Inheritance of carbon isotope discrimination in bread wheat (Triticum aestivum L.)

Reliable selection of families with increased grain yield is difficult in breeding programs targeting water-limited environments. Carbon isotope discrimination (Delta) is negatively correlated with transpiration efficiency, and low Delta is being used for indirect selection of high wheat yield in rainfed environments. Yet little is known of genetic control and opportunities for improving selection efficiency of Delta in wheat. Half-diallel and generation means mating designs were undertaken to provide estimates of the size and nature of gene action for Delta in a range of wheat genotypes varying for this trait. Significant (P < 0.01) differences were observed for leaf tissue Delta among parents (19.3 to 20.7 parts per thousand) and F-1 progeny (19.4 to 20.9 parts per thousand) in the half-diallel. General (GCA) and specific combining ability (SCA) effects were significant (P < 0.05), while Baker's GCA/SCA variance ratio of 0.89 was close to unity, indicating largely additive gene effects. GCA effects varied from -0.38 to +0.34 parts per thousand for low and high Delta genotypes `Quarrion' and `Gutha', respectively. GCA effects and parental means were strongly correlated (r = 0.95, P < 0.01) while directional dominance and epistasis contributed to small, non-additive gene action for Delta. Smaller Delta in F-1 progeny was associated with accumulation of recessive alleles from the low Delta parent. Narrow-sense heritability was high (0.86) on a single-plant basis. Generation means analysis was undertaken on crosses between low Delta genotype Quarrion and two higher Delta genotypes `Genaro M81' and `Hartog'. The F-1, F-2 and midparent means were not statistically (P > 0.05) different, whereas backcrossing significantly changed Delta toward the mean of the recurrent parent. Gene action was largely additive with evidence for additive x additive epistasis in one cross. Narrow-sense heritabilities were moderate in size (0.29 to 0.43) on a single-plant basis. Genetic gain for Delta in wheat should be readily achieved in selection among inbred or partially inbred families during the later stages of population development.