Identification of quantitative trait loci for traits conferring weed competitiveness in wheat (Triticum aestivum L.)

As weeds develop resistance to a broad range of herbicides, wheat (Triticum aestivum L.) cultivars with superior weed competitive capacity are needed to complement integrated weed management strategies. In this study, agronomic and morphological traits that enable wheat to compete effectively with weeds were identified. Halberd, Cranbrook, and 161 Cranbrook x Halberd doubled haploid (DH) lines were examined in field experiments conducted over two growing seasons. The weed species Lolium rigidum L. (annual ryegrass) was sown in strips perpendicular to the direction of wheat seeding. Various traits were measured during each season with competitive ability determined by both percent loss in wheat grain yield and suppression of ryegrass growth. Width of leaf 2, canopy height, and light interception at early stem elongation (Z31), and tiller number, height at maturity, and days to anthesis were important for competitive ability in 1999. In the previous year, length of leaf 2 and size of the flag leaf contributed to competitiveness. Seasonal effects appeared to have some impact on the relative contribution of crop traits to competitive ability. The morphological traits involved in maintaining grain yield differed from those that contributed to the suppression of ryegrass growth. Development of the Cranbrook x Halberd chromosomal linkage map enabled the putative identification of quantitative trait loci (QTL) associated with competitive ability in the DH population. Many of the QTL were mapped to similar positions in both years. Further, several traits, including time to anthesis, flag leaf size, height at stem elongation, and the size of the first 2 leaves, were mapped to similar positions on chromosomes 2B and 2D. Narrow-sense heritabilities on an entry-mean basis were typically high within each year for traits associated with weed competitive ability. However, large genotype x year interactions reduced these heritabilities, making genetic gain through phenotypic selection difficult. The identification of QTL repeatable over seasons indicates the potential for marker-assisted selection in a wheat breeding program selecting for improved grain yield and weed competitiveness.