Heat-induced changes in photosystem I activity as measured with different electron donors in isolated spinach thylakoid membranes.

Heat-induced changes in photosystem I (PSI) have been studied in terms of rates of oxygen consumption using various donors (DCPIPH2, TMPDred and DADred), formation of photo-oxidized P700 and changes in Chl a fluorescence emission at 77 K. Linear heating of thylakoid membranes from 35 degrees C to 70 degrees C caused an enhancement in PSI-mediated electron transfer rates (DCPIPH2-->MV) up to 55 degrees C. However, no change was observed in PSI rates when other electron donors were used (TMPDred and DADred). Similarly, Chl a fluorescence emission spectra at 77 K of heat-treated thylakoid membranes did not show any increase in peak at 735 nm, however, a significant decrease was observed as a function of temperature in the peaks at 685 and 694 nm. In DCMU-treated control thylakoid membranes maximum photo-oxidized P700 was generated at g = 2.0025. In heat-treated thylakoid membranes maximum intensity of photo-oxidized P700 signal was observed at approximately 50-55 degrees C without DCMU treatment. The steady-state signal of the photo-oxidized P700 was studied in the presence of DCPIPH2 and TMPDred as electron donors in DCMU-treated control and in 50 degrees C treated thylakoid membranes. We present here the first of such comparative study of PSI activity in terms of the rates of oxygen consumption and re-reduction kinetics of photo-oxidized P700 in the presence of different electron donors. It appears that the formation of the P700+ signal in heat-treated thylakoid membranes is due to an inhibited electron supply from PSII and not due to spillover or antenna migration.