Recent advances in crop research have the potential to accelerate genetic gains in wheat, especially if co-ordinated with a breeding perspective. For example, improving photosynthesis by exploiting natural variation in Rubisco's catalytic rate or adopting C₄ metabolism could raise the baseline for yield potential by 50% or more. However, spike fertility must also be improved to permit full utilization of photosynthetic capacity throughout the crop life cycle and this has several components. While larger radiation use efficiency will increase the total assimilates available for spike growth, thereby increasing the potential for grain number, an optimized phenological pattern will permit the maximum partitioning of the available assimilates to the spikes. Evidence for underutilized photosynthetic capacity during grain filling in elite material suggests unnecessary floret abortion. Therefore, a better understanding of its physiological and genetic basis, including possible signalling in response to photoperiod or growth-limiting resources, may permit floret abortion to be minimized for a more optimal source:sink balance. However, trade-offs in terms of the partitioning of assimilates to competing sinks during spike growth, to improve root anchorage and stem strength, may be necessary to prevent yield losses as a result of lodging. Breeding technologies that can be used to complement conventional approaches include wide crossing with members of the Triticeae tribe to broaden the wheat genepool, and physiological and molecular breeding strategically to combine complementary traits and to identify elite progeny more efficiently.

Grain morphology in wheat (Triticum aestivum) has been selected and manipulated even in very early agrarian societies and remains a major breeding target. We undertook a large-scale quantitative analysis to determine the genetic basis of the phenotypic diversity in wheat grain morphology. A high-throughput method was used to capture grain size and shape variation in multiple mapping populations, elite varieties, and a broad collection of ancestral wheat species. This analysis reveals that grain size and shape are largely independent traits in both primitive wheat and in modern varieties. This phenotypic structure was retained across the mapping populations studied, suggesting that these traits are under the control of a limited number of discrete genetic components. We identified the underlying genes as quantitative trait loci that are distinct for grain size and shape and are largely shared between the different mapping populations. Moreover, our results show a significant reduction of phenotypic variation in grain shape in the modern germplasm pool compared with the ancestral wheat species, probably as a result of a relatively recent bottleneck. Therefore, this study provides the genetic underpinnings of an emerging phenotypic model where wheat domestication has transformed a long thin primitive grain to a wider and shorter modern grain.

A genetic map of 92 RFLP loci and two storage protein loci was made using 94 doubled-haploid lines from a cross between the winter barley variety Igri and the spring variety Triumph. The markers were combined with data from two field experiments (one spring sown and one autumn (fall) sown) and a glasshouse experiment to locate a total of 13 genes (five major genes and eight quantitative trait loci (QTL)) controlling flowering time. Two photoperiod response genes were found; Ppd-H1 on chromosome 2(2H)S regulated flowering time under long days, while Ppd-H2 on chromosome 5(1H)L was detected only under short days. In the field experiments Ppd-H1 strongly affected flowering time from spring and autumn sowings, while Ppd-H2 was detected only in the autumn sowing. The glasshouse experiment also located two vernalization response genes, probably Sh and Sh₂, on chromosomes 4(4H)L and 7(5H)L, respectively. The vernalization response genes had little effect on flowering time in the field. Variation in flowering time was also affected by nine additional genes, whose effects were not specifically dependent on photoperiod or vernalization. One was the denso dwarfing gene on chromosome 3(3H)L. The remaining eight were QTLs of smaller effect. One was located on chromosome 2(2H), one on 3(3H), one on 4(4H), one on 7(5H), two on 6(6H), and two on 1(7H). Model fitting showed that the 13 putative genes, and their interactions, could account for all the observed genetical variation from both spring and autumn sowings, giving a complete model for the control of flowering time in this cross.Key words: barley, Hordeum vulgare, flowering time, photoperiod, vernalization, mapping.