Carbon Isotope Dicrimination: Potential in Screening Cool‐Season Grasses for Water‐Limited Environments

Carbon isotope discrimination (∆) has been shown to be negatively related to water use efficiency (WUE) in several C 3 crop species and has been proposed as a criterion to select for improved WUE in plant breeding programs. This study was conducted to determine if ∆ is related to WUE in crested wheatgrass [ Agropyron desertorum (Fischer ex Link) Schultes] and AItai wildrye [ Leymus angustus (Trin.) Pilger] and to evaluate the influence of drought on the magnitude of genetic variability and broad‐sense heritability of ∆ among clones of crested wheatgrass. In a greenhouse pot experiment, ∆ was negatively related to WUE in crested wheatgrass ( r = −0.73**, significant at P < 0.01) and Altai wildrye ( r −0.81**). In field studies, a line‐source sprinkler system operated in conjunction with an automated rainout shelter imposed a continuously variable water application on 29 vegetatively propagated clones of crested wheatgrass. Three water levels along the water application gradient were selected for sampling and forage dry weights and ∆ values were determined for two harvests in 1986 and one harvest in 1987. In the field experiment, ∆ declined as less water was applied and was positively associated with forage yield at each of the three harvests ( r = 0.59**, 0.44**, and 0.80**, respectively). Significant variability for ∆ was detected among the clones for the two peak‐season harvests, hut not for the late‐season harvest in 1986. Broad‐sense heritability for ∆ was 0.49 in the analysis of data combined over harvests and water levels. Genetic variances and broad‐sense heritabilities ( H ) for ∆ were generally greater at the high ( H = 0.52) and intermediate ( H = 0.52) water applications than at the low water application ( H = 0.37), and coefficients of variation for ∆ were typically less than 4.5%. Selection for ∆ appears to be a promising screening tool for the development of crested wheatgrass populations that are productive and make efficient use of limited water resources.