deltaC Values in Maize Leaf Correlate with Phosphoenolpyruvate Carboxylase Levels

Values of δ¹³C 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 δ¹³C 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 δ¹³C should arise through its effect on the carboxylation/diffusion balance in the mesophyll. In this case, δ¹³C 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 δ¹³C should occur through its effect on the extent of leakage of CO₂ from the bundle sheath cells. In this case, δ¹³C values should become more positive as ribulose bisphosphate carboxylase levels increase. Accordingly, the variation in δ¹³C values seen in maize leaves appears to be the result of variations in the level of phosphoenolpyruvate carboxylase.