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Carbon Assimilation in Photoheterotrophic Cells of Peanut (Arachis hypogaea L.) Grown in Still Nutrient Medium ¹

Department of Plant Physiology, School of Biological Sciences, Madurai Kamaraj University, Madurai 625021, India

The relative transport of photosynthetic and dark carboxylation products in photoheterotrophic cells of Arachis hypogaea L. var. TMV-3 at varied phases of growth were determined. Despite the presence of an equally competent photosynthetic apparatus as determined from ¹⁴CO₂ incorporation rates in the dark and light, pulse-chase experiments revealed little or no change in the radioactivity of the C₃ intermediates but rapid disappearance of label from the dark carbon assimilates (malate and other tricarboxylic acid cycle intermediates) with a simultaneous increase in the aminoacid pool in early log-phase (10 days old) cells. However, significant flow of carbon through the photosynthetic intermediates resulting in the accumulation of sugars occurred in the late log-phase (34 days old) cells. Limitation of exogenous sugar in the nutrient milieu and depletion of reserve carbohydrates stored in starch of the chloroplasts of the cells were considered as the decisive factors in promoting transport of C₃ cycle intermediates through the reductive pentose phosphate pathway in photoheterotrophic cells. The observed drain of radioactivity even from the small amounts of tricarboxylic acid cycle intermediates synthesized during photosynthesis into glutamate indicated that the transport of carbon through the nonautotrophic pathway is not controlled by these factors.