**6. The influence of earliness** *per se* **(***eps***) genes on the flowering time of bread wheat**

A third class of genes controlling flowering time of wheat is the earliness *per se* (*eps*) genes. *Eps* genes affect phenological development of wheat when all photoperiod and vernaliza‐

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 been identified.
