Changes in Levels of Intermediates of the C(4) Cycle and Reductive Pentose Phosphate Pathway during Induction of Photosynthesis in Maize Leaves.

Changes in the level of metabolites of the C(4) cycle and reductive pentose phosphate (RPP) pathway were measured simultaneously with induction of photosynthesis in maize (Zea mays L.) to evaluate what may limit carbon assimilation during induction in a C(4) plant.After 20 minutes in the dark, there was an immediate rise in photosynthesis during the first 30 seconds of illumination, followed by a gradual rise approaching steady-state rate after 20 minutes of illumination. Among metabolites of the C(4) cycle, there was a net increase in the level of C(3) compounds (the sum of pyruvate, alanine, and phosphoenolpyruvate) during the first 30 seconds of illumination, while there was a net decrease in the level of C(4) acids (malate plus aspartate). The total level of metabolites of the C(4) cycle underwent a sharp increase during this period. At the same time, there was a sharp rise in the level of intermediates of the RPP pathway (ribulose-1,5-bis-phosphate, 3-phosphoglycerate, dihydroxyacetonephosphate, and fructose-1,6-bisphosphate) during the first minute of illumination. The net increase of carbon among intermediates of the C(4) cycle and RPP pathway was far above that of carbon input from CO(2) fixation, and the increase in intermediates of the RPP pathway could not be accounted for by decarboxylation of C(4) acids, suggesting that an endogenous source of carbon supplies the cycles. After 3 minutes of illumination there was a gradual rise in the levels of intermediates of the C(4) cycle and in the total level of metabolites measured in the RPP pathway. This rise in metabolite levels occurs as photosynthesis gradually increases and may be required for carbon assimilation to reach maximum rates in C(4) plants. This latter stage of inductive autocatalysis through the RPP pathway may contribute to the final buildup of these intermediates.