Variation in heading date among ESW and KW wheat varieties
Figure 1 shows the mean and range of HDs of 121 ESW wheat varieties in 2015–2018, and 32 KW wheat varieties in 2017 and 2018, grown in Wooster, Ohio. The HDs of each variety in each of four crop years are provided in Supplemental Table S1. Significant differences were observed in the HDs of ESW and KW wheat varieties among the tested crop years. The HDs of ESW wheat varieties ranged from 138 to 148, 138–143, 127–140 and 139–149 in 2015, 2016, 2017 and 2018, respectively. The average HD of ESW wheat varieties was shortest in 2017 followed by 2016, 2015 and 2018. Early spring temperatures in Wooster, Ohio, were warmer in 2017 than in the other years when the growing degree days after January 1 accumulated more slowly (Supplemental Figure S1). This result agrees with Hu et al. (2005), who found that the HD of winter wheat is governed primarily by temperature. HDs of KW wheat varieties ranged from 113 to 119 and 130–136 in 2017 and 2018, respectively. KW wheat varieties headed on average 14 days earlier than did ESW wheat varieties in each crop year, indicating that the genetic background, in addition to crop year, significantly influences winter wheat HD.
The two-way ANOVA showed that variety, crop year and the interaction of the variety and crop year all significantly contributed to the variation in HD (p < 0.0001). Crop year exhibited the biggest influence on HD, followed by variety and the interaction between variety and crop year. This result agrees with the report of Würschum et al. (2018), which showed that wheat HD was mainly controlled by both genotype and environment, and much less by the interaction between genotype and environment, from the examination of 1110 winter wheat cultivars of worldwide origin. The HD was proved to be highly heritable as indicated by the significant correlations (r ranged from 0.52 to 0.73, p < 0.001) between HDs of ESW wheat varieties grown in different crop years, and its high estimated heritability of 0.86. Wheat cultivars exhibited similar responses to the different environments with regards to HD.
Allelic frequencies at vernalization loci
Considerable genetic variation in the phenology gene profile was observed among ESW wheat varieties. The phenology genes of each variety are summarized in Supplemental Table S1. Among the 149 ESW wheat varieties examined, 127 varieties possessed homogeneous alleles at all the tested loci, while 22 varieties carried heterogeneous alleles at one of the tested loci. Among the 32 KW wheat varieties examined, 30 varieties carried homogeneous alleles at all the phenology-related loci evaluated. The occurrence frequencies of the alleles at the phenology-related loci in the 149 ESW and 32 KW wheat varieties are summarized in Table 2.
Table 2
Frequency of different alleles at the vernalization (Vrn), photoperiod (Ppd), earliness per se (Eps) and reduced-height (Rht) loci in 149 eastern U.S. soft winter wheat and 32 Korean winter wheat varieties
Locus | Allele | Frequency (%) |
U.S. soft winter wheat | Korean winter wheat |
Vrn-A1 | vrn-A1, CNVa>2 | 94.0 | 12.5 |
| vrn-A1, CNV = 2 | 1.3 | 0.0 |
| vrn-A1, CNV = 1 | 0.7 | 81.3 |
| heterogeneous | 4.0 | 6.3 |
Vrn-B1 | vrn-B1-Neuse | 96.0 | 100.0 |
| vrn-B1-AGS2000 | 0.7 | 0.0 |
| null | 1.3 | 0.0 |
| heterogeneous | 2.0 | 0.0 |
Vrn-D3 | vrn-D3a | 10.1 | 46.9 |
| vrn-D3b | 89.9 | 53.1 |
Ppd-A1 | Ppd-A1a.1 | 32.9 | 0.0 |
| Ppd-A1b | 56.4 | 100.0 |
| heterogeneous | 10.7 | 0.0 |
Ppd-B1 | Ppd-B1a-Chinese Spring | 12.8 | 6.3 |
| Ppd-B1a-Sonora 64 | 6.7 | 3.1 |
| Ppd-B1b | 73.8 | 87.5 |
| null | 5.4 | 3.1 |
| heterogeneous | 1.3 | 0.0 |
Ppd-D1 | Ppd-D1a | 59.7 | 96.9 |
| Ppd-D1b | 16.1 | 0.0 |
| Ppd-D1b-Norstar | 12.8 | 0.0 |
| heterogeneous | 11.4 | 3.1 |
Eps-B1 | TaELF3-B1a | 92.6 | 100.0 |
| TaELF3-B1b | 3.4 | 0.0 |
| heterogeneous | 4.0 | 0.0 |
Eps-D1 | TaELF3-D1a | 73.2 | 100.0 |
| TaELF3-D1b | 19.5 | 0.0 |
| heterogeneous | 7.4 | 0.0 |
Rht-B1 | Rht-B1a | 63.8 | 65.6 |
| Rht-B1b | 33.6 | 31.3 |
| heterogeneous | 2.7 | 3.1 |
Rht-D1 | Rht-D1a | 38.9 | 53.1 |
| Rht-D1b | 56.4 | 46.9 |
| heterogeneous | 4.7 | 0.0 |
aCNV, copy number variation. |
All ESW and KW wheat varieties carried the winter allele at the evaluated Vrn loci, which was expected since only winter wheat varieties were included in this study. Little genetic variation was observed in the evaluated Vrn loci among ESW wheat (Table 2). The low variation at the Vrn loci was also observed by Tessmann et al. (2019) in 256 ESW wheat varieties and Cho et al. (2015) in 410 Korean wheat varieties. The copy number variation (CNV) of the vrn-A1 winter allele was significantly different between ESW and KW wheat varieties. Most ESW wheat varieties (94.0%) carried three or more copies of the vrn-A1 winter allele, while most KW wheat varieties (81.3%) carried a single copy of the vrn-A1 winter allele. Only three ESW wheat varieties (Jamestown, Ernie and VA08MAS-369) possessed two copies or a single copy of the vrn-A1 allele. Increased copy number at the Vrn-A1 locus has been reported to be associated with greater vernalization requirements, resulting in late heading (Díaz et al. 2012). The high frequency of a single copy of the vrn-A1 allele in KW wheat varieties may be related to the early heading of KW wheat varieties.
Little variation was also observed at the Vrn-B1 locus in ESW and KW wheats. Two winter alleles (vrn-B1-AGS2000 and vrn-B1-Neuse) were identified at the Vrn-B1 locus in ESW wheat (Table 2). The vrn-B1-AGS2000 allele was associated with lower vernalization requirements and earlier heading than the vrn-B1-Neuse allele (Guedira et al. 2014). Most of the ESW wheat varieties (96%) and all the KW wheat varieties (100%) carried the vrn-B1-Neuse allele, indicating that the Vrn-B1 locus may not contribute to the difference in HD between ESW and KW wheat varieties. Only one ESW wheat variety ‘AGS 2060’ carried the vrn-B1-AGS2000 allele. In addition, two ESW wheat varieties were found to have a null allele at the vrn-B1 locus. A similar result was reported in ESW wheat varieties by Tessmann et al. (2019).
Two winter alleles were identified at the Vrn-D3 locus in both ESW and KW wheat varieties (Table 2). The vrn-D3a allele, known to be associated with early heading (Chen et al. 2010), was carried by 10.1% of ESW wheat varieties. Tessmann et al. (2019) observed that around 30.0% of 256 ESW wheat varieties carried the vrn-D3a allele; however, the frequency of the vrn-D3a allele varied from 3.0–59.5% in ESW wheat varieties from different states calculated from Supplemental Table S5 reported by Tessmann et al. (2019). The frequency of the vrn-D3a allele in ESW wheat varieties in this study was lower than that found by Tessmann et al. (2019), probably due to the differences in ESW wheat varieties used in each study. These results indicate that the vrn-D3a allele is common in ESW wheat varieties and its frequency varies across states in the eastern region of the U.S.
The vrn-D3a allele was observed in 46.9% of KW wheat varieties. In addition, most ESW wheat varieties with the vrn-D3a allele possessed the photoperiod sensitive allele at the Ppd-D1 locus, while KW wheat varieties with the vrn-D3a allele had the Ppd-D1a photoperiod insensitive allele, indicating that different allelic combinations exist in ESW and KW wheat varieties.
Allelic frequencies at photoperiod loci
Allelic variations at the Ppd-A1, Ppd-B1 and Ppd-D1 loci in ESW wheat varieties were greater than those in KW wheat varieties. Most ESW wheat (56.4%) and all KW wheat (100%) carried the Ppd-A1b photoperiod sensitive allele. Guedira et al. (2014) found that the photoperiod insensitive allele Ppd-A1a.1 was common in ESW wheat varieties. Tessmann et al. (2019) also observed that 57.4% of ESW wheat varieties carried the Ppd-A1a.1 allele at the Ppd-A1 locus. The photoperiod insensitive allele Ppd-A1a.1 was, however, observed in 32.9% of 149 ESW wheat varieties. The Ppd-B1b photoperiod sensitive allele was carried by 73.8% of ESW wheat varieties and 87.5% of KW wheat varieties, and was most frequent in both ESW and KW wheat varieties. In addition, eight ESW wheat varieties and one KW wheat variety possessed a null allele at the Ppd-B1 locus. Tessmann et al. (2019) also reported that 7.4% of ESW wheat varieties carried the null allele at the Ppd-B1 locus. The Ppd-D1a photoperiod insensitive allele was the predominant allele at the Ppd-D1 locus with a frequency of 59.7% among the 149 ESW wheat varieties, followed by the Ppd-D1b photoperiod sensitive allele with a frequency of 16.1% (Table 2). The Ppd-D1b-Norstar photoperiod sensitive allele was observed in 12.8% of the 149 ESW wheat varieties. For KW wheat varieties, 96.9% carried the Ppd-D1a photoperiod insensitive allele; one KW wheat variety carried heterogeneous alleles at the Ppd-D1 locus. This result agrees with the report of Guo et al. (2010) who found that haplotype I (Ppd-D1a photoperiod insensitive allele) was most common in Asian wheat varieties. Cho et al. (2015) also observed that all KW wheat varieties developed since 1991 carried the Ppd-D1a allele.
Grogan et al. (2016) found a higher frequency of the Ppd-A1b photoperiod sensitive allele (98.0%) and lower frequencies of the Ppd-B1b photoperiod sensitive allele (57%) and the Ppd-D1a photoperiod insensitive allele (29.0%) in 299 U.S. great plains hard winter wheat varieties than in ESW wheat varieties. Guo et al. (2010) observed that the predominant allele at the Ppd-D1 locus was the photoperiod sensitive allele (Ppd-D1b, haplotype IV) among wheat varieties from the U.S. and Canada. On the contrary, in this study, the photoperiod insensitive allele Ppd-D1a was predominantly observed in ESW wheat varieties. Tessmann et al. (2019) also reported that 50.0% of ESW wheat varieties carried the Ppd-D1a allele. These results indicate that the occurrence of the photoperiod insensitive allele Ppd-D1a varies among different wheat classes in the United States.
Allelic frequencies at the earliness per se loci
The predominant alleles at the Eps-B1 and Eps-D1 loci were the late heading-related alleles TaELF3-B1a and TaELF3-D1a with frequencies of 92.6% and 73.2%, respectively, among the 149 ESW wheat varieties. All KW wheat varieties carried the late heading-related alleles TaELF3-B1a and TaELF3-D1a at both the Eps-B1 and Eps-D1 loci, indicating that the Eps-B1 and Eps-D1 loci may not be relevant to the early heading of KW wheat varieties.
Allelic frequencies at the reduced height loci
Most ESW (63.8%) and KW (65.6%) wheat varieties carried the wild type (tall) Rht-B1a allele at the Rht-B1 locus. The semi-dwarfing Rht-D1b allele was the predominant allele at the Rht-D1 locus in ESW wheat varieties with a frequency of 56.4%, while most KW wheat varieties (53.1%) carried the Rht-D1a allele at the Rht-D1 locus. Eight ESW wheat varieties (5.4%) and seven KW wheat varieties (21.9%) carried the tall alleles Rht-B1a and Rht-D1a at the Rht-B1 and Rht-D1 loci (Supplemental Table S1). The semi-dwarf alleles Rht-B1b and Rht-D1b were present at similar levels to those detected by Guedira et al. (2010) in 247 ESW wheat varieties released before 2008. Wilhelm et al. (2013) and Grogan et al. (2016) reported that the Rht-B1b and Rht-D1b alleles could promote wheat heading probably through their significant associations with the reduction in gibberellin sensitivity. The similar frequencies of the Rht-B1b and Rht-D1b alleles in ESW and KW wheat varieties suggest that the Rht-B1 and Rht-D1 loci may not contribute to the difference in HD between ESW and KW wheat varieties.
Combined analyses of alleles on heading date of ESW and KW wheat varieties over four crop years
The significance of crop year, and the Vrn, Ppd, Eps and Rht loci, and their interactions, on the HD of ESW wheat varieties was tested using 86 ESW wheat varieties (Table 3) with homozygous alleles at each locus. The GLM with crop year and the phenology-related loci as the independent variables and HD as a dependent variable explained 92.7% of the variation in the HD of ESW wheat, with most of the variation (87.0%) explained by crop year, and a smaller amount (5.7%) explained by genetic variation (Table 3). A similarly larger influence of crop year than genetic variation on HD was also observed by Grogan et al. (2016) in hard winter wheat varieties adapted to the great plains region of the U.S. Of the genetic variation, 27.8% was contributed by the Ppd-D1 locus, 8.9% by the Ppd-B1 locus, 7.4% by the Vrn-D3 locus, and 11.6% by the two-way interaction between the Ppd-B1 locus and the Ppd-D1 locus. A smaller influence of genetic variation on HD was explained by the Ppd-A1 locus (2.8%) and the Eps-D1 locus (3.7%) and by the two-way interactions between the Ppd-B1 (5.9%) locus or Ppd-D1 (10.4%) locus and the Ppd-A1 locus. These results agree with the report of Huang et al. (2018) which found that the Ppd-A1, Ppd-B1, Ppd-D1 and Vrn-D3 loci significantly affected the HD of ESW wheat under eastern U.S. environments. The Vrn-A1, Vrn-B1 and Eps-B1 loci exhibited insignificant effects on the HD of ESW wheat varieties, as there is limited allelic diversity at these loci.
Table 3
The influence of crop year, and the vernalization (Vrn), photoperiod (Ppd), earliness per se (Eps) and reduced-height (Rht) loci and their interactions, on the heading date of eastern U.S. soft winter wheat varieties in four crop years
Source | Degrees of freedom | Mean square | Proportion of total variance (%) | Proportion of genetic variance (%) |
Crop year | 3 | 2119.6*** | 87.0 | |
Vrn-A1 | 2 | 2.5 | 0.1 | 1.2 |
Vrn-B1 | 2 | 0.4 | 0.0 | 0.2 |
Vrn-D3 | 1 | 30.3*** | 0.4 | 7.4 |
Ppd-A1 | 1 | 11.7* | 0.2 | 2.8 |
Ppd-B1 | 3 | 12.2*** | 0.5 | 8.9 |
Ppd-D1 | 2 | 57.3*** | 1.6 | 27.8 |
Eps-B1 | 1 | 5.8 | 0.1 | 1.4 |
Eps-D1 | 1 | 15.3** | 0.2 | 3.7 |
Rht-B1 | 1 | 3.5 | 0.0 | 0.8 |
Rht-D1 | 1 | 2.7 | 0.0 | 0.6 |
Ppd-A1*Ppd-B1 | 3 | 8.1** | 0.3 | 5.9 |
Ppd-A1*Ppd-D1 | 1 | 42.9*** | 0.6 | 10.4 |
Ppd-B1*Ppd-D1 | 4 | 11.9*** | 0.7 | 11.6 |
Ppd-A1*Ppd-B1*Ppd-D1 | 1 | 0.1 | 0.0 | 0.0 |
Error | 262 | 2.0 | 7.3 | |
The sum of squares was used to calculate the proportion of total variance contributed by crop years and each locus, and the proportion of genetic variance by each locus. Eighty-six eastern U.S. soft winter wheat varieties with homozygous alleles at each locus were evaluated for heading date in four crop years and included in the analyses of variation. |
*** significant at p < 0.001, ** significant at p < 0.01, * significant at p < 0.05. |
The influences of crop year and the phenology-related loci on the HD variation in KW wheat were determined using 30 KW wheat varieties (Supplemental Table S2) with homozygous alleles at each locus. Most of the HD variation of KW wheat varieties (94.7%) was explained by crop year, and a smaller amount (1.7%) was explained by the phenology-related loci. Of the genetic variation in the HD of KW wheat varieties, 63.4% was accounted for by the Rht-D1 locus (Supplemental Table S2). Grogan et al. (2016) found that the Rht-B1 locus significantly affected the HD of hard winter wheat varieties grown in the U.S. great plains, but not the Rht-D1 locus. The Rht-B1 and Rht-D1 loci had an insignificant influence on the HD of ESW wheat varieties under the tested environments. It appears that the effects of the Rht-1 loci on wheat HD depend on both the allelic combinations at the phenology-related loci and the environment. The Vrn, Ppd and Eps loci did not significantly affect KW wheat HD; this mainly resulted from the lack of genetic diversity at these loci (Table 2).
Individual analysis of alleles on heading date of ESW and KW wheat varieties for each crop year
The GLMs with the phenology-related loci as the independent variables and HD as a dependent variable were tested for each crop year separately for ESW wheat varieties (Supplemental Table S3) and KW wheat varieties (Supplemental Table S4). The best-fit GLMs varied among crop years and not all phenology-related loci were significant. The model explained on average 42.0% of the variation in the HD of ESW wheat varieties, which ranged from a minimum of 38.0% in 2018 to a maximum of 47.1% in 2016 (Supplemental Table S3). The Ppd-D1 locus significantly affected the HD of ESW wheat in all four crop years and accounted for on average 13.3% of the variation in HD, which ranged from 9.2% in 2015 to 23.5% in 2017 (Supplemental Table S3). The largest effect of the Ppd-D1 locus on HD was observed in 2017 (Supplemental Table S3), probably due to the warm spring weather that year. The Vrn-D3 locus exhibited a significant influence on the HD of ESW wheat only in 2015 and 2016. The interaction between the Ppd-D1 locus and the Ppd-B1 locus, and between the Ppd-D1 locus and the Vrn-D3 locus, also contributed to small variations in the HD of ESW wheat varieties in 2018 and 2016, respectively. No significant effects of the Vrn-A1, Vrn-B1, Ppd-A1, Eps-B1, Eps-D1, Rht-B1 or Rht-D1 loci were observed on the HD of ESW wheats in any individual crop year.
Among the tested loci, only the Rht-D1 locus showed a significant influence on the HD of KW wheat varieties in both crop years, and accounted for 21.0% of the variation in the HD in 2017 and 18.2% in 2018 (Supplemental Table S4). The other loci showed no significant influence on the HD of KW wheats in a crop year.
Influence of alleles at the Vrn-D3, Ppd-B1 and Ppd-D1 loci on heading date of ESW and KW wheat varieties
Five loci, including the Vrn-D3, Ppd-A1, Ppd-B1, Ppd-D1 and Eps-D1 loci, were identified to significantly influences on the HDs of ESW wheat varieties in the combined analyses over four crop years (Table 3). The influences of the alleles at these five significant loci on the HD of ESW wheat were further evaluated using ESW wheat varieties with homozygous alleles at these loci. No significant differences were observed in HD between ESW wheat varieties carrying different alleles at the Ppd-A1 and Eps-D1 loci in any individual crop year. No significant differences in HD (except for 2018) were observed between ESW wheat varieties carrying the vrn-D3a allele or the vrn-D3b allele at the Vrn-D3 locus (Supplemental Figure S2), likely due to the presence of the Ppd-D1b photoperiod sensitive allele in those ESW varieties (Supplemental Table S1). The effect of the Ppd-D1b allele on HD might mask the effect of the vrn-D3a allele. In 2018, ESW wheat varieties carrying the vrn-D3a allele headed on average 1 day earlier than those carrying the vrn-D3b allele.
The effects of alleles at the Ppd-B1 and Ppd-D1 loci were also inconsistent in four crop years (Fig. 2). No significant differences in HD were observed between the ESW wheat varieties carrying different alleles at the Ppd-B1 locus in 2016 and 2018 (Fig. 2). ESW wheat varieties possessing the ‘Sonora 64’-type insensitive allele at the Ppd-B1 locus headed on average 1 or 2 days earlier than those with other alleles at the Ppd-B1 locus in 2015 and 2017. No significant differences in HD were also observed between ESW wheat varieties carrying different alleles at the Ppd-D1 locus in 2015 and 2016. ESW wheat carrying the Ppd-D1a insensitive allele headed on average 2 days earlier than those carrying the Ppd-D1b or Ppd-D1b-Norstar sensitive allele in 2017 (Fig. 2). A significant difference was observed only between ESW wheat carrying the Ppd-D1a allele and the Ppd-D1b-Norstar allele in 2018. The allelic variation at the Ppd-D1 locus showed variable influences on HD among crop years, suggesting differential expression under varying climatic conditions, especially temperature. The early heading of ESW wheat varieties with the photoperiod-insensitive allele Ppd-D1a is in agreement with results from the previous studies by Grogan et al. (2016) and Whittal et al. (2018).
For KW wheat varieties, only the Rht-D1 locus showed a significant influence on HD in each crop year (Supplemental Table S4). KW wheat varieties carrying the Rht-D1a allele headed on average 2 days earlier than those carrying the Rht-D1b allele in each crop year (Supplemental Figure S3). Tessmann et al. (2019) also observed a slight decrease (0.6%) in the HD of ESW wheat varieties with the Rht-D1a allele compared to those with the Rht-D1b allele under eastern U.S. environments. On the contrary, several other studies reported that the Rht-B1b and Rht-D1b alleles could promote wheat heading through their significant associations with the reduction in gibberellin sensitivity (Wilhelm et al. 2013; Grogan et al. 2016). It is not evident why the Rht-D1a allele shows an association with the early heading of winter wheat. The significant association may be due to the association of Rht loci with Ppd loci in determining wheat heading (Wilhelm et al. 2013).
Influence of allelic combinations on the heading date of ESW and KW wheat varieties
Considering the significant influence of allelic variation at the Vrn-D3, Ppd-B1 and Ppd-D1 loci on the HD of ESW wheats, as well as significant differences in the allelic frequencies at the Vrn-A1 locus between ESW and KW wheat varieties, the influences of allelic combinations at the Ppd-B1, Ppd-D1, Vrn-A1 and Vrn-D3 loci on the HD of winter wheat were determined using 127 ESW wheat varieties and 30 KW wheat varieties with homozygous alleles at these four loci. A total of 19 phenology gene profiles were observed in 127 ESW wheat varieties (Table 4). More than half (52.8%) of 127 ESW wheat varieties, however, possessed the same profile of Ppd-B1b/Ppd-D1a/vrn-A1(CNV > 2)/vrn-D3b. Around 26.0% of ESW wheat varieties possessed one of these four profiles: Ppd-B1b/Ppd-D1b/vrn-A1(CNV > 2)/vrn-D3b (10.2%); Ppd-B1b/Ppd-D1b-Norstart/vrn-A1(CNV > 2)/vrn-D3b (6.3%); null/Ppd-D1a/vrn-A1(CNV > 2)/vrn-D3b (4.7%); and Ppd-B1a-Sonora 64/Ppd-D1a/vrn-A1(CNV > 2)/vrn-D3b (4.7%). Fourteen other phenology gene profiles were observed in five or fewer varieties. The 30 KW wheat varieties carried six phenology gene profiles at the Ppd-B1, Ppd-D1, Vrn-A1 and Vrn-D3 loci (Table 4). Most KW wheat varieties possessed one of these two profiles: Ppd-B1b/Ppd-D1a/vrn-A1(CNV = 1)/vrn-D3a (40.0%) and Ppd-B1b/Ppd-D1a/vrn-A1(CNV = 1)/vrn-D3b (36.7%). Four other phenology gene profiles were observed in three or fewer varieties.
Table 4
Phenology gene profiles of 127 eastern U.S. soft winter wheat varieties and 30 Korean winter wheat varieties carrying homozygous alleles at the Ppd-B1, Ppd-D1, Vrn-A1 and Vrn-D3 loci sorted by frequency and their heading dates
Loci | Frequency (%) | Heading date |
Ppd-B1 | Ppd-D1 | vrn-A1 CNVa | vrn-D3 | 2015 | 2016 | 2017 | 2018 |
U.S. soft winter wheat | | |
b | a | > 2 | b | 52.8 | 143 | 140 | 133 | 145 |
b | b | > 2 | b | 10.2 | 143 | 141 | 135 | 146 |
b | b-Nord | > 2 | b | 6.3 | 144 | 140 | 135 | 147 |
null | a | > 2 | b | 4.7 | 143 | 140 | 134 | 145 |
a-S64e | a | > 2 | b | 4.7 | 140 | 139 | 131 | 143 |
a-CSf | b-Nor | > 2 | b | 3.9 | 142 | 140 | 132 | 144 |
b | b | > 2 | a | 3.1 | 141 | 139 | 133 | 143 |
a-CS | a | > 2 | b | 3.1 | 143 | 140 | 133 | 145 |
b | b-Nor | > 2 | a | 2.4 | 143 | 141 | 134 | 144 |
a-CS | b | > 2 | a | 1.6 | 141 | 139 | 130 | 142 |
a-CS | b | > 2 | b | 0.8 | 144 | 141 | 136 | 146 |
null | b | > 2 | b | 0.8 | 142 | 140 | 133 | 142 |
null | b-Nor | > 2 | b | 0.8 | 144 | 141 | 135 | 146 |
a-S64 | b | > 2 | a | 0.8 | 144 | 142 | 136 | 146 |
a-S64 | b | > 2 | b | 0.8 | 143 | 140 | 134 | 146 |
a-S64 | b-Nor | > 2 | b | 0.8 | nda | 141 | 136 | 146 |
b | a | 2 | b | 0.8 | 139 | 139 | 130 | 144 |
b | b-Nor | 2 | b | 0.8 | nd | 139 | 134 | nd |
b | b | 1 | a | 0.8 | 143 | 139 | 135 | 146 |
| | | | LSD (0.05)c | 4 | 2 | 4 | 4 |
Korean winter wheat | | | | | |
b | a | 1 | a | 40.0 | nd | nd | 116 | 133 |
b | a | 1 | b | 36.7 | nd | nd | 117 | 133 |
b | a | > 2 | b | 10.0 | nd | nd | 118 | 134 |
a-CS | a | 1 | b | 6.7 | nd | nd | 119 | 135 |
b | a | > 2 | a | 3.3 | nd | nd | 117 | 132 |
null | a | 1 | a | 3.3 | nd | nd | 119 | 135 |
| | | | LSD (0.05) | | | 4 | 4 |
aCNV, copy number variation. |
bnd, not determined. |
cLSD, least significant difference (at the 0.05 probability level). |
db-nor, ‘Norstar’-type sensitive allele. |
ea-S64, ‘Sonora 64’-type insensitive allele. |
fa-CS, ‘Chinese Spring’-type insensitive allele. |
Within ESW wheat varieties, the average HD of 19 phenology gene profiles ranged from 139 to 144 in 2015, 139–142 in 2016, 130–136 in 2017 and 142–147 in 2018 (Table 4). Within KW wheat varieties, the average HD of six phenology gene profiles ranged from 116 to 119 in 2017 and 132–135 in 2018 (Table 4). Within ESW wheat varieties or KW wheat varieties, relatively small variations in HD were observed among the 19 or six phenology gene profiles in each crop year, respectively, whereas each phenology gene profile exhibited considerable variation in HD among the crop years (Table 4). It appears that wheat varieties adapted to a specific location have HDs appropriate to the local climatic conditions to ensure the maximum yield potential.
Five predominant profiles of ESW wheat varieties had the same winter alleles at the Vrn-A1 and Vrn-D3 loci. Among these five predominant profiles, ESW wheat varieties carrying the profile of Ppd-B1a-Sonora 64/Ppd-D1a/vrn-A1(CNV > 2)/vrn-D3b headed 1–4 days earlier than those carrying one of the remaining four profiles. Two predominant profiles of KW wheat varieties had the same alleles at the Ppd-B1, Ppd-D1 and Vrn-A1 loci. No significant differences were observed in the average HD of KW wheat varieties between different phenology gene profiles.
The copy number variation at the Vrn-A1 locus was the major difference in the predominant phenology gene profiles between ESW and KW wheat varieties and might be responsible for the different HDs between ESW and KW wheat varieties under eastern U.S. environments. Additional variation in the HD of ESW wheat varieties could be achieved through the inclusion of other loci known to affect HD, such as the Vrn-A1 locus.