Wheat (Triticum aestivum L.), with the ability to be cultivated in a wide range of geographical latitudes (from 60°N latitude, to 48°E longitudes), climatic conditions and soil fertility, is one of the most widespread crops in the world (Kulkarni et al. 2017). The growth and productivity of wheat are often limited by drought stress, one of the main challenges for farmers and plant breeders. Unfortunately, global wheat production is also carried out in areas facing repeated threats of this tension. Meanwhile, drought stress is possible to occur at all plant growth stages in rain-fed areas, but it happens much more at the end of growing season (Kulkarni et al. 2017). In order to adapt wheat to different cultivation conditions, appropriate flowering and ripening time is necessary (Fletcher et al. 2019).
Drought escape is a classic adaptation mechanism which allows plants to complete its life cycle before the occurrence of drought stress (Shavrukov et al. 2017). Earliness is very important in the regions where drought occurs at the end of the crop season. New early cultivars are less exposed to end season drought stress and produce higher grain yield (Hill and Li. 2016). Early heading cultivars had higher grain yield not only in drought stress but also in irrigated conditions (Iqbal et al. 2006; Nitcher et al. 2014; Shavrukov et al. 2017). Nevertheless, there are also reports showing the negative effect of earliness on grain yield (Radhika and Thind 2014; Turner 1986). Flowering time is a complex trait mainly controlled by two important pathways including vernalization (VRN) and photoperiod (PPD). These genes are affected by environmental factors such as cold, temperature and day length (Cockram et al. 2007).
In the PPD pathway, dominant alleles including Ppd-A1a, Ppd-B1a and Ppd-D1a induce insensitivity to photoperiod (Law et al. 1978). Generally, Ppd-D1a allele is considered as the strongest allele, followed by Ppd-B1a and then Ppd-A1a (Scarth and Law 1984). In addition, Ppd-D1a is the major source of photoperiod insensitivity in bread wheat (Beales et al. 2007; Worland et al. 1998). Ppd-D1a is originally from an old Japanese cultivar named Akakomugi (Strampelli 1932; Worland et al. 1998).
Among the identified genes for flowering time, Ppd- D1 is the only gene that worth marker assisted selection or backcrossing (Langer et al. 2014). Deletion of 2089 base pairs in the Ppd-D1 locus, created Ppd-D1a allele which is insensitive to photoperiod (Beales et al. 2007). This gene is the most important factor influencing flowering time of European winter bread wheat cultivars and explains more than half of the genetic diversity of this trait. While only 2.3% of the genetic variation of this trait is under the control of Ppd- B1, which includes a small proportion of genetic diversity (Langer et al. 2014). Among 232 diverse winter wheat cultivars, the most common Ppd allele was Ppd-D1a (81%), ranging from 10 to 92% in the United States and Ukrainian cultivars, respectively, while there was no genetic diversity for Ppd-A1 (Fait and Balashova 2022). Ppd-D1a allele had very low frequency (2.6%) in wheat landraces, while dramatically increased during recent “green revolution” (Dragovich et al. 2021). Shcherban et al. (2015), reported that variation of photoperiod gene (Ppd-1) offers opportunities to adjust heading time and maximize grain yield. Several researches reported that Ppd-D1a decrease number of days to heading (Beales et al. 2007; Bentley et al. 2013; Chen et al. 2018; Fait and Balashova 2022; Langer et al. 2014; Shcherban et al. 2015; Wilhelm et al. 2013), grains number per spike (Chen et al. 2018; Kroupin et al. 2020; Worland et al. 1998) and plant height (Chen et al. 2018; Kroupin et al. 2020; Wilhelm et al. 2013).
Early heading reduces plant height, tillering and spike number in bread wheat (Wolde et al. 2019). Insensitive allele to photoperiod (Ppd-D1a) affected grain yield in a range of + 3 to -5% (Liu et al. 2020), whereas Chen et al. (2018) reported this allele increased grain yield by 8.2%. Effect of Ppd-D1a on yield component is also communicated in previous researches. This allele increased 1000-grain weight by 10- 16.9% (Chen et al. 2018; Liu et al. 2020). Yet, Kroupin et al. (2020) reported a negative effect of Ppd-D1a on 1000-grain weight. Ppd-D1a decreased grains number per spike by 6.5–10% (Chen et al. 2018; Kroupin et al. 2020; Liu et al. 2020), and had a slight effect on reducing plant height (Kroupin et al. 2020).
Effect of Ppd-D1a allele on earliness, yield and yield component has been evaluated in several studies. Nevertheless, the effect of this allele on mentioned traits was not consistent. In the present research, isogenic lines were developed for Ppd-D1a in two genetic backgrounds to investigate the precise pleiotropic effect of this allele on earliness, yield and yield components of bread wheat under rain-fed and well-watered conditions.