Inhibition of photosystems I and II and enhanced back flow of photosystem I electrons in cucumber leaf discs chilled in the light

Pre-illumination of cucumber leaf discs at a chilling temperature in low-ir radiance white light resulted in accelerated re-reduction of P700(+) [the s pecial Chl pair in the photosystem I (PSI) reaction centre] when the far-re d measuring light was turned off. Measurements (in methyl viologen or +/- D CMU conditions) of the re-reduction half time suggest that accelerated re-r eduction of P700(+) appeared to be predominantly due to charge recombinatio n and only partly due to reductants sustained by previous cyclic electron f low around PSI. Apparently, charge recombination in PSI was greatly enhance d by inhibition of forward, linear electron flow. Inhibition of PSII electr on transport was observed to occur to a lesser extent than that of PSI, but only if the measurement of PSII functionality was free from complications due to downstream accumulation of electrons in pools. We suggest that promo tion of controlled charge recombination and cyclic electron flow round PSI during chilling of leaves in the light may partly prevent further damage to both photosystems.