Photoreduction of NADP⁺ by a chloroplast photosystem II preparation: effect of light intensity

Recent work in this and other laboratories has demonstrated that, contrary to the favored Z scheme hypothesis, photosystem II (PS II) can photoinduce electron transfer from water to NADP + , without the participation of photosystem I (PS I). One proposed explanation for this conflict between hypothesis and observation was that PS II can reduce NADP + but only at high light intensities. We report here findings at variance with this proposal. A PS II preparation made from spinach chloroplasts by the two‐phase aqueous polymer partition method photoreduced NADP + without the involvement of PS I, at varying light intensities ranging from limiting to saturating.