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Changes in activities of the enzymes involved in the metabolism of active oxygen species were followed in homogenates prepared from wheat leaves (Triticum aestivum 1.) exposed to strong visible light (600 W m-*). The activities of superoxide dismutase (SOD), ascorbate peroxidase, and monodehydroascorbate reductase increased significantly on prolonged illumination of the leaves, indicating an increase in the rate of generation of active oxygen species. This increase was further exacerbated when high light stress was combined with low temperature (8°C). Our results indicate that the increase in activities of SOD and ascorbate peroxidase involved de novo protein synthesis that was sensitive to the nuclear-directed protein synthesis inhibitor cycloheximide. The activity of catalase, on the other hand, decreased on exposure to strong light, which could be due to its photolability, particularly at lower temperatures. Ascorbate and total carotenoid contents also increased on light treatment of the leaves. The induction of the enzymes except for catalase and increase in the levels of ascorbate and total carotenoids in response to the stress conditions indicate that they play an important role in the protection of higher plants from the damaging effects of toxic active species.

Effects of high light and temperature stress on the structure and function of the photosynthetic apparatus of wheat (Triticum aestivum) were studied. There was a decrease in the electron transport activity of chloroplasts isolated from photoinhibited and heat-stressed leaves. Chlorophyll fluorescence was measured in photoinhibited and heat-stressed leaves and the decrease in variable fluorescence and variable to maximum fluorescence ratio of the stressed leaves indicated a loss in the quantum yield of photosynthesis. The decrease in electron transport activity was accompanied by an increase in peroxidation of thylakoid lipids. Lipid peroxidation indicated the oxidative degradation of polyunsaturated fatty acyl residues of the thylakoid lipids. A negative correlation was observed between electron transport activity and lipid peroxidation. The electron transport activity was completely lost as the peroxidation level reached a threshold equivalent to 0.6 micromoles malondialdehyde. The threshold of lipid peroxidation for complete loss of activity was the same for both photoinhibition and heat treatment, suggesting that the nature of the environmental stress may be less important with respect to the relationship between electron transport and lipid peroxidation. Thus, it seems likely that lipids are required for sustaining the photosynthetic activity under environmental stress, and a loss in activity is observed as the lipids are degraded either by high light or high temperature stress.The photosynthetic activity of an organism can be severely reduced after exposure to light intensity in excess of that required to saturate photosynthesis. This phenomenon, referred to as photoinhibition (14,19,23) atoms of the oxygen-evolving complex (22, 27), denaturation of certain functional proteins (27), and the lateral phaseseparation of the nonbilayer-forming galactolipid monogalactosyldiacylglycerol of thylakoid membranes (13).Although a number of studies have been performed on thylakoid lipids, their exact functional role still remains unclear, probably because lipids, unlike proteins, do not exhibit any catalytic property (17,24). The lipids are, however, thought to allow the maintenance of appropriate protein conformation that may be required for the optimal chloroplast functioning. In the present paper, we have attempted to characterize the nature and the extent of damage to the photosynthetic electron transport chain in chloroplasts isolated from stressed leaves and correlate them with the peroxidative degradation of the fatty acyl residues of thylakoid lipids. MATERIALS AND METHODS Plant MaterialWheat seeds (Triticum aestivum var HD-2329, Indian Agricultural Research Institute, New Delhi) were surface-sterilized with 0.1% HgC12 solution. They were germinated on moist filter paper in dark at 25°C for 48 h. After germination, the seedlings were transferred to a plant growth chamber maintained at 25°C and grown there under 14/10 h light-dark cycles. Light intensity during growth of the seedlings was 75 W_ m-2. The ...

A highly purified oxygen evolving Photosystem II core complex was isolated from PS II membranes solubilized with the non-ionic detergent n-octyl-β-D-thioglucoside. The three extrinsic proteins (33, 23 and 17 kDa) were functionally bound to the PS II core complex. Selective extraction of the 22, 10 kDa, CP 26 and CP 29 proteins demonstrated that these species are not involved in the binding of the extrinsic proteins (33, 23 and 17 kDa) or the DCMU sensitivity of the Photosystem II complex.