Transitions between light-states 1 and 2 (State 1-State 2 transitions) were demonstrated in cells of Scenedesmus obliquus as characteristic, slow changes in the yield of variable chlorophyll fluorescence. The transitions were induced by 690 nm light 1 (absorbed predominantly by Photosystem I) and by 650 nm light 2 (absorbed predominantly by Photosystem II). Light-states 1 and 2 were probed further by chlorophyll fluorescence induction kinetic measurements which indicated a decrease of up to 20% in the absorption cross-section of Photosystem II on the transition from State 1 to State 2. No complementary change in the absorption cross-section of Photosystem I could be detected in measurements of the rate of P-700 photooxidation under 640 nm continuous illumination at limiting intensity. A marginal (approx. 3%) average increase in half-time of P-700 photooxidation on transition to State 2 was recorded but is probably within the limits of accuracy of the measurement. It is concluded that excitation energy is diverted away from the reaction centre of Photosystem II in State 2, but that the energy does not become redirected in such a way as to increase the rate of Photosystem I light-absorption in S. obliquus in vivo. This in turn suggests that the immediate phenomenon of State 1-State 2 transitions may reflect primarily regulation of excitation energy transfer from the light-harvesting chlorophyll a b complex to the Photosystem II reaction centre. This conclusion is discussed in view of earlier studies on the effect of thylakoid protein phosphorylation in vitro. © 1988.
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