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Carbon Isotope Discrimination: Relationship to Yield, Gas Exchange, and Water-Use Efficiency in Field-Grown Crested Wheatgrass

USDA-ARS, Crops Res. Lab., 1701 Center Ave., Fort Collins, CO 80526

Douglas A. Johnson and Kay H. Asay

USDA-ARS, Forage and Range Res. Lab., Utah State Univ., Logan, UT 84322-6300

Larry L. Tieszen

Biology Dep., Augustana College, Sioux Falls, SD 57197

^* Corresponding author.

In previous greenhouse experiments, stable C isotope discrimination () was negatively associated with water-use efficiency (WUE) in crested wheatgrass [Agropyron desertorum (Fischer ex Link) Schultes]. We conducted two field experiments to evaluate associations among , leaf WUE, and shoot WUE (dry weight change/plant transpiration) in clones of crested wheatgrass selected on the basis of low, medium, or high . The first experiment involved leaves of five clones from each class, with WUE_i and determined at two field sites. Mean values are always less for low- than high- clones, but did not differ significantly between medium- and high- clones. Negative correlations were obtained between and WUE_i at both the rainfed (r = –0.77**, df = 13, p < 0.01) and irrigated (r = –0.87**, df = 13) sites; however, WUE_i of low- clones exceeded that of high- clones only under rainfed conditions. The second experiment compared the performance of three low- and three high- clones under well-watered and draughted conditions. Plants were grown in pots imbedded in soil. Soil water content in the pots was maintained at either 0.16 or 0.06 kg H₂O kg^–1 soil. Within each water level, values for and leaf intercellular CO₂ concentration (c_i) were less for low- than high- clones and were negatively associated with both WUE_i and WUE_s. Even though shoot dry weight and decreased with drought, the two traits were not correlated for either well-watered or droughted clones. Across water levels, was negatively correlated with WUE_i (r = –0.93**, df = 10) and with WUE_s (r = –0.98**, df = 10). Results show in field-grown crested wheatgrass was consistent across sites and was consistently related to c_i, WUE_i, and WUE_s.

Received for publication March 8, 1991.

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