**4. The influence of photoperiod response genes on the flowering time of bread wheat**

Photoperiod is the day length and number of long days that a wheat cultivar must reach (a threshold) for floral initiation [38]. The duration of exposure to light can be categorized into three groups namely, short‐day (SD, 11‐14 h), long‐day (LD, 18 h), and day‐neutral (DN) or facultative [39]. The winter wheat and spring wheat varieties can be photoperiod‐sensitive or photoperiod‐insensitive. Photoperiod‐insensitive varieties are early flowering both under SD and LD conditions, in contrast to the photoperiod‐sensitive varieties that require exposure to LD for weeks before they can initiate flowering [38, 40]. Several genes controlling photoperiod response have been successfully identified in wheat (**Table 2**). The *Ppd‐1* genes (*Ppd‐A1*, *Ppd‐ B1*, and *Ppd‐D1*) induce flowering time irrespective of the day length in contrast to the *Ppd‐B2* gene reported on the short arm of chromosome 7B, which accelerates flowering time only under LD conditions [41, 42, 43]. The potency of the insensitivity of the photoperiod response genes has been ranked in the order:

*Ppd‐D1* > *Ppd‐B1* > *Ppd‐A1* [38, 44].


**Table 2.** Photoperiod response genes identified in bread wheat to date.

Photoperiod insensitivity is beneficial for crops grown in short‐growing seasons with high summer temperatures in order to avoid heat stress during grain‐filling stages [44, 45, 46]. Earlier flowering conferred by the *Ppd‐D1a* insensitive allele has broadened the adaptation of cultivars over a range of environments and increased yield potential in improved cultivars, in southern Europe, Asian, Mediterranean, and North African regions [11, 47, 48]. However, the desirability of this allele depends on the target environment. For example, in the northern parts of Europe, which do not experience late season stress, the *Ppd‐D1a* allele is not selected for due to the shortened vegetative phase associated with this allele, which results in consid‐ erably lower yield potential in these environments [5].
