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In osmotically shocked pea chloroplasts illuminated with modulated blue-green light (light 2), phosphorylation of the light-harvesting chlorophyll a b-protein complex (LHCP) accompanies the slow decrease in modulated fluorescence that indicates adaptation to light absorbed predominantly by Photosystem II (State 2). On subsequent additional illumination with continuous far-red light (absorbed predominantly by Photosystem I; light 1) both effects are reversed: modulated chlorophyll fluorescence emission increases (indicating adaptation towards State 1) and LHCP is dephosphorylated. Net phosphorylation and dephosphorylation of LHCP induced by light 2 and excess light 1, respectively, occur on the same time scale as the ATP-dependent chlorophyll fluorescence changes indicative of State 2 and State 1 transitions. The phosphatase inhibitor NaF (10 mM), stimulates the effect of blue-green light on …