SELECTIVE INHIBITION OF PHOTOSYNTHESIS

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Abstract
In the presence of ethanol or acetone at concentrations up to 10% by volume, photosynthetic fixation of C14O2 by Scenedesmus proceeds at nearly normal or enhanced rates, over periods of several minutes, into glycolic acid, glycine, alanine, serine, and an unknown compound. However, fixation into other compounds, such as phosphoglyceric, phosphopyruvic, and aspartic acids and the lipids, is suppressed over induction periods of several minutes. The results indicate that blocking of the photosynthetic cycle unmasks an alternate path of fixation. It is suggested that the initial step is the carboxylation of phosphoenolpyruvic acid. At a concentration of about 10−5 M, o-phenanthroline enhances the fixation into compounds which are connected with the tricarboxylic acid cycle, at the expense of the fixation in all other compounds. With increasing inhibition, alanine is in all cases the last labelled compound to vanish, as if some of it had an origin independent of that of the other products.

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