ON THE RELATIONSHIP BETWEEN THE QUANTUM YIELD OF PHOTOSYSTEM-II ELECTRON-TRANSPORT, AS DETERMINED BY CHLOROPHYLL FLUORESCENCE AND THE QUANTUM YIELD OF CO2-DEPENDENT O-2 EVOLUTION

• We tested the two empirical models of the relationship between chlorophyll fluorescence and photosynthesis, previously published by Weis E and Berry JA 1987 (Biochim Biophys Acta 894: 198-208) and Genty B et al. 1989 (Biochim Biophys Acta 990: 87-92). These were applied to data from different species representing different states of light acclimation, to species with C3 or C4 photosynthesis, and to wild-type and a chlorophyll b-less chlorina mutant of barley. Photosynthesis measured as CO2-saturated O2 evolution and modulated fluorescence were simultaneously monitored over a range of photon flux densities. The quantum yields of O2, evolution (OO2) were based on absorbed photons, and the fluorescence parameters for photochemical (q(p)) and non-photochemical (q(N)) quenching, as well as the ratio of variable fluorescence to maximum fluorescence during steady-state illumination (F(v)'/F(m)'), were determined. In accordance with the Weis and Berry model, most plants studied exhibited an approximately linear relationship between OO2/q(p) (i.e., the yield of O2 evolution by open Photosystem II reaction centres) and q(N) except for wild-type barley that showed a non-linear relationship. In contrast to the linear relationship reported by Genty et al. for q(p) X F(v)'/F(m)' (i.e., the quantum yield of Photosystem II electron transport) and OCO2, we found a non-linear relationship between qp x F(v)'/F(m)' and OO2 for all plants, except for the chlorina mutant of barley, which showed a largely linear relationship. The curvilinearity of wild-type barley deviated somewhat from that of other species tested. The non-linear part of the relationship was confined to low, limiting photon flux densities, whereas at higher light levels the relationship was linear. Photoinhibition did not change the overall shape of the relationship between q(p) x F(v)/F(m)' and OO2 except that the maximum values of the quantum yields of Photosystem II electron transport and photosynthetic O2 evolution decreased in proportion to the degree of photoinhibition. This implies that the quantum yield of Photosystem II electron transport under high light conditions may be similar for photoinhibited and non-inhibited plants. Based on our experimental results and theoretical analyses of photochemical and non-photochemical fluoresce quenching processes, we conclude that both models, although not universal for all plants, provide useful means for the prediction of photosynthesis from fluorescence parameters. However, we also discuss that conditions which alter one or more of the rate constants that determine the various fluorescence parameters, as well as differential light penetration in assays for oxygen evolution and fluorescence emission, may have direct effect on the relationships of the two models.

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