The relationship between changes in the redox state of plastoquinone and control of excitation energy distribution in photosynthesis

Abstract The hypothesis that excitation energy distribution between PS I and PS II is controlled by the redox state of the plastoquinone pool between the two photosystems was investigated using the green alga Chlorella vulgaris . Changes in the redox state of the pool were monitored by measurement of the area above the fluorescence induction curve on exposure to high-intensity light. In agreement with the hypothesis, exposure of state I adapted cells to light preferentially absorbed by PS II led to a reduction of the plastoquinone pool whilst exposure of State II adapted cells to light preferentially absorbed by PS I resulted in its oxidation. However, the limits within which these fluctuations occurred were much narrower than anticipated. The reasons for this are discussed in terms of the possible involvement of changes in the redox state of more specialised molecules associated with the main plastoquinone pool and the postulated role of plastoquinone as an electron shuttle between the two photosystems.