Far-red photosynthesis: Two charge separation pathways exist in plant Photosystem II reaction center

An alternative charge separation pathway in Photosystem II under the far-red light was proposed by us on the basis of electron transfer properties at 295 K and 5 K. Here we extend these studies to the temperature range of 77-295 K with help of electron paramagnetic resonance spectroscopy. Induction of the S₂ state multiline signal, oxidation of Cytochrome b₅₅₉ and Chlorophyll_Z was studied in Photosystem II membrane preparations from spinach after application of a laser flashes in visible (532 nm) or far-red (730-750 nm) spectral regions. Temperature dependence of the S₂ state signal induction after single flash at 730-750 nm (T_inhibition ~ 240 K) was found to be different than that at 532 nm (T_inhibition ~ 157 K). No contaminant oxidation of the secondary electron donors cytochrome b₅₅₉ or chlorophyll_Z was observed. Photoaccumulation experiments with extensive flashing at 77 K showed similar results, with no or very little induction of the secondary electron donors. Thus, the partition ratio defined as (yield of Y_Z/CaMn₄O₅-cluster oxidation):(yield of Cytb₅₅₉/Chl_Z/Car_D2 oxidation) was found to be 0.4 at under visible light and 1.7 at under far-red light at 77 K. Our data indicate that different products of charge separation after far-red light exists in the wide temperature range which further support the model of the different primary photochemistry in Photosystem II with localization of hole on the Chl_D1 molecule.