ABSTRACr keep the partial derivative:The hypothesis of optimal stomatal conductance predicts conductance shoud vary with changes of the vapor pressure difference between leaf and air (VPD) constant. The magnitude of the constant, A, may be largely determined by plant water relations (7,8,10,16), although other factors also may be involved (8, 10).Direct determination of dE/dA is usually done by altering the VPD to change g and measuring the resulting E and A (11). The hypothesis of optimal stomatal conductance has experimental verification where OE/OA was calculated and found to be relatively constant with respect to VPD (1,2, 11,13,16,(20)(21)(22) 29). As far as we know, the constancy of OE/OA has not been determined for a C4 plant or for changes in N, although some studies suggest this would be the case (10,26 Even though nitrogen nutrition affects leafwater relations (14, 25), Farquhar (10) hypothesized that X should remain the same for various nitrogen treatments. In a companion study, it was found that leaf water potential was equal between nitrate application treatments; however, there were differences in leafosmotic and pressure potentials ( 17). The objectives of this study are to test ifOE/OA is constant forAAmaranthuspowellii and if A is equal between treatments of different applied nitrate concentrations. MATERIALS AND METHODSPlants of Amaranthus powellii Wats. were germinated and grown in a controlled environment chamber and watered daily with nutrient solutions containing either 45, 10, 5, or 1 mm nitrate (17). Leaves just before or at full expansion on plants 28 to 30 d from sowing were used for all measurements. Gas exchange was measured in an open flow differential system (17). The VPD was altered by starting at high leaf cuvette humidities and decreasing the humidity of the air entering the cuvette by steps of about 5°C dewpoint, which did not result in a corresponding drop in the VPD because transpiration added to the cuvette humidity. The Ca was maintained at about 340 ,bar/bar by adjusting the flow rate of air through the leaf cuvette. The TL was maintained at 35°C and photosynthetic photon flux density was saturating ( 17).The gb was estimated by measuring the evaporation of water from moist filter paper.

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