Carbon Isotope Discrimination, Gas Exchange, and Water‐Use Efficiency in Crested Wheatgrass Clones

Abstract

Carbon isotope discrimination (Δ) has been proposed as a criterion in selecting for water‐use efficiency (WUE) in C3 crop species. This study was conducted to determine associations among Δ, leaf instantaneous WUE (WUEi), and shoot WUE (WUE3) 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 CO2 concentration of individual clones were correlated positively at each water level, the expected differences between Δ classes in Δ, WUEi, and WUE3 were obtained only under drought. A lower Δ in low than high Δ class under drought was associated with greater shoot dry weight, WUE3, and WUEi. Across water levels, large negative correlations were found between Δ and WUEi (r = ‐0.95**, df = 16, P < 0.01) or WUE3 (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.