THE EFFECTS OF SOME INHIBITORS OF PHOTOSYNTHESIS UPON THE PHOTOCHEMICAL REDUCTION OF A DYE BY ISOLATED CHLOROPLASTS

Abstract

Physiological research directed upon the activity of isolated chloroplasts has been spurred by the belief that the photochemical activity which chloroplasts exhibit in vitro is part of photosynthesis. If this is so, then those components of the photosynthetic apparatus which are active in isolated chloroplasts can thus be brought into closer range for study. The object of the present work was to find how closely the dye-reducing system of isolated chloroplasts is related to photosynthesis, by comparing the response of the former to various poisons and narcotics whose action upon the latter is already known, and to investigate the nature of the chloroplast apparatus through its activity. It is believed that the photochemical and enzymatic machinery of that part of the photosynthetic mechanism which brings about the oxidation of water and the liberation of oxygen can be removed from the living cell intact in the chloroplast structure in a form such that it will continue to function in vitro in the presence of externally supplied oxidants. It was the presence of such oxidants in aqueous leaf extracts which permitted the action of freed chloroplasts in Hill's initial experiments (7) wherein the evolution of oxygen was followed spectrophotometrically via the conversion of haemoglobin to oxyhaemoglobin. Such oxidants must also have been important in the original experiments by Haberlandt (6) and Ewart (3), and in those investigations by Molisch (16, 17) and Inman (13, 14) which followed. Hill (8)found that ferric potassium oxalate may be substituted for these unknown oxidants, and more recently Warburg and L?ttgens (20) struck a close analogy to photosynthesis through the successful use of pbenzoquinone which accepts hydrogen in its reduction, unlike Fe+++ in the Hill reaction but like anabolites in photosynthesis. Holt and French (10) found that chromate and ferricyanide can be reduced by the action of isolated chloroplasts with the evolution of oxygen, and developed a technique involving the visible reduction to their leucoforms of several redox indicators (referred to here as dyes). The reduction of such dyes is now followed photometrically by the employment of a photronic cell, with appropriate filters, connected to a galvanometer. This method was used in the poisoning studies reported below and will be described in more detail in a forthcoming paper by Holt, Smith and French (12). ? Present address : Carnegie Institution of Washington, Division of Plant Biology, Stanford, California.