Preharvest sprouting (PHS) and late maturity α-amylase (LMA) are the two major causes of unacceptably high levels of α-amylase in ripe wheat grain. High α-amylase activity in harvested grain results in substantially lower prices for wheat growers and at least in the case of PHS, is associated with adverse effects on the quality of a range of end-products and loss of viability during storage. The high levels of α-amylase are reflected in low falling number, the internationally accepted measure for grain receival and trade. Given the significant losses that can occur, elimination of these defects remains a major focus for wheat breeding programs in many parts of the world. In addition, the genetic, biochemical and molecular mechanisms involved in the control of PHS and LMA as well as the interactions with environmental factors have attracted a sustained research interest. PHS and LMA are independent, genetically controlled traits that are strongly influenced by the environment, where the effects of particular environmental factors vary substantially depending on the stage of grain development and ripening. This review is a summary and an assessment of results of recent research on these important grain quality defects.

Improved resistance to preharvest sprouting in modern bread wheat (Triticum aestivum. L.) can be achieved via the introgression of grain dormancy and would reduce both the incidence and severity of damage due to unfavourable weather at harvest. The dormancy phenotype is strongly influenced by environmental factors making selection difficult and time consuming and this trait an obvious candidate for marker assisted selection. A highly significant Quantitative Trait Locus (QTL) associated with grain dormancy and located on chromosome 4A was identified in three bread wheat genotypes, two white- and one red-grained, of diverse origin. Flanking SSR markers on either side of the putative dormancy gene were identified and validated in an additional population involving one of the dormant genotypes. Genotypes containing the 4A QTL varied in dormancy phenotype from dormant to intermediate dormant. Based on a comparison between dormant red- and white-grained genotypes, together with a white-grained mutant derived from the red-grained genotype, it is concluded that the 4A QTL is a critical component of dormancy; associated with at least an intermediate dormancy on its own and a dormant phenotype when combined with the R gene in the red-grained genotype and as yet unidentified gene(s) in the white-grained genotypes. These additional genes appeared to be different in AUS1408 and SW95-50213.

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Late maturity a-amylase (LMA) is a genetic defect that is commonly found in bread wheat (Triticum aestivum) cultivars and can result in commercially unacceptably high levels of a-amylase in harvest-ripe grain in the absence of rain or preharvest sprouting. This defect represents a serious problem for wheat farmers, and apart from the circumstantial evidence that gibberellins are somehow involved in the expression of LMA, the mechanisms or genes underlying LMA are unknown. In this work, we use a doubled haploid population segregating for constitutive LMA to physiologically analyze the appearance of LMA during grain development and to profile the transcriptomic and hormonal changes associated with this phenomenon. Our results show that LMA is a consequence of a very narrow and transitory peak of expression of genes encoding high-isoelectric point a-amylase during grain development and that the LMA phenotype seems to be a partial or incomplete gibberellin response emerging from a strongly altered hormonal environment.

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The level of grain dormancy and sensitivity to ABA of the embryo, a key factor in grain dormancy, were examined in developing grains of a white-grained wheat line, Novosibirskaya 67 (NS-67), and its red-grained near-isogenic lines (ANK-1A to -1D); a red-grained line, AUS 1490, and its white-grained mutant line (EMS-AUS). ANK lines showed higher levels of grain dormancy than NS-67 at harvest maturity. AUS 1490 grain also showed higher dormancy than EMS-AUS grain. These results suggest that the R gene for grain colour can enhance grain dormancy. However, the dormancy effect conferred by the R gene was not large, suggesting that it plays a minor role in the development of grain dormancy. Water extracts of AUS 1490 and EMS-AUS bran contained germination inhibitors equivalent to 1-10 microM ABA, although there was no difference in the amount of inhibitors between AUS 1490 and EMS-AUS. Thus, the grain colour gene of AUS 1490 did not appear to enhance the level of grain dormancy by accumulating germination inhibitors in its bran. Sensitivity to ABA of embryos was higher in grains collected around harvest-maturity for ANK lines and AUS 1490, compared with NS-67 and EMS-AUS. The R gene might enhance grain dormancy by increasing the sensitivity of embryos to ABA.

Flour and noodle colour influence the value of wheat (Triticum aestivum L.) and are obvious targets for breeders seeking to improve quality, end-product range, and marketability of wheat. The objective of this investigation was to identify quantitative trait loci (QTLs) associated with flour and noodle colour traits and with individual components of colour. One hundred and sixty-three doubled haploid lines derived from Sunco Tasman, white-grained, prime hard, and hard wheats adapted to the north-eastern region of Australia were used for the bulk of this study and were supplemented by doubled haploid populations derived from CD87 Katepwa and Cranbrook Halberd for comparisons of flour colour. Samples of Sunco Tasman, together with parental lines, were grown at Narrabri, NSW, in 1998 and 1999 and at Roma, Qld, in 1998 and used for visible light reflectance measurements of flour brightness (CIE L*) and yellowness (CIE b*), and white salted noodle (WSN) and yellow alkaline noodle (YAN) brightness, yellowness, and colour stability. Xanthophyll content and polyphenol oxidase (PPO) activity were measured spectrophotometrically. No consistent QTLs were identified for flour L* or initial L* of WSN and YAN. Xanthophyll content was very strongly associated with QTLs located on chromosomes 3B and 7A and these QTLs also had a major influence on flour b*, WSN b*, and YAN b*. Noodle brightness at 2, 24, and 48 h and the magnitude of change in noodle L* and b* with time were affected by QTLs on 2D, contributed by Tasman, and, to a lesser degree, 2A. The QTL on 2D was clearly associated with control of grain PPO, an enzyme implicated in darkening of Asian style noodles. QTLs located on 2B, 4B, and 5B and associated with control of grain size or flour protein content also appeared to influence a number of colour traits.

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