Relationship of carbon stable isotopes with water use efficiency in sugar beet under well-watered and drought conditions

Water use efficiency (WUE, the ratio of total dry matter to water use) became a breeding target only when it was found that carbon stable isotopes were inversely related to WUE. The current study conducted under well-watered and drought conditions in the field over two years at Broom’s Barn Research Station, UK, was undertaken to investigate the extent of genotypic differences for water use, yield, WUE, carbon isotope discrimination (Delta), and the associated traits. The results indicated that the varieties differed in WUE and sensitivity to water deficit stress at 5% probability level. However, this variation was not related to gas exchange characteristics. Variation in Delta as measured on leaf tissues was negatively correlated with WUE indicating that genetic variation in WUE could be indirectly exploited by measuring carbon isotope composition in plant organs, especially in leaf tissues. As variation in Delta and hence WUE may arise from variation in stomatal conductance, photosynthetic capacity or both, the traits theoretically associated with stomatal conductance (leaf ash content and canopy temperature) and photosynthetic capacity (chlorophyll content and specific leaf weight) were measured among which only specific leaf weight (leaf dry weight per unit area) was significantly associated with Delta (r= 0.77, P<0.01). This indicates that specific leaf weight could be used when there are not enough resources to use Delta directly and as an initial screening tool to cull the inferior genotypes in the early phases of breeding programmes aimed at improving WUE.