Photoinhibitory Stress Causes Accumulation of a 31 Kilodalton Protein in the Chloroplast Light-harvesting Apparatus
Photodamage to the photosynthetic apparatus of chloroplasts occurs when plants are exposed to high light levels under environmental conditions which inhibit assimilation of carbon dioxide. Such damage results in a decrease in the capacity for photosynthetic electron transport and photophosphorylation. Concomitant with these changes is a large decrease in chlorophyll a fluorescence emission from photosystem II (PS II), which is routinely used as a monitor of photoinhibition. (1–6) It has been suggested that the fluorescence decrease, and the reduced capacity for photochemistry, may be due to an increase in the radiationless dissipation of trapped light energy by PS II antennae chlorophylls or reaction centres. (4,6) On exposure of maize plants to a photoinhibitory photon flux density of 1500 μmol m-2 s-1 at 5°C for 6h, a 31 kD protein accumulated in the thylakoid membranes of the mesophyll chloroplasts. This protein was found to be a component of the light-harvesting complex (LHC II) associated with PS II. Evidence is provided which suggests that the protein is a precursor form of LHC II proteins which has not been processed prior to insertion into the thylakoid membrane and that its appearance is correlated with a dysfunction of LHC II which would account for a reduction in photochemical efficiency of PS II.
KeywordsThylakoid Membrane Photochemical Efficiency Excitation Energy Transfer Fluorescence Rise Chloroplast Biogenesis
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