Photoinhibition and photosynthetic acclimation of rice (Oryza sativa L. cv Jyothi) plants grown under different light intensities and photoinhibited under field conditions.

Thirty-days old rice (Oryza sativa L. cv. Jyothi) plants grown under the greenhouse (150-200 µmol m-2 s-1) or shade (600-800 µmol m-2s-1) were exposed to 7 days of full sunlight and compared with plants grown under direct sunlight (1200-2200 µmol m-2s-1).Transfer of greenhouse and shade plants to full sunlight for a day resulted in a decline in their photosynthetic efficiency (Fv/Fm) and an increase in non-photochemical quenching (qN). The decline in Fv/Fm was much greater in transferred greenhouse plants (33%) as compared to transferred shade-plants (20%). Sun-plants did not show much variation in the Fv/Fm ratio (4%) from their predawn measurements (control). The sun-grown plants showed a higher pool of xanthophyll pigments (violaxanthin + antheraxanthin + zeaxanthin). Transfer of greenhouse and shade-plants to full sunlight resulted in an increase in lutein, Chl a/b ratio, antheraxanthin (A) and zeaxanthin (Z) content. Increase in A and Z was correlated with the increase in the qN. The increase in the A and Z content was due to increase in the activity of violaxanthin de-epoxidase. Greenhouse and shade plants on exposure to sunlight showed an increase in lipid peroxidation (LPO). Prolonged exposure of greenhouse and shade plants up to 7 days resulted in recovery of the Fv/Fm, an increase in Z and A and a decline in the LPO. The study demonstrated that rice plants grown at lower light intensities initially underwent photoinhibitory damage on exposure to full sunlight, but were able to acclimate to the high irradiance by dissipating the excess light through various mechanisms such as an increase in lutein, high Chl a/b ratio and xanthophyll cycle, suggesting use of energy dissipation as a mechanism of protection against high irradiance, but to different extent and to some extent by different processes. The study was unique, as plants were grown and photoinhibited under natural conditions rather than the artificial light, as was the case in most of the studies so far. Results showed better adaptation of high-light grown plants and suggested role for chl a/b ratio and lutein, in addition to xanthophylls cycle in shade plants. Low-light grown plants could also completely adapt to full level of sunlight within 3 days of the treatment and xanthophylls cycle (measured as V, A and Z) and activity of de-epoxidase seemed to be important in this adaptation.

Effect of UV-B and high visual radiation on photosynthesis in freshwater (nostoc spongiaeforme) and marine (Phormidium corium) cyanobacteria.

Human activity is causing depletion of ozone in stratosphere, resulting in increased UV-B radiation and global warming. However, impact of these climatic changes on the aquatic organism (especially marine) is not fully understood. Here, we have studied the effect of excess UV-B and visible radiation on photosynthetic pigments, fatty acids content, lipid peroxidation, nitrogen content, nitrogen reductase activity and membrane proteins, induction of mycosporine-like amino acids (MAAs) and antioxidant enzymes superoxide dismutase (SOD) and ascorbate peroxidase (APX) in freshwater (Nostoc spongiaeform) and marine (Phormidium corium) cyanobacteria. UV-B treatment resulted in an increase in photosynthetic pigments in Nostoc and decrease in Phormidium, but high light treatment caused photobleaching of most of the pigments in both the species. Unsaturation level of fatty acids of both total and glycolipids remained unchanged in both the cyanobacteria, as a result of UV-B and high light treatments. Saturated fatty acids of total and glycolipids declined slightly in Nostoc by both the treatments. but remained unchanged in Phormidium. No changes in the unsaturated lipid content in our study probably suggested adaptation of the organism to the treatments. However, both treatments resulted in peroxidation of membrane lipids, indicating oxidative damage to lipids without any change in the level of unsaturation of fatty acid in the cell membrane. Qualitative and quantitative changes were observed in membrane protein profile due to the treatments. Cyanobacteria were able to synthesize MAAs in response to the UV-B treatment. Both treatments also increased the activities of SOD and APX. In conclusion, the study demonstrated induction of antioxidants such as SOD and APX under visible light treatment and screening pigment (MAAs) under UV-B treatment, which might protect the cyanobacteria from oxidative damage caused by high light and UV-B radiation.

Effect of UV-B on photosynthesis, membrane lipids and MAAs in marine cyanobacterium, Phormidium corium (Agardh) Gomont.

UV-B radiation (0.8 +/- 0.1 mW cm(-2)) and UV-B radiation supplemented with low intensity PAR (approximately 80 micro mol m(-2) s(-1)) affected photosynthesis at the level of antenna system as well as PS II reaction centre (Fo and Fm declined) in Phormidium corium (Agardh) Gomont. UV-B radiation resulted in decline in sugar content, peroxidation of membrane lipids as well as quantitative and qualitative changes in phosphoglycolipids and neutral lipids. Fatty acid profile did not show any qualitative changes due to the treatment, however, UV-B supplemented with low PAR resulted in slightly higher level of unsaturation. P. corium synthesized MAAs in response to UV-B. Quantity of MAAs increased when UV-B treatment was supplemented with low level PAR.

Protection against photooxidative damage provided by enzymatic and non-enzymatic antioxidant system in sorghum seedlings.

Effect of photoinhibition of sorghum leaves and isolated chloroplasts on chlorophyll fluorescence, peroxidation of thylakoid lipids and activity of antioxidant enzymes 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 GSH) resulted in significantly less damage to photosynthesis (Fv/Fm ratio) and peroxidation level. Photoinhibition also resulted in many fold increase in the activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX) and decrease in catalase. Data presented here suggest that photoinhibition resulted in production of oxygen radicals and photoinhibition of chloroplasts in the presence of low oxygen level or when supplemented with antioxidants decreased the damage to Fv/Fm ratio and peroxidation level to a great extent since former prevented the formation of oxygen radicals and later could scavenge the oxygen radicals thus the protection. Increase activity of SOD and APX may also be to metabolise the oxygen radicals produced during photoinhibition treatment, thereby, protecting the seedlings against photooxidative damage.