Response of Ulmus americana seedlings to varying nitrogen and water status. 2 Water and nitrogen use efficiency in photosynthesis.

Photosynthetic utilization of water and nitrogen in Ulmus americana L. seedlings was tightly linked with the relative availability of each resource. During periodic drying cycles, water use efficiency increased as predawn water potential fell from -0.5 to -2.0 MPa. During the later stages of such drying cycles, the relative contribution of stomatal limitations to the total net photosynthetic limitation appeared to be at its greatest, whereas biochemical limitations were predominant in well-watered plants grown under low nitrogen (N) availability. For any level of leaf water status, water use efficiency of photosynthesis (WUE) was always greater in plants with high leaf N content than in plants with low leaf N content. Photosynthetic nitrogen use efficiency (PNUE) was always greater in plants with low leaf N content than in plants with high leaf N content, for any level of water status. In combined N treatments and predawn water status classes, there was a significant inverse relationship between PNUE and WUE.