EFFECTS OF SHORT-TIME HEAT-STRESS ON THE PARAMETERS OF CATION-INDUCEDINCREASE OF CHLOROPHYLL FLUORESCENCE IN PEA THYLAKOID MEMBRANES

The half-time (tau1/2) of the Mg2+ induced chlorophyll fluorescence ri se in pea thylakoid membranes increases at elevated temperatures, the maximal effect being observed at 45-degrees-C. It was also observed th at the half-time of fluorescence rise decreased by increasing the Mg2 concentration in both control and heat treated chloroplasts, but the tau1/2 values of heat stressed membranes remain higher than in control (non-heated) preparations at all Mg2+ concentrations tested (1 - 10 m M). The second effect of thermal treatment was to change the size of t he final fluorescence level reached after addition of Mg2+. It should be noted that in heat treated chloroplasts higher cation concentration is required to reach the maximal fluorescence level than in non-heate d chloroplasts. Kinetic analysis of the salt induced increase of chlor ophyll fluorescence indicated that the rate of increase of the distanc e between PSII and PSI complexes is markedly reduced in heat treated c hloroplasts than in control. The salt induced time - dependent fluores cence changes are discussed in terms of heat - caused effects on the i ntra-thylakoid organization and physical properties of thylakoid membr anes and their possible influence on the diffusion controlled lateral movement of chlorophyll-protein complexes of PSI and PSII.