Light-Dependent Metabolism of Carbon Compounds by Mesophyll Chloroplasts from Plants with the C₄ Pathway of Photosynthesis

Abstract

This paper describes the capacities of intact mesophyll chloroplasts prepared from Atviplex spongiosa and maize for the light-dependent metabolism of carbon compounds. Illuminated chloroplasts converted pyruvate to phosphoenolpyruvate but the initial rate was maintained only when either 3-phosphoglycerate or methylviologen was added. Light-dependent oxygen evolution was observed when 3-phosphoglycerate or oxaloacetate was added to chloroplasts, and the concomitant production of dihydroxyacetone phosphate or malate was observed. The addition of ADP or NADP or both did not produce increases in these activities. Addition of the photophosphorylation uncoupler methylamine stimulated the rate of oxaloacetate reduction and inhibited the conversion of 3-phosphoglycerate to dihydroxyacetone phosphate. These chloroplasts were not capable of oxygen evolution upon the addition of bicarbonate and did not fix CO₂. The data also provided information about minimum rates of influx or efflux of metabolites through the chloroplast outer membrane. These studies confirmed that mesophyll chloroplasts of C₄ species are unique in terms of their cap- acities for carbon metabolism. These activities are discussed in relation to the role of mesophyll chloroplasts in C₄-pathway photosynthesis.