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Metabolism and Enzymology

Changes in Fluorescence Quenching Brought About by Feeding Dithiothreitol to Illuminated Leaves ¹

Robert Hill Institute, University of Sheffield, Sheffield S10 5BR, United Kingdom

When CO₂ is abruptly removed from the atmosphere surrounding an illuminated leaf, the primary electron-accepting plastoquinone of photosystem II (Q_A) (as measured by photochemical quenching, q_p) is rapidly reduced and then, after some seconds, becomes more oxidized. The reoxidation of Q_A^– is accompanied by an increase in pH (as measured by nonphotochemical quenching, q_N) with kinetics consistent with a causal relationship. The fact that, in such circumstances, Q_A can become more oxidized in the absence of CO₂ than in its presence indicates a diminished rate of reduction of Q_A, consequent upon impaired photosystem II efficacy. Dithiothreitol (DTT) feeding, which does not affect quantum yield or the maximum rate of photosynthesis, inhibits the reoxidation of Q_A^– but not the increase in the proton gradient. When leaves are reilluminated in high light following a dark interval of several minutes, DTT also abolishes the separation in time between the first maximum in q_P and the first maximum in the rate of O₂ evolution. It also diminishes subsequent oscillations. These results are held to demonstrate pH control of photosystem II and are consistent with DTT inhibition of the xanthophyll cycle and hydrogen peroxide reduction. They support the concept that oxygen and hydrogen peroxide are involved, as Hill oxidants, in a pH-related manner, during oscillatory behavior.