PHOTOSYNTHESIS AND METABOLISM OF ORGANIC ACIDS IN HIGHER PLANTS

Abstract

Plants were exposed to C14O2, their leaves were removed, and the distribution of C14 among the organic acids of the leaves was detd. Even after 24 hrs., the C14 was far from equally distributed among the organic acids of Bryophyllum calycinum. If a tricarboxylic acid cycle is operative in this plant, it is sluggish. In leaves of Bryophyllum, tomato, tobacco, barley, and rhubarb exposed to C14O2 in the light for 30 min., malic acid had the highest C14 specific activity and the highest total activity among the organic acids isolated. Malic acid appears to be a key compound in the organio acid metabolism of the plants examined. Plants allowed to fix C14O2 in the dark following a period of illumination, assimilated particularly high concentrations and total amts. of C14 into malic acid. Fixation in the dark, following depletion of the plants in the dark, also yielded malic acid of high activity. Plants were exposed to C14O2 in the light for 15 min., part of the leaves were removed for analysis, and the rest of the plant was kept for 4 hrs. in the dark without C14O2. In the dark the specific activity of citric and oxalic acids increased in barley and the specific activity of the other organic acids decreased. During the dark period, the C14 specific activity of all organic acids in Bryophyllum and all but the isocitric acid fraction in tobacco increased. Evidently a precursor of high specific activity was formed in the light by Bryophyllum and tobacco and was converted to organic acids in the dark. Thus the organic acids isolated are not first products of photosynthesis but are formed from precursors synthesized in earlier steps of the photosynthetic process.