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Metabolism and Enzymology

¹³C Values in Maize Leaf Correlate with Phosphoenolpyruvate Carboxylase Levels ¹

Department of Agricultural Chemistry, School of Agriculture, Chikusa, Nagoya 464, Japan, Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706

Values of ¹³C and levels of phosphoenolpyruvate carboxylase and ribulose 1,5-bisphosphate carboxylase/oxygenase were analyzed in segments from the fourth leaf of young maize (Zea mays L.) plants. The ¹³C values became significantly more negative from the base to the tip of the leaves. Phosphoenolpyruvate carboxylase levels and ribulose bisphosphate carboxylase levels both increased from the base to the tip. The principal effect of phosphoenolpyruvate carboxylase levels or ¹³C should arise through its effect on the carboxylation/diffusion balance in the mesophyll. In this case, ¹³C values should become more negative as phosphoenolpyruvate carboxylase levels increase, unless there are offsetting changes in stomatal aperture. The principal effect of ribulose bisphosphate carboxylase/oxygenase on ¹³C should occur through its effect on the extent of leakage of CO₂ from the bundle sheath cells. In this case, ¹³C values should become more positive as ribulose bisphosphate carboxylase levels increase. Accordingly, the variation in ¹³C values seen in maize leaves appears to be the result of variations in the level of phosphoenolpyruvate carboxylase.

¹ Supported by grant DE-FG02-86ER13534 from the U.S. Department of Energy.

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