Photoassimilation of CO2 by Isolated Bundle‐Sheath Strands of Zea mays

Abstract

AbstractThe role of malate decarboxylation as a source of CO2 and NADPH for the evolution of photosynthesis of isolated maize bundle‐sheath strands has been investigated. The bundle‐sheath cells were supplied with malate plus NADP, in the presence of intermediates of the Calvin cycle to increase the rate of CO2 fixation.The effects of malate addition on the rate of 3 phospoglycerate synthesis, with non‐saturating concentrations of bicarbonate, can be explained by an increase of the cellular pool of CO2 in the cells due to malate decarboxylation. The CO2 reacting with RuDP to give phosphoglycerate corresponds effectively to carbon atom 4 of malate. Malate addition produces an enhancement of the rate of CO2 incorporation which is much more important when the reducing power is the limiting process for the evolution of the Calvin cycle (with phosphoglycerate as added substrate and/or in the presence of DCMU. These results demonstrate the utilization of NADPH produced by malate decarboxylation for the regeneration of RuDP. NADPH can also reverse the reaction of malate decarboxylation and gives rise to a synthesis of malate by carboxylation of pyruvate.In contrast, the pattern of 14C distribution among compounds is not strongly modified by malate addition. This result suggests that PGA reduction in the whole leaf must occur also in mesophyll cells to allow correctregeneration of the reduced compounds of the photosynthetic cycle.