Abstract

The Norin-10 dwarfing genes, Rht-B1b (Rht1) and Rht-D1b (Rht2), are commonly used to reduce plant height and increase grain yield in wheat breeding programs. These dwarfing genes lower sensitivity of vegetative tissue to endogenous gibberellin to reduce cell and subsequent stem elongation. This reduction in cell elongation capacity reportedly results in a concomitant reduction in coleoptile length and early vigour (leaf area) thereby affecting seedling establishment and growth. A detailed genetic map from a cross between tall Halberd (Rht-B1a) and semidwarf Cranbrook (Rht-B1b) wheat cultivars was used to assess genetic factors affecting seedling growth. Parental and 150 doubled haploid progeny lines were characterised for seedling and height-related traits in controlled and field environments. Genotypic variation was large and predominantly under additive genetic control with evidence for transgressive segregation for some traits. Narrow-sense heritabilities were moderate to high (h² = 0.31–0.91) indicating a strong genetic basis for differences between progeny. Molecular marker analyses identified a number of significant (P < 0.05) quantitative trait loci (QTL) for each trait. A major QTL, mapping directly to the Rht-B1 locus on chromosome arm 4BS, accounted for up to 49% of the genotypic variance in peduncle length and plant height, and 27–45% of the genotypic variance in coleoptile length across different temperatures. Another QTL, located close to the RFLP marker XksuC2 on the long arm of chromosome 4B, accounted for 15–27% of the genotypic variance in coleoptile length. The influence of the XksuC2-linked QTL on coleoptile length was greatest at 19˚C and decreased with cooler temperatures. The same QTL affected reductions in leaf size, and both coleoptile tiller size and presence to affect overall seedling vigour. There was also some evidence for epistatic interactions influencing coleoptile tiller growth. Reductions in plant size at the Rht-B1b and XksuC2 loci were associated with presence of the Cranbrook 4B allele. The negative genetic effect of the Rht-B1b dwarfing gene on early growth of wheat confirms phenotypic evidence of a pleiotropic effect of Rht-B1b on establishment and early vigour. Genetic increases in coleoptile length and early leaf area development are likely to be limited in wheat populations containing the Rht-B1b dwarfing gene.