CHANGES IN CAROTENOID COMPOSITION AND PHOTOSYNTHESIS IN SORGHUM UNDER HIGH LIGHT AND SALT STRESSES

The effect of high light stress and salt stress, individually and in combination, was investigated in the leaves of Sorghum bicolor (L.). High light treatment at 5-degrees-C and 20-degrees-C caused an increase in zeaxanthin and a decrease in violaxanthin contents. Decreases in beta-carotene were also observed, which was followed by increased formation of 5,6-epoxide-beta-carotene. Violaxanthin + antheraxanthin + zeaxanthin (total) was not the same in control and stressed plants. Salt stress (no photoinhibition) also resulted in the formation of zeaxanthin but it was significantly less than that observed in the high light stressed plants. Neoxanthin content showed a slight decrease, while 5,6-epoxide lutein increased under salt stress. Changes in the xanthophyll cycle (violaxanthin + antheraxanthin + zeaxanthin) were reversed during recovery after photoinhibition. The CO2 assimilation rate and fluorescence F(v)/F(m) ratio were measured in intact leaves and isolated chloroplasts, respectively, from control and stressed plants. Formation of zeaxanthin and degradation of beta-carotene are apparently involved in protection against photoinhibition.