Photosynthetic metabolism in bundle sheath cells of the C 4 species Zea mays: Sources of ATP and NADPH and the contribution of photosystem II

Abstract

Rates of ATP and NADPH consumption during photosynthesis by isolated strands of Zea mays bundle sheath cells were calculated from an analysis of intermediates labeled from assimilated 14C0 2. The NADPH generated during decarboxylation of added malate (via malic enzyme) was also calculated so that the component of total NADPH produced by photoreduction could be deduced. Adding ribose 5-phosphate, malate, and aspartate to cells provided with HCO 3 − substantially increased the rates of ATP and NADPH utilization by the reactions of the photosynthetic carbon reduction cycle. The rate of NADP + photoreduction was reduced by adding malate and aspartate. The ratio of ATP produced by photophosphorylation to NADP + photoreduced was highest (5 to 11) in systems provided with malate and aspartate. Only low rates of oxygen evolution were observed in the presence of HCO 3 − and ribose 5-phosphate; these rates were further reduced when malate and aspartate were added. Observed rates of oxygen evolution were in close agreement with the rates of NADP + photoreduction estimated by analysis of labeled intermediates. Simultaneous measurements of oxygen evolution and oxygen uptake, using isotopic oxygen, indicated that the rate of pseudocyclic electron transport was low. Oxygen evolution was light dependent and inhibited by NH 4C1 and 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Cells provided with malate and sufficient 3-(3,4-dichlorophenyl)-l,1-dimethylurea to completely inhibit NADP + photoreduction retained a substantial capacity for ATP synthesis. Reducing the oxygen concentration partially reversed 3-(3,4-dichlorophenyl)-l,l-dimethylurea inhibition. We propose that these treatments modify the poising of the cyclic electron transport system and that cyclic photophosphorylation is responsible for up to 80% of the ATP produced in these cells.