Action of Ionizing Radiation on Photosynthetic Reactions of Isolated Chloroplasts

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Abstract
Chloroplast fragments, isolated from spinach leaves, were X-irradiated (140 kv, 12 ma) at 0[degree]C with a dose rate of 4000 r/minute. Since suspensions of isolated chloroplasts (or chloroplast fragments) exhibit a strong dilution effect, the relative inactivation being higher with increasing dilution of the irradiated material, the same chloroplast concentration during irradiation was used in all experiments. After irradiation, the ability of the chloroplasts to perform noncyclic photophosphorylation as well as different kinds of cyclic photophosphorylation was examined. In the noncyclic photophosphorylation the formation of adenosine triphosphate was much more sensitive towards X-rays than the accompanying reduction of triphosphopyridine nucleotide and the evolution of oxygen. E.g., the 37% doses were 90 and 660 kr, respectively, when the concentration of the chloroplasts during irradiation was 0.3 mg chlorophyll/ml. Since a partial uncoupling of the phosphorylation from the electron flow was observed in high chloroplast concentrations in the photosynthetic reaction, the radioresistance was tested under the conditions of high and low chloroplast concentration in the reaction; higher radiosensitivity of phosphorylation than of electron flow was obtained, whether or not the former was fully coupled with the latter. The radiosensitivity of the phosphorylations studied was: Non-cyclic > cyclic with flavin mononucleotide or vitamin Kg > cyclic with phenazine methosulfate. Conclusions from the results are discussed with reference to the conceptions of the pathways of photophosphorylation reactions.