The hypothesis of optimal stomatal conductance predicts conductance should vary with changes of the vapor pressure difference between leaf and air (VPD) to keep the partial derivative of transpiration rate (E) with respect to assimilation rate (A) constant. Stomatal conductance of Amaranthus powellii Wats. decreased strongly with increasing VPD for leaves with high total leaf nitrogen concentrations: whereas, it decreased slightly with increasing VPD for leaves with low leaf nitrogen concentrations. The calculated value was constant for leaves with high leaf nitrogen concentrations but was not constant for leaves with low leaf nitrogen concentrations. However, the predicted values of stomatal conductance, transpiration rate, and assimilation rate for a constant = 220 moles H2O/mole CO2 and the measured values fit the y=x line for all nitrate treatments. These data extend the experimental support for the optimal water use efficiency hypothesis for a C4 plant grown under different nitrate concentrations.