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 (Delta) in wheat (Triticum aestivum L.) suggest that selection of progeny with low Delta may increase TE and aerial biomass under water-limited conditions. This study investigated how early generation, divergent selection for Delta affected aerial biomass and grain yield among 30 low- and 30 high-Delta, 'Hartog'-like, BC2F46 progeny and the recurrent, high-Delta parent Hartog. Lines were evaluated in nine environments varying for seasonal rainfall (235437 mm) and hence grain yield (1.3-6.2 Mg/ha). Selection for low A in early generation progeny was associated with significantly (P < 0.01) smaller Delta, higher grain yield (+5.8%), aerial biomass (+2.7%), harvest index (+3.3%), and kernel size (+4.8%) in tested tines. Kernel number was the same for low- and high-Delta selected groups. Grain yield advantage of the low Delta 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 Delta, 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 Delta (h(2) = 0.63 +/- 0.10) than for either aerial biomass (0.06 +/- 0.05) or grain yield (0.14 +/- 0.04). Strong genetic correlations between Delta and both aerial biomass (r(g) = -0.61 +/- 0.14) and grain yield (-0.58 +/- 0.12) suggest A could be used for indirect selection of these traits in early generations. Selection of low Delta (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.