i.e. normal irrigation (equal to the evapotranspiration of the crop), 50%, 25% and 10% of the normal irrigation, comparatively. Maximum quantum yield of PSII (Fv/Fm) of soybean was generally higher while its actual quantum yield of PSII (△F/Fm') and CO2 assimilation rate (AN) were lower than those of cotton. Decrease in Fv/Fm, △F/Fm' and AN of soybean was larger in water stress treatments as compared with cotton. Decrease in △F/Fm' of soybean under water stress was accompanied by significant increase in non-photochemical quenching (NPQ) and significant decrease in photochemical reflectance index (PRI). Chlorophyll content decreased significantly only in soybean under higher water stress. Leaf temperature (TL) of soybean showed significantly larger increase in water stress treatments as compared with cotton. TL was highly negatively correlated with Fv/Fm, AN, PRI and △F/Fm' while it was highly positively correlated with NPQ of both crops. In soybean correlations between TL and AN, Fv/Fm and PRI were significant. It was concluded that soybean adapted to water stress by safely dissipating the excess excitation energy thermally with the down regulation of PSII activities to protect its photosynthetic apparatus from the photodamaging effect of water stress and higher TL. This photoprotective mechanism might be supported by the paraheliotropic leaf movement of the crop. Cotton, on the other hand, adapted to water stress by keeping its TL lower. Higher transpiration probably supported the crop to keep its TL lower under drought stress.
Key words: Chlorophyll fluorescence, Glycine max (L.) Merr., Gossypium hirsutum L., Leaf movement, Photoinhibition, Xanthophyll cycle.