Changes in Levels of Intermediates of the C(4) Cycle and Reductive Pentose Phosphate Pathway under Various Light Intensities in Maize Leaves.
The rate of CO(2) assimilation and levels of metabolites of the C(4) cycle and reductive pentose phosphate pathway in attached leaves of maize (Zea mays L.) were measured over a range of light intensity from 0 to 1,900 microEinsteins per square meter per second under a saturated CO(2) concentration of 350 microliters per liter and a limiting CO(2) concentration of 133 microliters per liter. The level of ribulose 1,5-bisphosphate (RuBP) stayed almost constant (around 60 nanomoles per milligram chlorophyll [Chl]) from low to high light intensities under 350 microliters per liter. Levels of 3-phosphoglycerate (PGA) increased from 100 to 650 nanomoles per milligram Chl under 350 microliters per liter CO(2) with increasing light intensity. The calculated RuBP concentration of 6 millimolar (corresponded to 60 nanomoles per milligram Chl) was about two times above the estimated RuBP binding-site concentration on ribulose bisphosphate carboxylase-oxygenase (Rubisco) of approximately 2.6 millimolar in maize bundle sheath chloroplasts in the light. The ratio of RuBP/PGA increased with decreasing light intensity under 350 microliters per liter CO(2). These results suggest that RuBP carboxylation is under control of light intensity possibly due to a limited supply of CO(2) to Rubisco through the C(4) cycle whose activity is highly dependent on light intensity. Pyruvate level increased with increasing light intensity as long as photosynthesis rate increased. A positive relationship between levels of PGA and those of pyruvate during steady-state photosynthesis under various conditions suggests that an elevated concentration of PGA increases the carbon input into the C(4) cycle through the conversion of PGA to PEP and consequently the level of total intermediates of the C(4) cycle can be raised to mediate higher photosynthesis rate.
Usuda
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Laboratory of Chemistry, Faculty of Medicine, Teikyo University, Ohtsuka, Hachioji, Tokyo, Japan, 192-03.