Changes in Levels of Intermediates of the C₄ Cycle and Reductive Pentose Phosphate Pathway under Various Concentrations of CO₂ in Maize Leaves

Abstract

The rate of CO₂ assimilation and levels of metabolites of the C₄ cycle and reductive pentose phosphate pathway in an attached leaf of maize (Zea mays L) were measured over a range of intercellular CO₂ concentration (Ci) of 10 to 190 microliters per liter. The CO₂ assimilation rate was saturated at a Ci of around 175 microliters per liter. The levels of ribulose 1,5-bisphosphate and fructose 1,6-bisphosphate decreased substantially with increasing Ci. The levels of 3-phosphoglycerate, phosphoenolpyruvate (PEP), and pyruvate increased with increasing Ci. The level of dihydroxyacetone phosphate increased moderately from Ci of 10 microliters per liter to 20 to 50 microliters per liter and stayed almost constant over the rest of the range of Ci investigated. The levels of fructose 6-phosphate did not show any significant changes over the range of Ci. The levels of glucose 6-phosphate decreased slightly with increasing Ci. Although photosynthetically inactive pools of malate, asparate, and alanine could mask real changes in levels of the photosynthetically active pools of these compounds, the apparent levels of these compounds and the total amount of intermediates in the C₄ cycle (malate, aspartate, pyruvate, PEP, and alanine) increased with increasing Ci. The results suggest that there is carbon input into the C₄ cycle from the reductive pentose phosphate pathway which increases the level of total intermediates of the C₄ cycle with increasing Ci.