Abstract
Changes in the level of metabolites of the C4 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 C4 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 C4 cycle, there was a net increase in the level of C3 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 C4 acids (malate plus aspartate). The total level of metabolites of the C4 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 C4 cycle and RPP pathway was far above that of carbon input from CO2 fixation, and the increase in intermediates of the RPP pathway could not be accounted for by decarboxylation of C4 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 C4 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 C4 plants. This latter stage of inductive autocatalysis through the RPP pathway may contribute to the final buildup of these intermediates.
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