Quantum efficiency of Photosystem II in relation to ‘energy’-dependent quenching of chlorophyll fluorescence

The balance between light-dependent reactions and electron-consuming reactions in intact sunflower leaves was varied by changing the incident light-flux at constant intercellular CO2 concentration. Measurements of fluorescence quenching were compared to measurements of the rate and apparent quantum yield of whole-chain electron transport at a number of steady-state conditions. The steady-state quantum yield declined with increasing light intensity, falling at the highest intensity to approx. 40% of the maximum value observed in low light. The coefficient for photochemical quenching, qQ, was near 1 in low light and only declined to 0.7 at the highest light, indicating that there was very little feedback from accumulation of reduced electron carriers. On the other hand, there was a large increase in qE, the coefficient for ‘energy’-dependent quenching, as the quantum yield fell. We found that these changes in the steady-state quantum yield, Φs, could be related to the changes in fluorescence quenching by an empirical equation, Φs = qQ(0.32 − 0.17 qE) which accounted for variation in Φs resulting from light saturation or changes in CO2 concentration. We develop a hypothesis that Photosystem (PS) II centers may be converted to an altered state (possibly mediated by the chloroplast ΔpH) which has very little variable fluorescence and a lowered photochemical yield. We develop a kinetic explanation for the properties of the altered form of PS II, and we propose that this mechanism (indicated by qE) functions together with the accumulation of reduced QA (indicated by qQ) to regulate the rate of net photochemistry by PS II when — with increasing light or decreasing CO2 — the potential rate of net photochemistry exceeds that for carbon metabolism. The latter mechanism apparently permits down-regulation of PS II to occur without strong accumulation of reduced QA, except during transients or under the most extreme conditions.

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