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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]

The major light-harvesting chlorophyll a/b protein (LHCIIb) of the plant photosynthetic apparatus is able to self-organise in vitro. When the recombinant apoprotein, Lhcb1, is solubilised in the denaturing detergent sodium (or lithium) dodecylsulfate (SDS or LDS) and then mixed with chlorophylls and carotenoids under renaturing conditions, structurally authentic LHCIIb forms. Assembly of functional LHCIIb, as indicated by the establishment of energy transfer between complex-bound chlorophyll molecules, occurs in two apparent kinetic steps with time constants of 10 to 30 seconds and 50 to 300 seconds, depending on the reaction conditions. Here, we use circular dichroism (CD) in the far-UV range to monitor the folding of the LHCIIb apoprotein as it is complexed with pigments. The α-helix content in the protein''s secondary structure increases in two apparent kinetic steps with time constants similar to those observed for the establishment of chlorophyll energy transfer. When the carotenoid concentration in the reaction mixture is reduced, the time constants of α-helix formation increase, as do those for the appearance of chlorophyll energy transfer. This indicates that both processes, pigment assembly and secondary structure formation, are tightly coupled. A substantial amount of α-helix is present in dodecylsulfate-solubilised LHCIIb apoprotein and appears to be distributed among various protein domains. [Copyright &y& Elsevier]