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Interaction of Water Supply and N in Wheat ¹

Agricultural Engineering Research Center, Colorado State University, Foothills Campus, Fort Collins, Colorado 80523

The purpose of this study was to investigate effects of N nutrition and water stress on stomatal behavior and CO₂ exchange rate in wheat (Triticum aestivum L. cv Olaf). Wheat plants were grown hydroponically with high (100 milligrams per liter) and low (10 milligrams per liter) N. When plants were 38 days old, a 24-day water stress cycle was begun. A gradual increase in nutrient solution osmotic pressure from 0.03 to 1.95 mega Pascals was achieved by incremental additions of PEG-6,000. Plants in both N treatments adjusted osmotically, although leaf water potential was consistently lower and relative water content greater for low N plants in the first half of the stress cycle. Leaf conductance of high N plants appeared greater than that of low N plants at high water potentials, but showed greater sensitivity to reductions in water potential as indicated by earlier stomatal closure during the stress cycle. The apparent greater stomatal sensitivity of high N plants was associated with a curvilinear relationship between leaf conductance and leaf water potential; low N plants exhibited more of a threshold response. Trends in [CO₂]_INT throughout the stress cycle indicated nonstomatal effects of water stress on CO₂ exchange rate were greater in high N plants. Although estimates of [CO₂]_INT were generally lower in high N plants, they were relatively insensitive to leaf water potential-induced changes in leaf conductance. In contrast, [CO₂]_INT of low N plants dropped concomitantly with leaf conductance at low leaf water potentials. Oxygen response of CO₂ exchange rate for both treatments was affected less by reductions in water potential than was CO₂ exchange rate at 2.5% O₂, suggesting that CO₂ assimilation capacity of the leaves was affected more by reductions in leaf water potential than were processes related to photorespiration.