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Water Use Efficiency and Dry Matter Distribution in Nitrogen- and Water-Stressed Winter Wheat

Cotton Physiology and Genetics Res., USDA-ARS, Delta States Res. Ctr., Stoneville, MS 38776

^* Corresponding author.

Water use efficiency (WUE) and the distribution of dry matter to the roots in wheat (Triticum aestivum L.) are potential selection criteria for improving yield under water stress. The objectives of this research were (i) to compare methods of measuring WUE, and (3) to determine the distribution of dry matter between shoots and roots in water- and N-stressed wheat. Winter wheat was grown in a potting mixture for 6 wk in growth chambers with varying levels of N and water. Nitrogen applied to the mixture varied from 10 to 100 mg N per plant. Water supply was maintained at 15 to 35% of the mixture water holding capacity (stressed) to above 50% of capacity (well watered). Cumulative transpiration, steady-state CO₂ and H₂O exchange rates, and dry matter were determined. The ratio of dry matter production (mmol C) to transpiration (mol H₂O) averaged 1.39 and 1.94 mmol C mol^–1 H₂O for the 10 and 100 mg N treatments, respectively. The ratio of the steady state CO₂ exchange rate to the H₂O exchange rate was 1.97 and 4.24 mmol C mol^–1 H₂0 for the 10 and 100 mg N treatments, respectively. The gas exchange WUE values tended to be higher than the gravimetric WUE values, but the two were correlated (r = 0.76**). Results supported the hypothesis that gas exchange WUE values could provide an accurate prediction of WUE values obtained from gravimetric analysis. Shookroot ratio was consistently reduced by N stress, but not by water stress. The results supported previous observations that optimal leaf N concentration promoted higher WUE. Mild water stress did not consistently affect WUE, but a more severe water stress consistently decreased WUE, especially under suboptimal N supply.

Received for publication April 14, 1988.

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