Comparison of Three Photochemical Activities of Chloroplasts",2

The Hill reaction is one of the simplest chloroplast reactions in which one can measure in unambiguous terms both the reducing and the oxidizing equivalents produced in the process of photosyntnesis. In its simplest aspect, the oxidizing power is measured as oxygen production, while the reducing power is measured in terms of reduction of some electron acceptor (9). Two other photochemical reactions catalyzed by plant chloroplasts are the photochemical oxidation of ascorbic acid in the presence of the dye 2,6-dichlorophenolindophenol (DPIP), and the photochemical reduction of the dye indigo carmine (IC), which photoreduction requires the presence of both ascorbic acid and DPIP (10, 12). These reactions also can be described in terms of the classical Hill reaction, in which case one could consider the ascorbate-DPIP couple as reacting with the photochemical oxidizing system, allowing the photochemical reducing system to react with oxygen and indigo carmine respectively (10). Although there is to date no decisive experimental proof to show that such is the case, isotopic evidence does support this hypothesis (4). This paper describes the effect of various chloroplast treatments upon the ability of chloroplasts to catalyze a conventional Hill reaction, the photochemical oxidation of ascorbic acid in the presence of DPIP, and the photochemical reduction of indigo carmine, the purpose being to see if one can in this manner correlate these 3 photochemical activities of the chloroplast.