CARBON ISOTOPE DISCRIMINATION - RELATIONSHIP TO YIELD, GAS-EXCHANGE, AND WATER-USE EFFICIENCY IN FIELD-GROWN CRESTED WHEATGRASS

In previous greenhouse experiments, stable C isotope discrimination (DELTA) 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 DELTA, leaf WUE, and shoot WUE (dry weight change/plant transpiration) in clones of crested wheatgrass selected on the basis of low, medium, or high-DELTA. The first experiment involved leaves of five clones from each DELTA-class, with WUE(i) and DELTA determined at two field sites. Mean DELTA-values are always less for low-DELTA than high-DELTA clones, but did not differ significantly between medium-DELTA and high-DELTA clones. Negative correlations were obtained between DELTA 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-DELTA clones exceeded that of high-DELTA clones only under rainfed conditions. The second experiment compared the performance of three low-DELTA and three high-DELTA clones under well-watered and droughted 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 DELTA and leaf intercellular CO2 concentration (c(i)) were less for low-DELTA than high-DELTA clones and were negatively associated with both WUE(i) and WUE(s). Even though shoot dry weight and DELTA decreased with drought, the two traits were not correlated for either well-watered or droughted clones. Across water levels, DELTA was negatively correlated with WUE(i) (r = -0.93**, df = 10) and with WUE(s) (r = -0.98**, df = 10). Results show DELTA in field-grown crested wheatgrass was consistent across sites and was consistently related to c(i), WUE(i), and WUE(s).