Relationships of grain δ13C and δ18O with wheat phenology and yield under water-limited conditions

Stable carbon isotope composition (delta C-13) of dry matter has been widely investigated as a selection tool in cereal breeding programmes. However, reports on the possibilities of using stable oxygen isotope composition (delta O-18) as a yield predictor are very scarce and only in the absence of water stress. Indeed, it remains to be tested whether changes in phenology and stomatal conductance in response to water stress overrule the use of either delta C-13 or delta O-18 when water is limited. To answer this question, a set of 24 genotypes of bread wheat ( Triticum aestivum) were assayed in two trials with different levels of deficit irrigation and a third trial under rainfed conditions in a Mediterranean climate ( northwest Syria). Grain yield (GY) and phenology (duration from planting to anthesis and from anthesis to maturity) were recorded, and the delta C-13 and delta O-18 of grains were analysed to assess their suitability as GY predictors. Both delta C-13 and delta O-18 showed higher broad-sense heritabilities (H-2) than GY. Genotype means of GY across trials were negatively correlated with delta C-13, as previously reported, but not with delta O-18. Both isotopes were correlated with grain filling duration, whereas delta O-18 was also strongly affected by crop duration from planting to anthesis. We concluded that delta O-18 of grains is not a proper physiological trait to breed for suboptimal water conditions, as its variability is almost entirely determined by crop phenology. In contrast, delta C-13 of grains, despite being also affected by phenology, still provides complementary information associated with GY.