Relationships between carbon isotope discrimination and grain yield in winter wheat under well-watered and drought conditions

The association of carbon isotope discrimination of grain (Delta C-13) with yield performance under rain-fed and well-watered conditions was analysed using a doubled-haploid (DH) winter wheat population, derived from the cross between cvars Beaver x Soissons, within field experiments at two site-seasons. The aim of this work was to quantify associations between Delta C-13 and yield responses to drought and to test effects of major genes (the semi-dwarf genes, Rht-B1b, Rht-D1b, an awn suppressor gene, B1 and the 1BL. 1RS wheat-rye chromosome translocation) segregating in the population for associations with Delta C-13 and drought performance. Carbon isotope discrimination, through its negative relationship with transpiration efficiency, may be used as a surrogate for this trait. Grain Delta C-13 was positively associated with grain yield under both irrigated and unirrigated conditions in each site-season and, overall, explained 0.34 of the phenotypic variation in grain yield amongst DH lines under drought and 0.14 under well-watered conditions. There was a positive association between specific leaf lamina N content (SLN) at anthesis and Delta C-13 under drought amongst DH lines in one site-season, suggesting higher SLN may confer increased stomatal conductance via higher photosynthetic capacity, hence increased grain Delta C-13. Overall the Rht-D1b (semi-dwarf) lines had slightly higher Delta C-13 of grain (20.0 parts per thousand) than the Rht-B1a/Rht-D1a (tall) group of lines (19.8 parts per thousand). There were no significant differences between the Rht-B1b (semi-dwarf) or the Rht-B1b/Rht-D1b (dwarf) lines and the tall lines. Comparing their performance under irrigated and unirrigated conditions, the Rht groups of lines (Rht-B1b semi-dwarf, Rht-D1b semidwarf and dwarf and tall groups) responded no differently to drought for Delta C-13. The Rht-D1b semi-dwarf lines had higher grain yield (9.50 t/ha) than the tall lines (8.76 t/ha), while the yield of the Rht-B1b semi-dwarf and dwarf lines did not differ significantly from the tall lines. In each site-season, the presence of the 1BL. 1RS chromosome increased grain Delta C-13 (P < 0.001), with an overall increase from 19.7 parts per thousand in the 1B lines to 20.0 parts per thousand in the 1BL. 1RS lines (P < 0.001). However, the 1BL. 1RS and 1B lines responded similarly to drought. The effect of the presence/absence of awns on grain Delta C-13 was not statistically significant in either site-season. Overall, the present results show that Rht-D1b confers higher Delta C-13 and grain yield, and the 1BL. 1RS translocation confers higher Delta C-13. This implies that modern UK wheat cultivars may have lower water-use efficiency during the grain filling period than their predecessors, and therefore may require more water to fulfil their yield potential.