Abstract
The influence of short-time heat stress (5 min at 50 °C) on the photo-oxidation of the reaction center pigment of photosystem I (P700) under continuous illumination of pea chloroplasts was studied. It was found that heating leads to a considerable increase in P700 +, measured by the amplitude of the electron spin resonance signal I. Fluorescence emission spectra (77 K) of heat-treated chloroplasts indicate an enhancement of spillover. Photosystem II activity (H 2O—2,6-dichlorophenolindophenol) is strongly inhibited at heat-stressed chloroplasts. Low light intensity measurements showed a higher rate of light-induced oxidation of P700, related to an increase in the photosystem I absorption cross-section. The data are discussed with respect to the heat-induced structural reorganization of membrane pigment—protein complexes.
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