Effects of Short-Time Heat Stress on the Parameters of Cation induced Increase of Chlorophyll Fluorescence in Pea Thylakoid Membranes

Abstract

The half-time (τ 1/2 ) of the Mg 2+ induced chlorophyll fluorescence rise in pea thylakoid membranes increases at elevated temperatures, the maximal effect being observed at 45°C. It was also observed that the half-time of fluorescence rise decreased by increasing the Mg 2+ concentration in both control and heat treated chloroplasts, but the τ 1/2 values of heat stressed membranes remain higher than in control (nonheated) preparations at all Mg 2+ concentrations tested (1–10 mM). The second effect of thermal treatment was to change the size of the final fluorescence level reached after addition of Mg 2+ . It should be noted that in heat treated chloroplasts higher cation concentration is required to reach the maximal fluorescence level than in non-heated chloroplasts. Kinetic analysis of the salt induced increase of chlorophyll fluorescence indicated that the rate of increase of the distance between PSII and PSI complexes is markedly reduced in heat treated chloroplasts than in control. The salt induced time — dependent fluorescence changes are discussed in terms of heat — caused effects on the intra-thylakoid organization and physical properties of thylakoid membranes and their possible influence on the diffusion controlled lateral movement of chlorophyll-protein complexes of PSI and PSII.