Abstract
The effect of medium pH on state 1-state 2 transitions was investigated in an intact leaf using the photoacoustic technique. Oxygen evolution yield in a leaf adapted to state 2 was independent of external pH. Emerson enhancement of modulated oxygen evolution during adaptation to state 2 was found to be pH dependent, with minima at pH 6.0–6.5 and 8.0–8.5. These minima correspond to two maxima for the balance of excitation energy distribution between photosystems I and II. A similar pH dependence was found for the initial rate of ATP-induced fluorescence quenching in isolated thylakoids. These results suggest that protein phosphorylation is maximal at pH 6.0–6.5 and 8.0–8.5. It is proposed that these pH maxima in protein phosphorylation reflect the participation of two kinases in the transition to state 2.
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