Carbon isotope discrimination and water‐use efficiency of six crops grown under wet and dryland conditions

Mustard ( Sinapis alba L.), Argentine canola ( Brassica napus L. cv. Westar), Polish canola ( Brassica campestris L. cv. Tobin), pea ( Pisum sativum L.), durum wheat ( Triticum durum L. cv. Kyle) and soft wheat ( Triticum aestivum L. cv. Fielder) were grown at Outlook, Saskatchewan, Canada, under irrigated and dryland conditions. Carbon isotope discrimination (Δ) and water‐use efficiency ( W ), defined as grams of above ground dry matter produced per kilogram water used, were negatively correlated in the six field‐grown crops. In irrigated plants Δ remained relatively constant (20–21‰) throughout the growing season. However, in dryland plants, Δ declined in response to the progressive depletion of stored soil water (Polish canola, 20‐2‐18‐8‰; mustard, 19.9–18 5‰; pea, 19.9–17 2‰ durum wheat, 19.7–16.4‰; Argentine canola, 19.4–17.6‰; soft wheat, 19.0–17.4‰). Although there were genetic differences in Δ among the species, water availability was the major factor controlling Δ.