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Articles

C₄ Photosynthesis: Light-dependent CO₂ Fixation by Mesophyll Cells, Protoplasts, and Protoplast Extracts of Digitaria sanguinalis¹

^a Department of Horticulture, University of Wisconsin, Madison, Wisconsin 53706

Mesophyll cells, protoplasts, and protoplast extracts of Digitaria sanguinalis were used for comparative studies of light-dependent CO₂ fixation. CO₂ fixation was low without the addition of organic substrates. Pyruvate, oxaloacetate, and 3-phosphoglycerate induced relatively low rates (10 to 90 µmoles/mg chlorophyll·hr) of CO₂ fixation when added separately. However, a highly synergistic relationship was found between pyruvate + oxaloacetate and pyruvate + 3-phosphoglycerate for inducing light-dependent CO₂ fixation in the mesophyll preparations. Highest rates of CO₂ fixation were obtained with protoplast extracts. Pyruvate, in combination with oxaloacetate or 3-phosphoglycerate induced light-dependent rates from 150 to 380 µmoles of CO₂ fixed/mg chlorophyll·hr which are equivalent to or exceed reported rates of whole leaf photosynthesis in C₄ species. Concentrations of various substrates required to give half-maximum velocities of CO₂ fixation were determined, with the protoplast extracts generally saturating at the lowest substrate concentrations. Chloroplasts separated from protoplast extracts showed little capacity for CO₂ fixation. The results suggest that CO₂ fixation in C₄ mesophyll cells is dependent on chloroplasts and extrachloroplastic phosphoenolpyruvate carboxylase.

The stimulation of pyruvate-induced CO₂ fixation by oxaloacetate and 3-phosphoglycerate is thought to be due to induction of noncyclic electron transport which generates ATP for the conversion of pyruvate to phosphoenolpyruvate by pyruvate Pi dikinase. The primary products of the substrate-induced CO₂ fixation were oxaloacetate and malate, which provides further evidence for carbon fixation through the -carboxylation pathway. High rates of light-dependent CO₂ fixation with a significant percentage of ¹⁴C fixed into malate suggest an efficient operation of both photosystems I and II.

The substrate inductions are discussed with respect to the proposed role of the mesophyll cell in C₄ photosynthesis, and schemes suggesting the stoichiometry of energy requirements for photosynthetic carbon metabolism in C₄ mesophyll cells are presented.