Despite considerable gains in wheat flour milling yield through conventional breeding strategies and milling technologies, the theoretical maximum yield still has not been attained. Discovery of genes in wheat that control flour yield would provide ameans for breeders to develop new wheats that fulfill their potential in relation to this trait. A more targeted approach is to improve the understanding of the role of grain microstructure in determining high flour yield. Previous research has shown that increased flour yield in hard wheat is associated with increased endosperm rheology index, calculated from strength and stiffness as measured by the SKCS (Osborne et al, 2005; Osborne et al, 2007). Edwards et al (2007) demonstrated, through microscopyand particle size analysis of the crushed material from the SKCS 4100 and a first break roll stand, that the SKCS data for unconditioned wheat averaged over 300 grains provided a useful indicator of milling performance of a wheat sample. The Pina-Dl andPinb-Dl alleles, tightly linked to the Ha locus on the short arm of Chromosome 5D, determine the hardness phenotype (Greenwell and Schofield, 1986). However, this does not fully account for the observed genetic variation in hardness, especially within each hardness class, and it is thought that additional modifying genes account for the range of hardness within hard or soft classes (Martin et al, 2001). Others have investigated the relationship between endosperm starch granule size and hardness. Igrejaset al (2002) reported that harder wheat had a higher content of small starch granules but could not find a QTL for starch granule size on the 5D Chromosome; they concluded that "starch size distribution is influenced by genes which have yet to be analysed". The aim of the present study was to test the hypothesis that a genetic association exists in hard wheat between starch granule type and flour yield.
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