Light Intensity Dependence of P700 Photooxidation in Heat-Stressed Pea Chloroplasts

Abstract

The short heat stress-induced alterations of photosynthetic membrane structure are well established^{1, 2, 3}. A considerable decrease of PS II activity^{1, 4, 5} and an increase of PS I-associated reactions (PS I-mediated electron transport^{1, 5}, P700 photooxidation^{3, 6}) have been observed within the temperature range where heat-induced damages in the chloroplasts organization occur. Whereas the inhibition of PS II activity could be attributed to the heat damage of the PS II pigment-protein complex, the nature of PS I activity increase remains still controversial. It is generally accepted that PS I-related activity increase after heat-treatment could be due to the redistribution of excitation energy in favor to PS I⁷, and/or to the exposure of new electron acceptor sites in the electron transport chain⁵. It has been recently supposed that the increase of P700 photooxidation efficiency in heat - treated chloroplasts could be partially due to the increase of absorption cross section of PS I via the heat - induced rearrangements of the thylakoid protein complexes ^{3, 6}. In the present study the ESR spectroscopy of the light - induced ESR signal I (P700⁺) and the kinetics investigations of P700⁺ formation under limiting light conditions are applied to asses the effects of heat - treatment on the efficiency of P700 photooxidation and PS I antenna size.