Gradual disassembly of photosystem 2 in vivo induced by excess irradiance.: A hypothesis based on changes in 77 K fluorescence spectra of chlorophyll a in barley leaves

Effects of short-term exposure to different irradiances on the function of photosystem 2 (PS2) were studied for barley grown at low (LI; 50 mu mol m(-2) s(-1)) and high (HI; 1100 mu mol m(-2) s(-1)) irradiances. HI bar-ley revealed higher ability to down-regulate the light-harvesting within PS2 after exposure to high irradiance as compared to LI plants. This ability was estimated from the light-induced decreases of F685/F742 and E476/E436 in emission and excitation spectra of 77 K chlorophyll (Chl) a fluorescence in vivo which was 65 and 10 % for HI plants as compared to 30 and 2 % for LI plants, respectively. For LI plants this protective down-regulation of the light-harvesting of PS2 was saturated at 430 mu mol m(-2) s(-1), and progressive PS2 photodamage was induced at higher irradiances, After exposure of LI segments to 2 200 mu mol m(-2) s(-1) a pronounced maximum at 700 nm appeared in emission spectrum of 77 K CN a fluorescence. Based on complementary analysis of 77 K excitation spectra measured at the emission wavelength 685 nm we suggest that this emission maximum may be attributed to the formation of aggregates of light-harvesting complexes of PS2 (LHC2) with part of PS2 core during progressive PS2 photodamage. Our results can be explained assuming different contributions of LHC2 and PS2 core to the total nonradiative dissipation of absorbed excitation energy for the LI and HI barley.