Heat-induced reversible changes in Photosystem 1 absorption cross-section of pea chloroplasts and sub-chloroplast preparations : Evidence from excitation fluorescence spectra.

Reversible changes in the room temperature fluorescence quenching at 685 nm and light scattering level at 577 nm, indicating about 15% of granal unstacking, induced by high temperature treatment (40°C, for 5 min) of pea chloroplasts were shown. Analysis of the low temperature excitation fluorescence spectra of the 735 nm Photosystem 1 (PS 1) band (F735), in the 635-725 nm region, has revealed the involvement of light-harvesting (LHC 2, maxima at 650 and 676 nm) and the proximal Photosystem 2 antenna (maxima 668, 687 nm) in heat-induced enhancement of the PS 1 long wavelength antenna absorption cross-section. It was found that the two PS 1 sub-chloroplast preparations, achieved by the digitonin method, possessed different characteristics of this enhancement. For the heavier fraction (100 000 g) the additional absorption cross-section was formed mostly at the expense of PS 2 antennas (apparently spillover), but for the lighter PS 1 fraction (145 000 g) the changes have indicated an α-transfer mechanism, i.e., participation of only LHC 2 in the energy transfer towards PS 1. This may indicate the heterogeneous character of the temperature-induced energy redistribution across the PS 1-containing chloroplast membrane compartments. The model of heat-induced changes in the pigment-protein complex arrangement is discussed in terms of domain organisation of the thylakoid membrane.