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CROP PHYSIOLOGY & METABOLISM

Seeding Date Alters Carbohydrate Accumulation in Winter Wheat

Agriculture and Agri-Food Canada, Lethbridge Research Centre, PO Box 3000, Lethbridge, AB T1J 4B1, Canada

* Corresponding author (gaudetd{at}em.agr.ca)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Early seeding and accumulation of soluble carbohydrates is critical for the full expression of snow mold [caused principally by Typhula incarnata Lasch ex. Fr., Typhula ishikariensis Imai, Sclerotinia borealis Bub. & Vleug., Microdochium nivalis (Ces. ex Berl. & Vogl.) Samuels and Hallet] resistance in winter wheat (Triticum aestivum L. em Thell). Experiments were conducted during two growing seasons in the field at Lethbridge, AB, to study how early seeding affects the quantity of simple sugars and fructan, and degree of fructan polymerization in both resistant and susceptible cultivars. Different planting dates were employed to obtain plants at different developmental stages prior to winter dormancy. Leaf and crown tissue samples were collected from 14 winter wheat cultivars differing in snow mold resistance throughout the autumn, winter, and early spring. Snow mold resistant cultivars accumulated moderate levels of simple sugars and high levels of fructans across seeding dates and maintained a higher degree of polymerization of fructans compared with snow mold susceptible cultivars. Early seeded treatments generally accumulated lower levels of simple sugars and higher levels of more highly polymerized fructan in the autumn and winter than did late seeded treatments. The closest correlations between snow mold resistance and fructan content (r = 0.87) or degree of polymerization (r = 0.74) were observed in the later seeded treatments, suggesting that early seeding masked the expression of genotypic snow mold resistance. These results demonstrate an association between early seeding and fructan accumulation in relation to snow mold resistance in winter wheat and provide a physiological basis for the higher level of snow mold resistance among early seeded treatments.

Abbreviations: ANOV, analysis of variance • SE, standard error, DP, degree of polymerization of fructan

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
IN NORTHERN AGRICULTURAL AREAS, winter cereals must survive prolonged exposure to subzero temperatures. In some regions, a deep and persistent snow cover provides protection but coincidently provides a dark and humid environment with soil temperatures between 0°C and -7°C that favours the growth of snow molds which can severely damage crops (Gaudet et al., 1989). As a consequence, winter wheat often fails to survive even when plants are not exposed to lethal cold temperatures (Tomiyama, 1955; Bruehl, 1982; Gaudet and Bhalla, 1988). The most important species of snow mold fungi are Typhula incarnata, Typhula ishikariensis, Sclerotinia borealis, Microdochium nivalis, and cottony snow mold caused by unidentified Low Temperature Basidiomycetes (LTB). Resistance to the majority of these fungi is correlated (Bruehl, 1982; Gaudet and Kozub, 1991). The prevalence and severity of the different snow mold species is influenced by the duration of snow cover and the average subnival temperatures (Bruehl, 1982; Gaudet et al., 1989; Nissinen, 1996).

During the autumn and early winter, the gradual decrease in average ambient temperature induces a hardening process in plants, which is essential for the development of both snow mold resistance and freezing resistance (Årsvoll, 1977; Levitt, 1980; Tronsmo, 1985; Gaudet, 1994). Cold acclimated winter cereals exhibit a reduction in growth rate and height, a decrease in leaf surface area, and an increase in plant dry matter (Krol et al., 1984). Concomitant with the dry matter increase is a decrease in relative water content and an increase in cytoplasmic content (Krol et al., 1984; Yoshida et al., 1998). Winter cereals and grasses accumulate large quantities of soluble carbohydrate, predominantly fructans, in cell vacuoles of the leaf and crown tissues in response to hardening conditions during autumn and early winter (Pollock and Cairns, 1991).

Fructans in winter cereals are polymers of fructose, possessing ß -2-1 and ß -2-6 linkages (Bancal and Gaudillère, 1989; Hurry et al., 1994, 1995). Synthesis of fructans is mediated by sucrose:sucrose fructosyltransferases (SST) and sucrose:fructan fructosyltransferase (SFT), which converts two sucrose molecules into 1- or 6-kestose plus glucose, respectively. Sucrose:fructan fructosyltransferase continues the elongation process, transfering fructose from sucrose to the 1-kestose, 6-kestose, or neokestose, and/or fructans. Additional oligosaccharide elongation continues through addition of fructose via fructan:fructan fructosyltransferase (FFT) (Duchateau et al., 1995). During the winter and early spring, fructans are slowly metabolized by various ß-fructosidases (invertase, fructan endohydrolase, and fructan exohydrolase) (Olien and Clark, 1993; Livingston and Henson, 1998). Increased levels of dry matter and soluble sugars have been associated with both snow mold resistance and freezing tolerance (Kiyomoto and Bruehl, 1977; Levitt, 1980). Snow mold resistant cultivars accumulate higher levels of carbohydrates, especially fructan, in crowns and metabolize them more slowly during the winter and early spring than do susceptible cultivars (Kiyomoto and Bruehl, 1977; Amano, 1987; Bengtsson, 1989; Yoshida et al., 1998).

Resistance to snow molds in winter cereals and grasses is also related to the developmental stage of the plant, and hence plant age. Under controlled environment conditions, older hardened winter wheat plants express higher resistance to snow molds (Gaudet and Chen, 1987; Gaudet and Kozub, 1991; Gaudet, 1994; Nakajima and Abe, 1996) than do younger plants similarly hardened. Even susceptible cultivars can develop substantial resistance to snow mold if growing conditions permit the development of many tillers and large crowns (Gaudet and Chen, 1987; Gaudet and Kozub, 1991; Nakajima and Abe, 1996). The maximum expression of snow mold resistance has been associated with early seeding of winter wheat in the Pacific northwestern USA which permits the development of large plants with numerous tillers (Bruehl, 1967, 1982). Bruehl (1967) demonstrated that optimum date for planting winter wheat to survive attack from snow molds was mid- to late-August. When sowing was delayed until mid-September, cultivars normally resistant to snow molds became susceptible. A similar relationship between seeding date and resistance has been demonstrated in the deep snow regions of central and northern Alberta where snow mold damage is common (Gaudet et al., 1989).

To date, little is known about the biochemical and molecular nature of snow mold resistance. Because early seeding of winter wheat is critical for the full expression of snow mold resistance, knowledge of how early seeding affects carbohydrate accumulation is essential in understanding how snow mold resistance develops in both resistant and susceptible cultivars when they are seeded early. We studied the effect of seeding date on the quantity of simple sugars and fructan, and the degree of polymerization of fructan in the crowns and leaves of winter wheat cultivars exhibiting different levels of snow mold resistance throughout the autumn, winter and early spring of 1997-1998 and 1998-1999 in the field at Lethbridge, AB.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Genetic Materials
The relative snow mold ratings for cultivars used in this study were previously determined under field and controlled environment conditions (Bruehl, 1967; Gaudet and Kozub, 1991) and are presented in Table 1. Doubled haploid lines were generated by the corn hybridization technique (Suenaga and Nakajima, 1989). Lines 268-4a-3 and 272-2a-2 were derived from the F₁ of a Norstar/PI181268 cross; doubled haploid line 3-8a-3 was derived from the F₁ of a CI14106/ Norstar cross whereas line 63-4a-1 was derived from a Norstar/CI14106//Norstar cross.

Carbohydrate Extraction
Plants were thoroughly washed and crowns (2-cm section of stem tissues directly above the roots) and leaves were sampled and processed separately. Both fresh and dry weights were recorded. Samples were freeze dried and ground with a Wiley mill. Following storage at -20 or -40°C for 4 to 10 mo, 0.1 g of crown and leaf tissue from each replicate was extracted three times in hot (95°C) water according to Harrison et al. (1997) and the supernatant from the three successive extractions were combined. The samples were then frozen at -20 or -40°C until employed in carbohydrate determination assays. Fructose, glucose, and sucrose were quantified enzymatically with a sugar determination kit (Boehringer-Mannheim, Indianapolis, IN). Total simple sugars were determined as the sum of fructose, glucose, and sucrose. For quantification of fructan, carbohydrates were hydrolysed following treatment for 15 min with 10% (v/v) sulfuric acid according to Somani et al. (1987); total fructan was determined as the sum of the fructose and glucose content following acid hydrolysis, minus the prehydrolysis sucrose, fructose, and glucose content. The fructose:glucose ratio of the total fructan represented the degree of polymerization (DP). Results are expressed as milligrams of carbohydrates per gram of dry weight.

Statistical Analysis
To establish the response of carbohydrate levels during sampling dates, linear and quadratic equations for simple sugars, fructan, and DP were fitted to each of the three replicates over the seven and eight sampling dates within years. Intercepts, linear, and quadratic coefficients for each parameter were then analysed by ANOV. Because cultivar x seeding date interactions were evident, ANOV also was conducted within cultivars to determine seeding date effects. Pearson correlations were calculated between the raw means and the relative resistance rating of the cultivars for each year x seeding date x sampling date treatment. Pairwise t-tests were conducted among seeding dates within each sampling date x cultivar treatment combination.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
There was a progressive decrease in daily average temperatures from mid-September to mid-January in both years. The weather during both the autumn and winter was relatively mild in both years with similar average monthly soil temperatures at crown level (2 cm), and snow cover was sparse and transient. A notable difference between the two winters was observed during September to November in 1997, which was cooler (21% fewer degree-days) than the same period in 1998. Soil temperatures of -2°C and lower observed during 1997-1998 at a 2-cm depth in the middle of November and again in early December. In contrast, freezing temperatures below -2°C were only observed during 1998-1999 after 19 December

Seedling emergence for PI173438 during the autumn of 1998 was poor and was likely a reflection of poor seed quality. Consequently, not all sampling dates are represented. In general, plants survived the winter in good condition with slight winterkill (<5%) in the very tender cultivar Cappelle Desprez observed in March and April sampling dates in 1999 only. While obvious growth was not evident on 14 April, soil temperatures were sufficiently high during both years to permit increased metabolic activity in winter wheat crowns.

Similar patterns for the accumulation of simple sugars and fructans, and for changes in degree of polymerization, were observed in both leaves and crowns during the two years of study. However, average levels of simple sugars, fructan, and values for degree of polymerization, were 22, 67, and 34% higher, respectively, in crowns than in leaves in 1997-1998, and 40, 81, and 26% higher, respectively, in crowns than in leaves 1998-1999. Data are presented for crowns only.

Simple Sugars
Patterns for total simple sugars in crowns for eight representative cultivars, during 1997-1998 and 1998-1999, are shown in Fig. 1 and 2, respectively. Differences were observed among cultivars and seeding dates in their intercepts, linear, and quadratic coefficients for fructan in leaves and crowns in both years (P < 0.01). The patterns of accumulation and loss of total simple sugars were similar among cultivars during both winters but some differences were apparent during 1997-1998. For example, in CI14106, Blizzard, PI173438, Cappelle Desprez and Omar, maximum simple sugar levels were observed in November followed by a gradual decline during the winter, whereas in PI181268 and Norstar, maximum levels were observed from December to February (Fig. 1). Maximum levels of simple sugars observed in most cultivars on 19 November coincided with the occurrence of below freezing soil temperatures beginning on 15 November. During the 1998-1999 winter, maximum levels were generally observed during the January and February sampling dates followed by a rapid decline during March and April (Fig. 2). During 1998-1999, simple sugar levels attained maximum levels on 14 January, which followed the occurrence of below freezing soil temperatures, first recorded on 19 Dec. 1998.

View larger version (31K): [in this window] [in a new window]   Fig. 1. Simple sugar (fructose + glucose + sucrose) levels in crown tissues of winter wheat cultivars during the autumn, winter, and early spring of 1997-1998 following seeding on 23 August or 20 September at Lethbridge, AB. Vertical bars represent standard error values.

View larger version (33K): [in this window] [in a new window]   Fig. 2. Simple sugar (fructose + glucose + sucrose) levels in crown tissues of winter wheat cultivars during the autumn, winter, and early spring of 1998-1999 following seeding on 8 August, 6 September, or 23 September at Lethbridge, AB. Vertical bars represent standard error values.

Differences were observed in total simple sugars among cultivars (P < 0.0001). The lowest average levels of total simple sugars during both winters were observed in the snow mold resistant cultivar PI173438, and the highest average levels in snow mold susceptible cultivars Norstar, Gaines, and Omar (Fig. 1, 2). However, correlation coefficients between snow mold resistance and levels of the individual simple sugars, or the total simple sugar generally were nonsignificant (data not shown) in leaves or crowns among seeding and sampling dates, in either winter.

Fructans
Patterns for fructan accumulation in crowns among the representative cultivars, during 1997-1998 and 1998-1999, are shown in Fig. 3 and 4. The patterns of fructan accumulation and metabolism in crowns were similar in both winters although average fructan levels were, on average, 18% lower in 1997-1998 compared with1998-1999. This difference may have been due to the occurrence of 21% fewer degree days recorded during September to November in 1997 than recorded in 1998. Fructan levels in crowns for all seeding dates, increased to a maximum level between late December and early February and decreased rapidly during March and April. Differences were observed among cultivars and seeding dates in their intercepts, linear, and quadratic coefficients for fructan levels in crowns in both years (P < 0.01). In general, the snow mold resistant cultivars such as CI14106, Blizzard, PI173438, and PI181268 exhibited high average fructan levels throughout the winter compared with low levels observed in the snow mold susceptible cultivar Cappelle Desprez (Fig. 3, 4). The snow mold susceptible cultivars Gaines and Omar initially developed moderate levels of fructans during November and December, but levels rapidly decreased during January to March.

View larger version (29K): [in this window] [in a new window]   Fig. 3. Fructan levels in crown tissues of winter wheat cultivars during the autumn, winter, and early spring of 1997-1998 following seeding on 23 August or 20 September at Lethbridge, AB. Vertical bars represent standard error values.

View larger version (33K): [in this window] [in a new window]   Fig. 4. Fructan levels in crown tissues of winter wheat cultivars during the autumn, winter, and early spring of 1998-1999 following seeding on 8 August, 6 September, or 23 September at Lethbridge, AB. Vertical bars represent standard error values.

During both winters, fructan levels in the leaves and crowns were positively associated with the snow mold resistance ratings for the winter wheat cultivars (Tables 2, 3). In general, the correlation coefficients were similar for fructans in both crowns and leaves and followed the same patterns during both 1997-1998 and 1998-1999. During 1997-1998, correlation values were highest (P < 0.0001) from 19 November to 30 December in the late seeded treatments; the highest values were r = 0.80 in leaves and r = 0.81 in crowns. Among early seeded treatments, correlation values reached r = 0.54 in the crowns and r = 0.66 in the leaves of the 8 October sample (Table 2). During 1998-1999, high correlation values (P < 0.0001) between fructan levels and snow mold resistance ratings were observed in the both the normal and late seeded treatments. The highest correlation values observed were r = 0.82 and r = 0.87 in crowns during the 15 February and 18 March sampling dates, respectively, in the crowns of the normal seeding date. In leaves, the highest correlation value was r = 0.83, observed during the 15 Dec. 1998 sampling date in the late seeded treatments.

View larger version (29K): [in this window] [in a new window]   Fig. 5. Degree of polymerization values of fructans in crown tissues of winter wheat cultivars during the autumn, winter, and early spring of 1997-1998 following seeding on 23 August or 20 September at Lethbridge, AB. Vertical bars represent standard error values.

View larger version (34K): [in this window] [in a new window]   Fig. 6. Degree of polymerization values for fructans in crown tissues of winter wheat cultivars during the autumn, winter, and early spring of 1998-1999 following seeding on 8 August, 6 September, or 23 September at Lethbridge, AB. Vertical bars represent standard error values.

During both winters, DP values in both the leaves and crowns were positively associated with the snow mold resistance ratings for the winter wheat cultivars (Tables 2, 3). Correlation coefficients between the snow mold resistance ratings and DP values were similar in both crowns and leaves and followed the same patterns in 1997-1998 and 1998-1999. In 1997-1998, they were highest (P < 0.0001) from 19 November to 14 April in the late seeded treatments (r = 0.63 in leaves and r = 0.56 in crowns). Among early seeded treatments, correlation values were generally lower (Table 2). In 1998-1999, the highest correlation values (P < 0.0001) between DP levels and snow mold resistance ratings were observed during 17 November to 20 April in the normal seeded treatment, and during the 15 December sampling date in the late seeded treatment. The highest values observed were r = 0.74 in crowns and r = 0.73 in leaves (Table 3).

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Previous studies have demonstrated that enhanced snow mold resistance among winter wheat cultivars is associated with (i) the accumulation of high levels of soluble carbohydrates, especially fructans, in crowns during autumn and (ii) elevated fructan content remaining in the spring, after snow mold infection (Kiyomoto and Bruehl, 1977; Kiyomoto, 1987; Yoshida et al., 1998). This study demonstrated that both the quantity and nature of carbohydrate accumulation and utilization in winter wheat leaves and crowns at Lethbridge, AB, was influenced by plant genotype, developmental stage of the winter wheat plant prior to dormancy, stage of winter dormancy, and environmental conditions prevailing throughout the autumn and winter. Snow mold resistant cultivars accumulated moderate levels of simple sugars, and high levels of fructan across seeding dates, and maintained a higher degree of polymerization of fructan compared with snow mold susceptible cultivars. Early seeded treatments generally accumulated lower levels of simple sugars, and higher levels of more highly polymerized fructan than did late seeded cultivars. These results suggest that the developmentally based snow mold resistance in winter wheat first reported by Bruehl (1967) is related to the accumulation and maintenance of high levels of more highly polymerized fructans in leaf and crown tissues and is consistent with the hypothesis that these complex carbohydrates are involved in the expression of both the genotypically based and developmentally based snow mold resistance (Bruehl, 1967, 1982; Årsvoll, 1977; Gaudet and Chen, 1987; Gaudet and Kozub, 1991; Nakajima and Abe, 1996; Yoshida et al., 1998).

The pattern of simple sugar metabolism throughout both winters of this study at Lethbridge, AB, is consistent with those reported for winter wheat in the Pacific Northwest USA (Kiyomoto and Bruehl, 1977; Kiyomoto, 1987) and northern Japan (Yoshida et al., 1998). In the present study, the later seeded treatments accumulated higher quantities of simple sugars during second stage hardening than the early seeded treatments. These results suggested that crown tissues of early seeded treatments were less capable of accumulating elevated simple sugar levels in response to below freezing temperatures. These results may provide a physiological rationale for seeding date recommendations for winter wheat in western Canada. Recommended seeding dates for southern Alberta range from 1 September to 15 September (Andrews et al., 1960; Pittman and Andrews, 1961). The smaller response in simple sugar accumulation during second stage hardening in early seeded treatments may contribute to the lower winter survival of early seeded treatments in southern Canadian prairies (Andrews et al., 1960; Pittman and Andrews, 1961).

Simple sugars were not associated with any directional change in snow mold resistance ratings in either early or late seeded treatments. The moderately susceptible cultivars Norstar and Gaines and the resistant cultivars CI14106 and PI181268 maintained high levels of simple sugars throughout the winter, whereas the resistant cultivar PI173438 and susceptible cultivars maintained relatively low levels of simple sugars throughout the winter. These results were consistent with those observed by Yoshida et al. (1998) for simple sugar level in cultivars grown in northern Japan.

Snow mold resistant cultivars accumulated high levels of fructan in both early and late seed treatments in response to hardening conditions in the autumn and early winter, and maintained the higher levels longer in the late winter and early spring. Conversely, the snow mold susceptible cultivars developed lower levels in the autumn, appeared unable to maintain the levels of fructan into the late winter and early spring, or both. Our results support those of Kiyomoto and Bruehl (1977) for winter wheats grown in the Pacific Northwest USA, and Yoshida et al. (1998) for winter wheat grown in northern Japan, and are consistent with previous studies demonstrating shoot dry matter increases with early seeding of winter wheat in western Canada (Fowler and Gusta, 1977; Fowler, 1982). Our results are the first to demonstrate an association between early seeding and fructan accumulation in plant tissues and provide a physiological basis for the higher level of snow mold resistance among early seeded treatments first reported by Bruehl (1967). In controlled environments, both snow mold resistance and fructan levels have been shown to be higher in older, hardened winter wheat plants than in young plants that were similarly hardened (Gaudet and Kozub, 1991; Gaudet et al., 2000). Thus, it appears that older plants accumulate higher fructan levels during autumn hardening than younger plants exposed to the same hardening conditions, while younger plants appeared to be more capable of accumulating simple sugars.

Total fructan levels across treatments were higher during 1998-1999 than in 1997-1998. This may have been due to the 21% increase in the number of growing degree days recorded during the autumn of 1998-1999 compared with 1997-1998; this would have permitted additional plant development and carbohydrate accumulation during the autumn. In the late seeded treatments, the additional growing degree days during 1998-1999 may have accounted for the smaller effect of seeding date on fructan levels among cultivars compared with 1997-1998.

In the present study, the association between high fructan levels during the winter, and snow mold resistance ratings was more pronounced in the later seeded treatments than in the early seeded treatments, despite the fact that early seeded treatments accumulated higher fructan levels. These results suggested that the higher average fructan levels in the earliest seeded treatments obscured the relationship between genotypic snow mold resistance among cultivars and fructan content in tissues. Similarly, genotypic differences in snow mold resistance also become obscured in larger, older plants under controlled environment conditions (Gaudet and Chen, 1987; Gaudet and Kozub, 1991).

The degree of polymerization of fructan in winter wheats during the late fall and early winter ranged between DP-8 and DP-10 which are consistent with values previously reported (Suzuki and Nass, 1988). Plant genotype, developmental stage, stage of dormancy during the winter, and environmental conditions influenced the DP of fructan in winter wheat crowns during both winters. This would be expected since the DP at any given time basically reflects the average size of fructan molecules (Suzuki and Nass, 1988; Pollock and Cairns, 1991). During 1997-1998, there were conspicuous cultivar x seeding date interactions whereby the DP in some cultivars such as CI14106, Gaines, and Cappelle Desprez were higher in early seeded than late seeded treatments; however, these differences were less evident during 1998-1999. This suggested that environmental conditions also strongly influence the interconversion between simple and complex carbohydrates. Moderately high correlation coefficients between snow mold resistance ratings and DP in the later seeded treatments between r = 0.50 and r = 0.64 during 1997-1998, and from r = 0.60 to r = 0.73 during 1998-1999 indicated that there was a tendency for snow mold resistant cultivars to develop and maintain higher DP of fructans throughout the winter and early spring. This suggested that one of the components of snow mold resistance may be related to maintainance of a higher DP of fructan.

The prominent decrease in degree of polymerization of fructan closely paralleled the increase in simple sugar levels following the occurrence of below freezing temperatures near -3°C at crown level during both winters and resulted in the onset of the second-phase of cold hardening described by Olien and Clark (1995) and Livingston and Henson (1998). The increase in levels of simple sugars and fructan, and decrease in the average DP of fructan in winter cereals crowns, following subfreezing temperatures resulted from an increase in the level and activity of invertase and fructan exohydrolase in the apoplast (Livingston and Henson, 1998).

Currently, there are no satisfactory hypotheses to explain the nature of snow mold resistance, although the accumulation and maintenance of high levels of polymerized fructans, throughout the winter and early spring, are clearly implicated. The concept of sugar-based resistance is not a novel one. Disease susceptibility can be correlated with sugar status in leaves for some diseases (for review, see Vanderplank, 1984). "Low sugar diseases" are those in which resistance increases with sugar levels in the leaves, and decreases when source-sink induced losses of sugars occur (Vanderplank, 1984). For snow mold diseases of winter cereals and grasses, the sink-induced loss of resistance corresponds to the depletion of carbohydrate in plant crowns during the winter and early spring. Resistance in winter cereals appears to be correlated not only with the fructan content in leaves and crowns but also the degree of polymerization of the fructan. Snow mold fungi might have a decreased ability to metabolize fructan polymers compared with mono- and disaccharides, and resistant cultivars that maintain a higher proportion of soluble carbohydrate as fructan would then be less susceptible.

Snow mold resistance in the wheat cultivar PI173438 is conditioned by the additive effect of two or three loci that are relatively highly heritable (Amano, 1982; Iriki and Kuwabara, 1993). However, studies on loci number do not provide any information on the number of genes involved and whether different alleles exist for each gene. Therefore, many genes may be operative in other wheats and their relatives. Because of the complex nature of the inheritance and unpredictable nature of snow mold development in the field, it is very difficult to screen cultivars reliably for snow mold resistance (Bruehl, 1982). Screening techniques have been developed to classify lines reliably under controlled environment conditions using snow mold chambers, but these techniques are costly, require extensive growth cabinet facilities, and do not permit the screening of the large number of lines that are necessary in a breeding program (Bruehl, 1982; Gaudet and Kozub, 1991). The closest correlations (ranging from r = 0.80 to r = 0.87) observed in the leaves and crowns during the last seeding date during 1997-1998 second and last seeding date during 1998-1999 may be sufficiently high to employ fructan content in leaf or crown tissues as a selection criterion in a breeding program, especially if the goal is to eliminate highly susceptible lines during early segregating generations. Studies are underway to determine the potential of employing fructan levels as a screening tool in a breeding program.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Understanding how levels of various sugars are modulated by genetic and environmental factors prevailing during the autumn, winter, and early spring, and how snow mold resistance is mediated by these sugars, are essential prerequisites to understanding the physiological basis of snow mold resistance. This study demonstrated that both the quantity and nature of carbohydrate accumulation and utilization in winter wheat leaves and crowns at Lethbridge, AB, was influenced by plant genotype, developmental stage of the winter wheat plant prior to dormancy, stage of winter dormancy, and environmental conditions prevailing throughout the autumn and winter. Snow mold resistant cultivars accumulated moderate levels of simple sugars, and high levels of fructan across seeding dates, and maintained a higher degree of polymerization of fructan compared with snow mold susceptible cultivars. Early seeded treatments generally accumulated lower levels of simple sugars, and higher levels of more highly polymerized fructan than did late seeded cultivars. Thus, elevated fructan content and the degree of polymerization appear to be implicated in the development-based resistance expressed in winter wheat reported by Bruehl (1967)(1982) and Gaudet et al. (1989), in early-seeded treatments under field conditions. The results of these studies may be useful in developing field-based screening tests to develop of snow mold resistant winter wheat cultivars adapted to the deep snow regions in western Canada.

	ACKNOWLEDGMENTS

 
The authors express their thanks to Ken Amos and Michelle Heikoop for their excellent technical help, and Toby Entz for statistical advice.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Partial support from the Alberta Agricultural Research Institute and Agriculture and Agri-Foods Canada Matching Investment Initiative is acknowledged. LRC Contribution No. 3870033.

Received for publication June 13, 2000.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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