Mechanism of Heat Induced Stimulation of PSI Activity in Pea Chloroplasts
It was well documented1,2 that short time heat stress (5 min at 50o) results in an increase of PSI-mediated electron transport rate. An explanation of this phenomenon has been given by Thomas et al.2, who proposed an exposure of new electron donor sites within cyt b6/f complex for the artificial electron donor. Considerable increase (50%) of light-induced ESRI signal (P700+) in heat stressed chloroplasts was also reported3. Recently, it was demonstrated4, that even at very low light intensity the amplitude of P700+ remains higher and that under limiting light conditions P700 photo-oxidation occurs faster in heat-treated chloroplasts. It was supposed3,4, that the mechanism of heat-induced stimulation of P700 photoconversion includes a redistribution of the excitation light energy in favor to PSI via an increase of PSI absorption cross section and enhancement of the spillover. This assumption was made on the basis of the well established ‘ heat-indu*-ced structural reorganization of the thylakoid membranes — randomization of the pigment-protein complexes of PSII and PSI, dissociation of LHCa/b protein complex from core complex of PSII and its possible lateral migration along the thylakoid membrane to the PSI-enriched stroma lamellae. In the present study the effects of heat-stress on the energy distribution and PSI electron transport rate in total thylakoids and stromal thylakoid fractions were compared.
KeywordsChlorophyll Fluorescence Thylakoid Membrane Fluorescence Rise Stromal Thylakoid Stromal Fraction
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