Action of Ionizing Radiation on Photosynthetic Reactions of Isolated Chloroplasts

When chloroplasts from spinach are irradiated with high doses of ,or X-rays in vitro, the photosynthetic phosphorylation with FMN2 or vitamin K as a cofactor (that is, cyclic photophosphorylation according to Arnon, 1) is much more inhibited than the Hill reaction with ferricyanide (2, 3). Moreover, the suspensions of isolated chloroplasts exhibit a strong dilution effect, the relative inactivation being higher with increasing dilution of the irradiated chloroplasts (3, 4). The dilution effect in suspensions of particles which was found with isolated mitochondria by Yost and Robson (5) has been discussed previously (4). Because both the dilution effect and protection effects influence markedly the response of chloroplast suspensions to irradiation, it was thought desirable to examine two sites of one reaction in one suspension of chloroplasts in order to analyze in more detail the effect of X-rays on photosynthetic reactions. The finding of Arnon et al. (6) that photophosphorylation can be coupled to a Hill reaction makes it possible to measure electron transport and coupled ATP formation in one reaction of one chloroplast suspension. In this reaction the phosphorylation is linked to a reduction of TPN accompanied by oxygen evolution according to the following equation (1).