Abstract
Imbalance for linear electron flow in light 2 (e.g. λ < 690 nm) in favor of PS II, may be regulated by a mechanism which changes the ratio between competing linear and cyclic electron flows. In order to examine this possibility, artificial PS I-cyclic electron flow cofactors were added to broken chloroplasts, which performed linear whole electron transport in light 2 excitation with methylviologen as an electron acceptor. A significant increase of the apparent photosystems imbalance for linear electron transport activities in favor of PS II, monitored by modulated fluorimetry, occurred upon addition of oxidized or reduced (with excess ascorbate) forms of PMS and TMPD as well as with the reduced form of DAD. It is suggested that these effects arise from a competition with PS II on electron donation to PS I, and establishment of a steady-state where cyclic electron flow competes with the linear one. Most convincing was the observation of a similar increase of this imbalance in favor of PS II, reproduced in leaf discs treated with PMS, performing in this case a more direct measurement of oxygen evolution by photoacoustic spectroscopy. The wavelength dependence of the imbalance for linear electron transport activities was checked in chloroplasts for the case of PMS, and had maxima at wavelengths where absorption of chlorophyll b predominates. These observations suggest that the final photoactivity distribution between the two photosystems for the linear electron flow (affected by the cofactor) depends on the basal photosystems imbalance of quarta absorbed in each photosystem due to the pigment distribution pattern between PS I and PS II, as influenced by the cationic level and the abolition of membrane energization. The relevance to a possible regulation mechanism in-vivo, which controls the ratio between cyclic to non-cyclic electron flows, is discussed.
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More From: Biochimica et Biophysica Acta (BBA) - Reviews on Bioenergetics
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