Light intensity-dependent reversible down-regulation and irreversible damage of PSII in soybean leaves

Light intensity dependence of reversible down-regulation and irreversible damage of PSII was studied by determination of chlorophyll fluorescence, PSII electron transport, D1 protein level, and oligomeric state of PSII complexes. After illumination with moderate light (700 μmol photons m−2 s−1, saturated for photosynthesis) for 3 h, all of the light-saturated PSII electron transport rate, D1 protein level and proportion of PSII dimer in total PSIIs in soybean leaves grown at 250–300 μmol m−2 s−1 had no significant change. Although PSII photochemical efficiency (Fv/Fm) declined significantly, it could recover completely after 4 h in the dark. After illumination with strong light (2000 μmol m−2 s−1) for 3 h, however, the significant decreases in all parameters above were observed, and Fv/Fm could not restore completely after 4 h in the dark. Moreover, the chlorophyll fluorescence parameter F685/F735 measured at 77 K dropped significantly in thylakoids from all soybean leaves illuminated at 700 (L2), 1200 (L4) and 2000 (L7) μmol m−2 s−1 for 3 h. After subsequent dark recovery for 3 h, F685/F735 in thylakoids from soybean leaves illuminated with L2 could recover to the level of dark control, but not in those from soybean leaves illuminated with L4 and L7. Those results indicate that the illumination with moderate light results in reversible down-regulation of some PSIIs, while illumination with strong light leads to damage of some PSIIs, namely, both reversible down-regulation and irreversible damage of PSII are light intensity-dependent. [Copyright &y& Elsevier]