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

Abstract

Heat-induced changes in photosystem I (PSI) have been studied in terms of rates of oxygen consumption using various donors (DCPIPH₂, TMPD^red and DAD^red), formation of photo-oxidized P700 and changes in Chl a fluorescence emission at 77 K. Linear heating of thylakoid membranes from 35 °C to 70 °C caused an enhancement in PSI-mediated electron transfer rates (DCPIPH₂→MV) up to 55 °C. However, no change was observed in PSI rates when other electron donors were used (TMPD^red and DAD^red). 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 ∼50–55 °C without DCMU treatment. The steady-state signal of the photo-oxidized P700 was studied in the presence of DCPIPH₂ and TMPD^red as electron donors in DCMU-treated control and in 50 °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.