Distribution of excitation energy between photosystem I and photosystem II in red algae. III. Quantum requirements of the induction of a state 2-state 1 transition.
1. The light-induced redistribution of excitation energy between both photosystems (state 1-state 2 phenomenon) is investigated in Halymenia latifolia and in eight other marine red algae by measurements of slow fluorescence kinetics and of O2-exchange in monochromatic and in flashing light. 2. A light 1 pulse (443 nm) of 0.2 s and of medium intensity is sufficient to induce complete transfer from state 2 (maximum energy transfer) to state 1 (minimum energy transfer). At inducing light periods of 3 min, light intensities as low as 2.10(-13) einstein.cm-2.s-1 gave half-maximum effect. This low energy effect is strictly to be distinguished from another, somewhat similar effect restricted to higher light intensities (more than 10(-10) E.cm-2.s-1). 3. The low-energy effect is definitely dose-dependent over a wide range of inducing illumination times. In the mean of all experiments with Halymenia, a photon fluence of 2.7.10(-11) E.cm-2 gave a half-maximum transfer to state 1. The dose-effect curves are always found distinctively S-shaped. 4. On the basis of light flash experiments it is calculated that in Halymenia, Stenogramme and in Phycodrys, 2-4 photons per electron transport chain, absorbed in surplus by Photosystem I, are sufficient to induce a half-maximum transition to state 1. 5. The quantum requirement for the induction of the inverse transition to state 2 starting with state 1 is in the same range; it tends to be slightly higher. 6. The results are interpreted as revealing a close connection between the redox state of the electron transport chain (or of some single component of it) and the probability of energy transfer between Photosystem II and Photosystem I.
D004307:Dose-Response Relationship, Radiation; D004734:Energy Metabolism; D004735:Energy Transfer; D005453:Fluorescence; D008027:Light; D010788:Photosynthesis; D011789:Quantum Theory; D000461:Rhodophyta / Q000378:metabolism* / Q000528:radiation effects