On the relationship between the quantum yield of Photosystem II electron transport,as determined by chlorophyll fluorescence and the quantum yield of CO2-dependent O2 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 (ØO₂) 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 ØO₂/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×F'_v/F'_m (i.e., the quantum yield of Photosystem II electron transport) and ØCO₂, we found a non-linear relationship between q_p×F'_v/F'_m and ØO₂ 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×F'_v/F'_m and ØO₂ 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.Abbreviations F₀ and F'₀ fluorescence when all Photosystem II reaction centres are open in dark- and light-acclimated leaves, respectively - F_m and F'_m fluorescence when all Photosystem II reaction centres are closed in dark and light, respectively - F_v variable fluorescence equal to F_m-F₀ - F_s steady state level of fluorescence in light - F'_v and F'_m variable (F'_m-F'₀) and maximum fluorescence under steady state light conditions - HEPES N-2-hydroxyethylpiperazine-N-2-ethane-sulphonic acid - Q_A the primary, stabile quinone acceptor of Photosystem II - q_N non-photochemical quenching of fluorescence - q_p photochemical quenching of fluorescence - ØO₂ quantum yield of CO₂-saturated O₂ evolution based on absorbed photons