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Published online 31 May 2007
Published in Crop Sci 47:1031-1039 (2007)
© 2007 Crop Science Society of America
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CROP BREEDING & GENETICS

The Effect of Vernalization Genes on Earliness and Related Agronomic Traits of Spring Wheat in Northern Growing Regions

Muhammad Iqbala, Alireza Navabia, Rong-Cai Yanga,c, Donald F. Salmonb and Dean Spanera,*

a Dep. of Agricultural, Food and Nutritional Science, Univ. of Alberta, Edmonton, AB T6G 2P5, Canada
b Field Crop Development Centre, Alberta Agriculture, Food & Rural Development, Lacombe, AB T4L 1W8, Canada
c Alberta Agriculture, Food & Rural Development, Edmonton, AB T6H 5T6, Canada

* Corresponding author (Dean.Spaner{at}ualberta.ca).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
Vernalization response (Vrn) genes play a major role in determining the flowering times of spring-sown wheat (Triticum aestivum L.). The objective of this study was to investigate the effect of Vrn genes on flowering and maturity times and important agronomic traits in a set of reciprocal whole-chromosome substitution lines and eight western Canadian spring wheat cultivars of known Vrn genes, grown over three seeding dates in Alberta, Canada over 2 yr. The genotype carrying spring habit alleles at Vrn-A1, Vrn-B1, and Vrn-D5 flowered and matured the earliest, and had the highest grain protein content but the lowest grain yield. Genotypes with spring habit alleles Vrn-A1 and Vrn-B1 were early maturing and high yielding. Genotypes with the spring habit Vrn-D5 allele either singly or in combination with Vrn-A1 were late maturing. The spring habit allele of Vrn-A1 was not completely epistatic to Vrn-B1 and Vrn-D5 for flowering or maturity time. The spring habit allele of Vrn-B1, however, was epistatic to that of Vrn-D5 for these traits. In northern wheat growing regions, breeding preference should be given to Vrn genotypes with three spring habit alleles or those with spring habit alleles of Vrn-A1 and Vrn-B1. Genotypes carrying spring habit Vrn-D5 allele singly or in combination with Vrn-A1 should be planted as early in the growing season as possible to realize their full yield potential.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
DESPITE THE EXISTENCE of considerable genetic variation for flowering and maturity time in western Canadian adapted spring wheat, the genetic basis of these differences is poorly understood (Iqbal et al., 2006). Environmental conditions during the growing season vary greatly with location and year, and adaptation to the various ecoregions requires cultivars of different maturity potential. The growing season in western Canada is short (95–125 d), and the development of early maturing cultivars is important to avoid frost damage that can lower both yield and quality (Iqbal et al., 2006).

The cultivation of hexaploid wheat in a wide range of environmental conditions has resulted mainly from direct selection for timing of anthesis (Gororo et al., 2001). Growth and developmental phases (tillering, stem elongation, ear emergence, anthesis, and ripening) of wheat are controlled by vernalization (Vrn) and photoperiod response, and earliness per se genes (Kosner and Pankova, 1998). These genes, along with their interaction with growth temperatures (Gororo et al., 2001), play a significant role in wheat's adaptation and yield potential in many environments. Vernalization response, or high temperature inhibition of reproductive development, is widespread in temperate plant species (Flood and Halloran, 1986). Winter wheat requires exposure to a continuous cold treatment (vernalization) before reproductive initiation. Spring wheat generally does not have such a requirement, but some cultivars do respond to cold treatment by flowering early (Levy and Peterson, 1972; Jedel et al., 1986; Iqbal et al., 2006).

Vernalization sensitivity or insensitivity in hexaploid wheat is controlled by alleles at the major vernalization loci, Vrn-A1, Vrn-B1, Vrn-D1, and Vrn-D5 (Pugsley, 1971, 1972), located on the long arms of group 5 chromosomes (Law and Worland, 1997; McIntosh et al., 2003). Winter wheat possesses recessive alleles at all loci, while spring wheat has dominant alleles at one or more of these loci. The spring habit allele of Vrn-A1 locus is epistatic and confers complete insensitivity to vernalization; spring habit alleles of Vrn-B1, Vrn-D1, and Vrn-D5 confer low sensitivity to vernalization (Pugsley, 1971, 1972).

Whole-chromosome substitution lines have been used effectively in the inheritance studies of various quantitative traits of wheat (Law, 1966, 1967). These lines differ only in the particular chromosome(s) substituted, facilitating the determination of the chromosomal location of genes, provided no other genes on the substituted chromosome influence the trait of interest (Kosner and Pankova, 1998). Such lines have proved useful in studying the genetics of heading time in the hexaploid wheat. Halloran and Boydell (1967) determined the chromosomal location, number and nature (major or minor) of the Vrn genes in wheat using chromosome substitution lines. Law et al. (1976) studied the role of chromosome 5A and 5D in the genetic control of ear-emergence time of wheat using ‘Chinese Spring’/‘Hope’ single chromosome substitution lines. Miura and Worland (1994), using chromosome substitution lines in the Chinese Spring background, identified genes controlling ear emergence time on homoeologous group 3 chromosomes. Kosner and Pankova (1998) used single chromosome substitution lines to detect allelic variants at the recessive vrn loci of winter wheat. Major and Whelan (1985) reported significant effects of the substituted chromosomes 5A, 5B, and 5D carrying specific Vrn gene(s) on relative maturity in the greenhouse by using ‘Cadet’/‘Rescue’ spring wheat reciprocal substitution lines. However, the effect of the particular substituted chromosome (carrying specific Vrn genes) in the Cadet/Rescue substitution lines on flowering and maturity time and other traits of agronomic importance under field conditions is not known.

An evaluation of spring wheat genotypes with different Vrn gene combinations under different seeding dates may improve our knowledge of the effect of these genes, not only on flowering and maturity time but also on related agronomic and quality traits in northern growing regions, including western Canada. The present study was conducted to (i) investigate the effect of planting time on flowering or maturity time and important agronomic traits in a set of chromosome substitution lines of known Vrn genes; (ii) uncover the effect of specific Vrn genes on flowering or maturity and important agronomic traits, and (iii) investigate whether some Vrn gene combinations offer advantages over others in terms of maturity and grain yield in northern spring wheat growing regions.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
The genetic material consisted of a set of six reciprocal whole-chromosome substitution lines of homoeologous group 5 chromosomes in two Canadian hard red spring wheat cultivar (Cadet and Rescue) backgrounds (Kosmolak et al., 1980), and eight Canadian spring wheat cultivars, Barrie (McCaig et al., 1995), Cutler (Briggs et al., 1991), Intrepid (DePauw et al., 1999), Park, Thatcher, Marquis, Taber (Knox et al., 1992), and Foremost (Thomas et al., 1997), representing the range of maturity in Canadian spring wheats. The chromosome substitution lines and their parents represent eight haplotypes arising from all possible allelic combinations at Vrn-A1, Vrn-B1, and Vrn-D5 loci. The Vrn genotypes of the chromosome substitution lines and the two parents, previously determined by Roberts and Larson (1985), were verified and those of the eight Canadian spring wheat cultivars were determined using genome specific primers for Vrn-A1, Vrn-B1, and Vrn-D1 genes (Iqbal, unpublished data, 2006) (Table 1). The first letter in each code of the chromosome substitution line shows the recipient parent, the second letter defines the donor parent of the specific substituted chromosome, and the last letter specifies the substituted chromosome. For example, CR5A means that chromosome 5A of Rescue has been substituted into Cadet.


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  		Table 1. Vernalization genotypes of a set of reciprocal whole chromosome substitution lines and eight Canadian spring wheat cultivars.

 
Field trials were conducted at two locations in Edmonton, AB (53°34'N, 113°31'W), during the summers of 2004 and 2005. Soils at the experimental sites were Black Chernozems (AAFRD, 2004). Total seasonal precipitation and average temperatures for the 2 yr are presented in Table 2. At each of the two locations, the 16 genotypes were planted on three seeding dates, 2 wk apart in a split-plot design with four blocks. Seeding dates were assigned to main-plots and genotypes within seeding dates to subplots. Seeding dates and genotypes were replicated four times. Seeding dates were 7 May, 20 May, and 3 June in 2004, and 3 May, 17 May, and 31 May in 2005. Plot size was six rows, 4 m long with row spacing of 0.23 m. Planting density was 350 seeds m–2. Fertilizer was applied according to soil test recommendations. During 2004, 100 kg ha–1 46–0–0 (N–P2O5–K2O) and 40 kg ha–1 11–52–0 (N–P2O5–K2O) was applied with the seed, in addition to 85 kg ha–1 46–0–0 that was banded into soils at both locations in fall 2003. During 2005, 160 kg ha–1 46–0–0 and 30 kg ha–1 11–52–0 was applied before seeding, and 30 kg ha–1 11–52–0 was applied with the seed at both locations. Weeds were controlled by the application of post emergence herbicide MCPA Amine 500 (Nufarm Agricultural Inc., Calgary, AB) at 1235 mL ha–1.


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  		Table 2. Average air and soil temperatures and total precipitation during 2004 to 2005 at the Edmonton Research Station of University of Alberta, Canada.

 
Data were recorded on days from seeding to anthesis and physiological maturity, plant height, number of spikes per square meter, grains per spike, grain weight, and grain yield and grain protein content. Time of anthesis was determined visually as the day when 75% of the heads in a plot dehisced anthers. Physiological maturity was visually determined as the number of days from seeding to when 75% of the peduncles in a plot completely lost green color. The number of calendar days from seeding to anthesis and maturity, in each year, was converted to growing degree days by summing the average daily temperatures (over a base temperature of 0°C) from the date of seeding to the date when anthesis or maturity was recorded. The number of spikes in a randomly chosen 0.5 by 2 m row plot area was counted and recorded as spikes per square meter. Grain number per spike was calculated from 10 randomly sampled heads immediately before harvest. Grain protein content (%) was determined using near-infrared reflectance (NIR) spectroscopy using a Monochromator NIR Systems model 6500 (NIRSystems, Inc., Silver Springs, MD).

Due to the significant interaction of year with seeding date and genotype, data for individual years were analyzed separately in the MIXED procedure of SAS (SAS Institute, 2003) with location, seeding date, seeding date x location, genotype, genotype x location, genotype x seeding date, and genotype x seeding date x location considered fixed, and block (location) and block (location x seeding date) considered random. Least square means were estimated by genotypes for all seeding dates and comparisons within and between seeding dates made using an ESTIMATE statement. The genetic effects of Vrn genes were determined (using an ESTIMATE statement) assuming that the substituted chromosomes differ only in Vrn genes and that the genetic background has no effect. Genetic effects of Vrn genes were determined using pooled data from the three seeding dates. Due to the nonestimable means of the winter-type line (CR5A), maturity and yield data were used only from first seeding date in both years, while grain weight and grain protein data were used from first seeding date in 2004, and from first and second seeding dates in 2005. To study the response of genotypes to seeding dates, a Shifted Multiplicative Model (SHMM) (Seyedsadr and Cornelius, 1992) with a cluster method (Crossa et al., 1993) was used following Navabi et al. (2006). For grouping the genotypes into subsets with minimum crossover interactions, an arbitrary cut-off point was chosen at a distance of 1.5 in the dendrogram.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION AND CONCLUSIONS
 REFERENCES
 
Mean seasonal temperatures did not vary greatly between the 2 yr (Table 2). Mean air and soil temperatures in the week following first seeding date were low in both years. Seeding date had a significant (P < 0.05) effect on all traits, except grain weight, in both years, and grain protein in 2005 (Table 3). Genotypes differed (P < 0.01) for all traits in both years. Genotypes x seeding dates were also significant (P < 0.01) for all traits, but were of lower magnitude than one or both of the main effects, as indicated by the F values (Table 3). Seeding dates had a narrower range (25°C days) for days to anthesis in 2005 compared to 2004 (104°C days).


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  		Table 3. Analysis of variance for eight traits measured on 10 genotypes grown over three seeding dates in central Alberta, Canada, during 2004–2005.

 
During 2004, most genotypes took the minimum degree days to anthesis and maturity when seeded earliest and the maximum degree days at the second seeding date (Table 4). Plant height increased for most of the genotypes with delayed seeding (data not given). Grains per spike generally decreased with delayed seeding. Grain weight did not differ with seeding date for half of the genotypes (Table 4). Most genotypes yielded the most grain when seeded on the second date. Grain yields were lower when seeded earliest in 2004 which was most likely because of the delayed harvest. Cool and wet weather conditions (including snow) resulted in the delayed harvest of first seeding date. Grain protein content of most genotypes was lowest when seeded last (Table 4).


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  		Table 4. Least squares means for five traits of 16 genotypes planted on three seeding dates (SD) at two locations in Alberta, Canada, during 2004.

 
During 2005, most genotypes flowered earlier in the third and later in the first seeding date (Table 5). Nearly all genotypes matured earlier when planted earliest and late when planted last. Plant height did not vary between first and second seeding date but generally increased for the third seeding date (data not given). Grain weights were generally the greatest when seeded last. Grain yields of all genotypes differed with seeding date, exhibiting a decrease with delayed seeding (Table 5). Grain protein content was generally the lowest when planted latest for genotypes whose grain protein content differed with seeding date (Table 5).


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  		Table 5. Least squares means for five traits of 16 genotypes planted on three seeding dates (SD) at two locations in Alberta, Canada, during 2005.

 
The earliest flowering and maturing genotypes during both years were those possessing three spring habit Vrn alleles (RC5A), spring habit alleles Vrn-A1 and Vrn-B1 (Park, CR5B) or spring habit Vrn-A1 only (Cutler, Intrepid) (Tables 5, 6). Though Cadet, Marquis, and Thatcher also carry spring habit allele at Vrn-A1, they did not flower and mature as early as the other carriers of spring habit Vrn-A1. RC5A also had the highest grain protein in all three seeding dates during both years (Tables 4, 5). CR5B, besides being early flowering or maturing, was also among the top three yielding chromosome substitution lines in 2005. Taber (vrn-A1 Vrn-B1 vrn-D1), CR5D (Vrn-A1 vrn-B1 Vrn-D5), and RC5B (vrn-A1 vrn-B1 Vrn-D5) were the latest flowering or maturing spring habit genotypes in both years. The carrier of Vrn-A1 Vrn-D5 (CR5D) and vrn-A1 Vrn-D5 (RC5B) had the highest grain weights among the chromosome substitution lines in both years. The cultivar Park, with two spring habit Vrn alleles, was the earliest maturing among the eight Canadian spring wheat cultivars. It also had a high grain yield and protein content, compared to cultivars of the same quality class (Barrie, Intrepid, Marquis, and Thatcher) (Tables 4, 5).

The additive effects of Vrn-A1, Vrn-B1, and Vrn-D5 on days to anthesis and maturity were significant and negative in both years (Table 6). The negative additive effects of all three genes resulted in earlier anthesis and maturity of genotypes carrying spring habit alleles of the three genes (Table 7). Among the three genes, Vrn-B1 had the greatest, while Vrn-D5 had the least effect on accelerating anthesis and maturity in both years, except in 2004, where the effect of Vrn-B1 on maturity was lower than that of Vrn-A1 (Table 6). All three digenic interaction effects were significant for days to anthesis in both years and for days to maturity in 2004. The only significant digenic interaction for days to maturity in 2005 was that of Vrn-A1 and Vrn-D5. The digenic interactions were positive, thereby resulting in the inhibition of the additive effects of the three genes. The inhibitory effect was the greatest in the carrier of spring habit Vrn-A1 and Vrn-D5 alleles and the least in the carrier of spring habit Vrn-A1 and Vrn-B1 alleles (Tables 6, 7). The effects of trigenic interaction were significant and negative on days to anthesis and maturity in both years. Due to this effect, the genotype carrying spring habit alleles at three Vrn loci was the earliest flowering and maturing in both years (Table 7). Some of the genetic effects of Vrn genes were also significant on plant height, grain weight, grain yield, and grain protein (Table 6).


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  		Table 6. Genetic effects of vernalization response genes on maturity and related agronomic traits measured on a set of chromosome substitution lines tested in Alberta, Canada, during 2004 to 2005.

 

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  		Table 7. Least squares means for six traits of eight haplotypes of Vrn genes grown in three seeding dates in Alberta, Canada, during 2004 to 2005.

 
The shifted multiplicative model with cluster method classified the 16 genotypes into three main groups for days to anthesis (Fig. 1) and maturity. The winter-type line CR5A and Taber exhibited similar responses to seeding dates in three of the four environments, where their flowering time (°C days) increased with delayed seeding. Most genotypes did not group into any discernable patterns with respect to their vernalization genes. For example, 14 of the 16 genotypes clustered together in the Michener 2004 environment (Fig. 1). Most of these genotypes differ in vernalization response genes, but their close grouping suggests similar responses to seeding dates. Similar results were observed for the other three environments (Fig. 1).


Figure 1
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  		Figure 1. Dendrograms resulting from a Shifted Multiplicative Cluster Analysis for days to anthesis of 16 genotypes grown on three seeding dates over four environments at Alberta, Canada.

 

   DISCUSSION AND CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
REFERENCES
 
Although significant genotype x seeding date interactions were observed within each year and location for flowering and maturity time in the present study, these interactions did not follow a pattern that could be ascribed to the vernalization response of the genotypes. With few exceptions, the relative flowering and maturity times (°C days) or ranks of the genotypes did not change from one seeding date to another. These results suggest that vernalization responses of sensitive spring wheats are generally not altered under western Canadian growing conditions. The presence of different Vrn genes did alter flowering and maturity times and related agronomic traits of the chromosome substitution lines tested. We identified specific combinations of Vrn genes that may be advantageous in northern spring wheat growing regions.

No clear pattern, in terms of degree days required for flowering and maturity, was observed for the three seeding dates in the present study. In terms of calendar days, the time to flowering and maturity of spring-sown wheat in western Canada generally decreases with delayed seeding (Nass et al., 1975; Baker, 1990; Cutforth et al., 1990). We did not observe this pattern in terms of growing degree days in the present study. However, for the line CR5A (carrying winter habit alleles vrn-A1, vrn-B1, and vrn-D5), the degree days required to flower increased with delayed seeding, and this pattern was more pronounced in 2004. Both vernalization sensitive and insensitive genotypes responded similarly to seeding dates, indicating that the change in flowering and maturity times of the genotypes between seeding dates was not due to the vernalization response of the genotypes. With the exception of first seeding date in 2004, grain yield decreased with delayed seeding, which supports the results reported by Ciha (1983) and Hucl (1995) and reinforces the importance of early seeding of spring wheat in northern regions.

The ideal genetic material for examining the interaction between Vrn genes would have been a set of near isogenic lines of major Vrn genes. However, such lines are currently not available in Canadian spring wheat backgrounds and their development is cumbersome. Nevertheless, results of this study provide useful information regarding the interaction of major Vrn genes in wheat. Pugsley (1971, 1972) reported the spring habit Vrn gene at Vrn-A1 to be epistatic to those at Vrn-B1, Vrn-D1, and Vrn-D5. However, in this study, Vrn-A1 was not found completely epistatic to Vrn-B1 and Vrn-D5, as was evident from the differences in the flowering and maturity times of chromosome substitution lines (with similar genetic background) having Vrn-A1, but differing in either Vrn-B1 or Vrn-D5. This lack of epistatic effect of Vrn-A1, to the best of our knowledge, has not been previously reported. The combination of Vrn-D5 with Vrn-A1 resulted in later flowering and maturity. The spring or winter habit allele at Vrn-D5 is not known for the eight Canadian spring wheat cultivars. The difference in the flowering and maturity times of these cultivars may be due to the presence of Vrn-D5 and/or different earliness per se genes. The flowering and maturity times of genotypes with spring habit alleles at Vrn-B1 and Vrn-D5 were not different from genotypes with spring habit allele at Vrn-B1, suggesting that the spring habit allele of Vrn-B1 is probably epistatic to the Vrn-D5 allele. Shindo et al. (2003) reported that the spring habit allele of Vrn-B1 was epistatic to the spring habit allele of Vrn-D1.

The present study identified combinations of Vrn genes that may offer advantage in northern spring wheat growing regions. Genotypes carrying spring habit alleles at three Vrn loci may provide extreme earliness in regions with a very short growing season. Although grain yields of such genotypes may be comparatively lower than their late counterparts, these genotypes may be less prone to early fall frost. Genotypes with spring habit alleles at Vrn-A1 and Vrn-B1 appear to be a preferred Vrn genotype in high northern latitudes, as these combine both earliness and acceptable grain yields. Genotypes carrying Vrn-B1 singly or in combination with either Vrn-A1 or Vrn-D5 flower and mature comparatively earlier than the carriers of Vrn-D5 either singly or in combination with Vrn-A1. The latter genotypes may have a high number of grains per spike and grain weights, due to their longer vegetative and grain fill period, and may yield well in regions with comparatively longer growing seasons. Such genotypes may be planted as early in the growing season as possible to avoid yield declines, which were observed in the later seeding dates in this study.

Photoperiod genes also play an important role in the flowering and maturity times of wheat. However, due to the long spring and summer days in high northern latitudes, the photoperiod requirement of sensitive spring wheat is most likely fulfilled (Klaimi and Qualset, 1973; Knott, 1986; Marshall et al., 1989) and flowering time is, therefore, not greatly affected by photoperiod response genes. Nevertheless, photoperiod sensitivity may offer a yield advantage in some regions of the high northern latitudes (Dyck et al., 2004). Variation in the flowering and maturity times of the genotypes with similar Vrn genes in the present study suggests that these genotypes most likely differ in earliness per se genes. The greatest additive effects observed for Vrn-B1 in this study may possibly be confounded with the effects of earliness per se genes, as Cadet is known to have a longer basic vegetative phase than Rescue (Roberts and Larson, 1985). Earliness per se genes are generally considered to have minimal influence on developmental rate compared to photoperiod and vernalization genes. However, in the short growing season of high northern latitudes, these genes may cause flowering and maturity time variation large enough to escape drought stress during grain filling or early fall frosts in some regions. The incorporation of earliness per se genes is therefore necessary, along with a desirable Vrn gene combination, in the development of early maturing cultivars for the spring wheat growing regions of high northern latitudes. We did not find the spring habit Vrn-D1 allele in 42 Canadian spring wheats (including genotypes of this study) characterized for Vrn genes (Iqbal, unpublished data, 2006). However, incorporation of Vrn-D1 has been strongly recommended in spring wheats to increase grain yield and improve adaptation to late drought and heat stress (Stelmakh, 1993). It may, therefore, be useful to incorporate Vrn-D1 either singly or in combination with other Vrn genes in high northern latitude spring wheats. Polymerase chain reaction markers have recently become available for the major vernalization response genes Vrn-A1, Vrn-B1, and Vrn-D1 in wheat (Yan et al., 2004; Fu et al., 2005). The use of marker-assisted selection for these genes may, therefore, help in the development of early maturing spring wheat cultivars with higher grain yield potential.


   ACKNOWLEDGMENTS
 
We gratefully acknowledge the financial support to the senior author from the Ministry of Education, Government of Pakistan. This research was also supported by research grants to the University of Alberta wheat breeding program from the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Alberta Agricultural Research Institute (AARI), and the Alberta Crop Industry Development Fund Ltd. (ACIDF), Canada. We would also like to express our gratitude to Klaus Strenzke and Cliff Therou for technical assistance.


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
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Received for publication September 26, 2006.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 




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