Light-induced changes in the fluorescence yield of chlorophyll A in vivo. IV. The effect of preillumination on the fluorescence transient of Chlorella pyrenoidosa.

The fluorescence transient of Chlorella pyrenoidosa, excited by saturating blue light, has a base level O, hump I, dip D, peak P, and at 1.5 sec a quasi-steady level S (12). With 2 sec exciting exposures and 4 min dark periods, preillumination-1 (lambda >/= 690 nm, intensities 1-750 ergs/sec-cm(2) incident), replacing the dark periods, lowers I more effectively than preillumination-2 (650 nm </= lambda </= 680 nm) in both aerobic and anaerobic cells. Results indicate that the intersystem electron transport pool A as well as the primary electron acceptor of pigment system II Q (fluorescence quencher) is normally being reduced at I. Preillumination-1 lowers and delays P. Preillumination-2 (absorbed by both pigment systems) also lowers P, but delays P only at low intensity; at high intensity it hastens P. Preillumination-1 raises S while preillumination-2 lowers S. With 30 instead of 2 sec exciting light exposures, preillumination-1 causes a large S increase, and at low intensity a P increase. The S effects seem to be of a long-term nature (26-29) rather than rapid changes in the redox state of Q. As exciting light intensity increases, fluorescence yield at P increases three-fold maximally. The ratio of P (anaerobic) to O (aerobic) is 5.5. These high ratios restrict the Franck-Rosenberg model of photosynthesis (13), which is based on fluorescence yield doubling.