1. From leaves of barley and spinach, cellular components were isolated and brought together under various conditions to investigate the fate of ascorbic acid as affected by the components in the light and dark.2. A new colorimetric method for assaying ascorbic acid and some other reducing substances was devised, measuring the color of molybdenum-blue developed by the substances in the presence of excess amounts of phosphomolybdate and inorganic acid.3. The photooxidation of ascorbic acid by green and yellow filtrates, prepared from green and etiolated leaves of barley, was studied by the ordinary as well as the new colorimetric method. In the presence of oxygen, the oxidation of ascorbic acid was found to be accelerated by light in the green filtrate, but not in the yellow filtrate.4. The oxidation of the endogenous reducing substance contained in the supernatant fraction of spinach leaf extracts was studied in the presence of washed chloroplasts (spinach). In the presence of oxygen, the rate of oxidation in the light was markedly higher than in the dark. From the changes in absorption spectrum accompanying the reaction, the endogenous reducing substance in question was identified as ascorbic acid.5. The occurrence of an endogenous precursor of ascorbic acid in spinach leaf extracts was disclosed. The photoreduction of this precursor into ascorbic acid was studied in the precence of spinach chloroplasts. A specific inhibition of this reaction by phosphoglycerate and glycerophosphate was discovered.6. The experimental results obtained were discussed in connection with the role of ascorbic acid in photosynthesis.
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