Selection for reduced carbon isotope discrimination increases aerial biomass and grain yield of rainfed bread wheat

Genetic gain is characteristically slow when selecting directly for increased grain yield under water-limited conditions. Genetic increases in grain yield may be achieved through increases in aerial biomass following selection for greater transpiration efficiency (TE as aerial biomass/water transpired). Strong negative correlations between TE and carbon isotope discrimination (Δ) in wheat (Triticum aestivum L.) suggest that selection of progeny with low Δ may increase TE and aerial biomass under water-limited conditions. This study investigated how early generation, divergent selection for Δ affected aerial biomass and grain yield among 30 low- and 30 high-Δ, 'Hartog'-like, BC₂F_4:6 progeny and the recurrent, high-Δ parent Hartog. Lines were evaluated in nine environments varying for seasonal rainfall (235-437 mm) and hence grain yield (1.3-6.2 Mg/ha). Selection for low Δ in early generation progeny was associated with significantly (P < 0.01) smaller Δ, higher grain yield (+5.8%), aerial biomass (+2.7%), harvest index (+3.3%), and kernel size (+4.8%) in tested lines. Kernel number was the same for low- and high-Δ selected groups. Grain yield advantage of the low Δ group increased with reductions in environment mean yield (r = -0.89, P < 0.01) and total seasonal rainfall (r = -0.85, P < 0.01) indicating the benefit of low Δ, and therefore high TE for genetic improvement of grain yield in lower rainfall environments. Narrow-sense heritability on a single-plot basis was much greater for Δ (h² = 0.63 ± 0.10) than for either aerial biomass (0.06 ± 0.05) or grain yield (0.14 ± 0.04). Strong genetic correlations between Δ and both aerial biomass (r_g = -0.61 ± 0.14) and grain yield (-0.58 ± 0.12) suggest Δ could be used for indirect selection of these traits in early generations. Selection of low Δ (high TE) families for the advanced stages of multiple-environment testing should increase the probability of recovering higher-yielding wheat families for water-limited environments.