Photoreduction of nadp+ in photosystem ii of higher plants: requirement for manganese

The effects of reversible manganese extraction on NADP⁺ photoreduction were studied with higher plant subchloroplast preparations of photosystem II (PS II). Under anaerobic conditions, when the reaction centers (RCs) of PS II are “closed” (i.e. in the state [P680 Pheo] [Formula Omitted]), and in the presence of ferred oxin -ferred oxin-NADP⁺ reductase, NADP⁺ reduction is observed at a rate of 0.8-1.1 nmol/mg × chlorophyll × h. After complete removal of manganese from PS II, the rate of NADP⁺ reduction is reduced 40–50-fold. Upon the addition of Mn at a concentration of approx. 4 Mn atoms per reaction center, the NADP⁺ reducttion is restored up to 85–90% of the initial value. When half of this amount of Mn is combined with about 40 times of the equivalent concentration of other divalent ions (Ca²⁺, Sr²⁺, Mg²⁺ etc.) the reaction is also reactivated. Dinoseb (10⁻⁶ M) an inhibitor of electron transfer in PS II prevents NADP⁺ photoreduction. It is concluded that under conditions when the first quinone acceptor, Q_A, is in its reduced state [Formula Omitted], electrons are transferred from reduced pheophytin (Pheo⁻) to NADP⁺, indicating that PS II can reduce NADP⁺ withoutthe participation of PS I. On the basis of these and literature data, an alternate pathway for electron phototransfer in PS II reactioncenters of higher plants is suggested. Some problems concerning the Z-schemearediscussed.