Abstract:
Effect of photoinhibition of sorghum leaves and isolated chloroplasts on chlorophyll fluorescence, peroxidation of thylakoid lipids, activity of antioxidants enzymes and content of xanthophylls and ABA were studied. Photoinhibition of intact leaves and isolated chloroplasts decreased Fv/Fm ratio and qP, while qN increased. Photoinhibitory damage was more at 5 degrees C than at 30 degrees or 50 degrees C. Peroxidation of thylakoid lipids was 5 times greater when photoinhibited at 50 degrees C compared to control. Photoinhibition of chloroplasts under low oxygen condition or when supplemented with anti-oxidants (beta-carotene, ascorbate and glutathione) resulted in less damage to photosynthesis (Fv/Fm ratio) and peroxidation level of thylakoid lipids. Photoinhibition also resulted in many fold increase in the activity of superoxide dismutase (SOD), ascorbate peroxidase (APX) and decrease in catalase. Results also showed that seedlings grown on ABA supplemented growth medium had better energy dissipation (greater qN) and much greater level of de-epoxidation (high level of Z) than control seedlings. The level of endogenous ABA in seedlings grown on ABA supplemented medium or otherwise remained same. Photoinhibition of isolated chloroplasts in the presence of low oxygen level (anaerobic conditions) or when supplemented with antioxidants such as ascorbate, glutathione and (3-carotene, decreased the damage to photosynthesis and lipid peoroxidation to a great extent since former prevented formation of oxygen radicals and later could scavenge the radicals leading to protection. The results indicate that increased activity of SOD and APX help metabolise the oxygen radicals, thereby protecting the seedlings against photooxidative damage and also suggest a possible role of ABA in energy dissipation, probably by stimulating the xanthophyll cycle.