**7. The influence of height‐reducing genes on the flowering time of bread wheat**

Among the most important growth habit parameters influencing adaptation and yield poten‐ tial of bread wheat to various environments is plant height. The most common genes for reduced height (*Rht*, also called semidwarf) in wheat have been mapped on *Rht‐B1* and *Rht‐ D1* loci on chromosomes 4B and 4D, respectively [71]. Another potentially valuable height reducing gene, designated *Rht*8, has been mapped on chromosome 2D of bread wheat [72, 73]. The alleles of the two genes, *Rht‐B1b* and *Rht‐D1b*, inhibit cell elongation due to insensitivity to the growth hormone gibberellic acid in contrast to the *Rht*8 gene. The primary mechanism of height reduction caused by these alleles (*Rht‐B1b* and *Rht‐D1b*) is a reduction in the rate of stem development and dry matter accumulation in vegetative tissue, leading to increased partitioning of water and nutrients to the spike [74]. Consequently, more fertile florets and more seeds per spike are produced.

tion requirements have been satisfied [1, 11]. However, genes of all three classes (*Vrn*, *Ppd*, and *eps*) exert pleiotropic effects on other aspects of plant growth and development [1]. Whereas the major *Vrn* and *Ppd* genes govern the gross adaptation to environments, the *eps* genes have been shown to largely fine‐tune the flowering time of wheat varieties for their regional adaptations [1, 61–63]. Sufficient information is now available on the effect of *eps* genes in determining flowering time of wheat. Genetic analyses show that these loci have been mapped only as QTL effects rather than major genes because of their relatively small effect [1, 6]. This makes it difficult to undertake a comparative analysis of *eps* effects with confidence. Nevertheless, comparative genetic studies indicate that most wheat chro‐ mosomes harbor *eps* genes [1, 11, 64]. Worland [11] reported the likelihood of the existence of these genes on chromosome groups 2, 3, 4, 6, and 7. It was suggested in the same study that these genes fine‐tune flowering time probably by determining the amount and rate at which vegetative and floral primordia are produced. A detailed mapping in bread wheat has detected *eps* loci on chromosomes of homologous group 2 and on the short arm of chro‐ mosome 3A [65, 66]. A locus on chromosome 2B is orthologous with the *eps2* gene in barley (*Hordeum vulgare*) [6, 11, 64], whereas the one on chromosome 3A is orthologous with the *Eps*‐3*Am* gene in einkorn wheat (*Triticum monococcum*) [67]. The locus on chromosome 3A has been reported to also have significant effects on plant height, thousand kernel weight, and number of grains per plant [66]. However, no *eps* genes have been cloned as yet in bread wheat [12, 68] as compared to barley [6, 61, 69] and einkorn wheat [67]. More than 90 QTL for heading date, with most of them believed to play a role in fine‐tuning flowering time, have been reported to be spread over almost the entire wheat genome [62, 70]. Recently, Zikhali et al. [12] validated the presence of an *eps* effect on 1DL in hexaploid wheat. Some qualities of *eps* genes, such as high heritability and their independency on the environment, display a platform for this class of genes to be efficiently used in breeding programs to modify the flowering time of wheat by advancing/shortening its life cycle [61]. With further studies, it will be possible to fine‐tune flowering time to regional climatic variations using these loci (including those of *Vrn* and *Ppd*) once their primary and pleiotropic effects have

**7. The influence of height‐reducing genes on the flowering time of bread** 

Among the most important growth habit parameters influencing adaptation and yield poten‐ tial of bread wheat to various environments is plant height. The most common genes for reduced height (*Rht*, also called semidwarf) in wheat have been mapped on *Rht‐B1* and *Rht‐ D1* loci on chromosomes 4B and 4D, respectively [71]. Another potentially valuable height reducing gene, designated *Rht*8, has been mapped on chromosome 2D of bread wheat [72, 73]. The alleles of the two genes, *Rht‐B1b* and *Rht‐D1b*, inhibit cell elongation due to insensitivity to the growth hormone gibberellic acid in contrast to the *Rht*8 gene. The primary mechanism of height reduction caused by these alleles (*Rht‐B1b* and *Rht‐D1b*) is a reduction in the rate of stem development and dry matter accumulation in vegetative tissue, leading to increased

been identified.

82 Wheat Improvement, Management and Utilization

**wheat**

*Rht*8 genotypes were reported to compare very well with *Rht‐B1b* and *Rht‐D1b* genotypes in hot and dry environments (i.e. short growing season) [73]. This was evident in a study conducted by Lanning et al. [71] under terminal drought stress in Montana and Washington. In the study, the *Rht*8 semidwarf lines appeared to have superior seed characteristics (signifi‐ cantly higher kernel weight and grain protein content) relative to the *Rht‐B1b* and *Rht‐D1b* lines which even had reduced grain protein content relative to the wild‐type. However, other studies reported higher yield potential associated with *Rht‐B1b* and *Rht‐D1b* genotypes under high input growing conditions (i.e., irrigated) as compared to *Rht*8 and standard height geno‐ types [75, 76]. The performance of semidwarf wheat lines was evaluated relative to standard height lines using a recombinant inbred line (RIL) population grown in both rain‐fed and irrigated conditions in Montana [77]. Semidwarf lines containing *Rht‐D1b* were discovered to have superior yield as compared to standard height lines. Moreover, McNeal et al. [78] observed that semidwarf wheat lines containing either *Rht‐B1b* or *Rht‐D1b* outyielded tall lines in Montana, except in very low yield potential environments where tall lines were supe‐ rior. From these results, it can be concluded that when opting for high yield potential under normal or high input growing conditions, *Rht‐B1b* and *Rht‐D1b* genotypes are the best, but when planting in hot and dry (low yielding) environments, *Rht*8 genotypes should be selected for. The success of the *Rht* genes has resulted in their wide deployment in wheat breeding programs globally [79].

A moderate but significant correlation between flowering time and plant height has been reported in bread wheat [5, 80–82]. Shorter genotypes tend to flower earlier than the taller ones [5]. This effect was proposed to be mainly due to the *Rht‐B1b* allele, suggesting a possible effect of the *Rht‐B1* gene on heading date in wheat. Similar results were also reported by Wilhelm et al. [80], confirming the significant effect of *Rht‐B1* on flowering time and suggested a possibility of genes controlling plant height to also affect flowering time. However, other studies report that earliness is often associated with reduced height and potentially reduced resource capture, therefore, reduced yield [42, 83]. This suggested negative correlation between earliness and yield remains a challenge in wheat breeding programs, posing a need to modify flowering time to suit local climatic conditions while maintaining or even increasing yield potential. The biggest challenge is to incorporate all or as many of the favorable and/or agricultural important traits as possible in one cultivar [84, 85].
