**5. The interaction of** *Vrn* **and** *Ppd* **alleles and its impact on flowering time of bread wheat**

Various studies have been conducted to study the interaction between *Vrn‐1* and *Ppd‐1* active alleles and the impact of their different combinations on the flowering time of bread wheat. Cultivars containing different combinations of *Vrn‐1* and *Ppd‐1* alleles will respond differently under different environmental conditions and thus, display different heading dates or flowering times [3, 9, 23, 51, 52]. It was discovered that *Vrn‐A1* genotypes (either single or in combination with other *Vrn* alleles) are the earliest in flowering followed by *Vrn‐B1* and then *Vrn‐D1* genotypes [53]. In fully vernalized winter wheat, *Ppd‐D1a* allele advanced flowering time by up to 24 days [54, 55]. However, in the presence of an active allele of *Vrn‐1*, the flowering time of wheat was reduced by at least 30 days [54, 56]. It was reported that wheat genotypes with all three dominant alleles of *Vrn‐1* genes (*Vrn‐A1, Vrn‐ B1*, and *Vrn‐D1*) head quite early compared to mono‐ or di‐dominant gene combinations [28]. Similar information was also reported by various authors [5, 9, 57–59]. From the results of the above‐mentioned studies, it was shown which combinations of alleles perform better than others. In agreement with Zhang et al. [53], it was demonstrated in the other studies mentioned above that *Vrn‐B1* and/or *Vrn‐D1* alleles are less effective in advancing flowering time as compared to the *Vrn‐A1* allele. The *Vrn‐1* genotypes are reported to be marginally early in flowering time in the following order:

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

**Major gene series Gene(s) comprised Gene location Reference**

respectively

respectively

*Vrn‐2* Not specified chromosomes 4B, 4D and 5A [35]

*Vrn‐4 Vrn‐D4* Chromosome 5D [37]

[31, 34]

[36]

*Vrn‐1 Vrn‐A1, Vrn‐B1* and *Vrn‐D1* Long arms of chromosomes 5A, 5B and 5D,

*Vrn‐3 Vrn‐A3, Vrn‐B3* and *Vrn‐D3* Short arms of chromosomes 7A, 7B and 7D,

**Table 1.** Vernalization genes/class of genes identified in bread wheat to date.

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

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

**Gene Gene location Reference**

*Ppd‐B2* Short arm of chromosome 7B [50]

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

*Ppd‐D1* (formerly *Ppd1*) Chromosome 2D [27, 38, 40, 44, 45, 49] *Ppd‐B1* (formerly *Ppd2*) Chromosome 2B [27, 38, 40, 44, 45, 49] *Ppd‐A1* (formerly *Ppd3*) Chromosome 2A [27, 38, 40, 44, 45, 49]

**bread wheat**

genes has been ranked in the order: *Ppd‐D1* > *Ppd‐B1* > *Ppd‐A1* [38, 44].

80 Wheat Improvement, Management and Utilization

*Vrn‐A1 Vrn‐B1 Vrn‐D1 > Vrn‐A1 Vrn‐B1*, *Vrn‐A1 VrnD1* or *Vrn‐A1 > Vrn‐B1* or *Vrn‐D1*.

An epistatic interaction between the *Vrn‐A1* and *Vrn‐D1* active alleles was demonstrated in a study [9]. The same study confirmed an additive/complementary interaction for flowering time between the photoperiod‐insensitive *Ppd‐D1a* allele and the *Vrn‐1* active alleles [57, 60]. Moreover, it was noted that although genotypes carrying *Vrn‐1* and *Ppd‐D1a* alleles are early flowering under both SD and LD conditions, the flowering time is delayed by low tempera‐ tures under SD conditions. Overall, it is concluded that *Ppd‐1* and *Vrn‐1* genes participate in a similar pathway to control flowering time [10]. This implies that even though vernalization and photoperiod responses are independent processes, fulfillment of both requirements is necessary for early flowering of bread wheat. Flowering will be delayed if these processes did not occur [28] and the extent of this delay will depend on the *Ppd* gene present in the variety, as well as the environmental conditions [43].
