Carbon isotope discrimination (Δ) has been proposed as a criterion in selecting for water‐use efficiency (WUE) in C 3 crop species. This study was conducted to determine associations among Δ, leaf instantaneous WUE (WUE i ), and shoot WUE (WUE 3 ) in clones of crested wheatgrass [ Agropyron desertorum (Fischer ex Link) Schultes] previously selected for low, medium, and high Δ. Nine clones (three in each Δ class) were grown in a greenhouse in pots weighed every third day and brought to either 0.03 or 0.12 kg kg −1 gravimetric water content. Leaf gas exchange rates and water potentials were determined at 2‐wk intervals until plant harvest at 12 wk. Under drought, the ranking of Δ classes for stomatal conductance agreed with previous and present Δ classifications, and midday leaf water potentials averaged ≈0.4 MPa lower in the low than in the medium or high Δ class. Shoot dry weight and Δ decreased with drought and were correlated positively under well‐watered conditions ( r = 0.77*, df = 7, P < 0.05), but not under drought. Although Δ and leaf intercellular CO 2 concentration of individual clones were correlated positively at each water level, the expected differences between Δ classes in Δ, WUE i , and WUE 3 were obtained only under drought. A lower Δ in low than high Δ class under drought was associated with greater shoot dry weight, WUE 3 , and WUE i . Across water levels, large negative correlations were found between Δ and WUE i ( r = ‐0.95**, df = 16, P < 0.01) or WUE 3 (r = −0.89**, df = 16). These results indicate that selection for low carbon isotope discrimination will improve water‐use efficiency in crested wheat‐grass under drought.
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Read et al. (1991) studied this question.