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Published online 20 May 2008
Published in Crop Sci 48:1147-1154 (2008)
© 2008 Crop Science Society of America
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Cold Acclimation Threshold Induction Temperatures in Cereals

D. Brian Fowler*

Dep. of Plant Sciences, Univ. of Saskatchewan, 51 Campus Drive, Saskatoon, SK, Canada S7N 5A8

* Corresponding author (Brian.Fowler{at}usask.ca).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
To acquire a competitive advantage and ensure survival when exposed to low-temperature extremes, cool season plants must be programmed to respond to temperatures favorable for growth and environmental cues that signal seasonal changes. This project was initiated to determine (i) the cold acclimation threshold induction temperatures (ITs) in wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), and rye (Secale cereale L.) and (ii) their relationship to plant freezing tolerance at full acclimation. A wide range of genotypic specific IT and initial rapid acclimation responses that were inversely related to decreases in temperatures below the threshold was observed both within and among species, indicating that cereals monitor temperature with a high level of precision. Hardy wheat cultivars had a 5.7°C warmer activation temperature than tender genotypes when the vernalization gene was neutralized in near-isogenic lines, and a 12°C difference in IT of hardy rye compared with tender barley cultivars emphasized the high cold adaptation potential of rye. This early response to decreasing temperatures means that hardy rye had a longer time to prepare for the extremes of winter and was in a better position to cope with unexpected frosts during the growing season. Differences in IT were closely related to the differences in freezing tolerance at full acclimation. However, a longer vegetative stage also meant that winter habit genotypes were more responsive to extended periods at acclimation temperatures in the threshold range.

Abbreviations: CBF, C-repeat binding factor • IT, threshold induction temperature • NIL, near-isogenic line • LT50, low-temperature tolerance • PPFD, photosynthetic photon flux density • QTL, quantitative trait locus


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
TO BE SUCCESSFUL and ensure survival when exposed to cold stresses, cool season plants must make continuous adjustments to ever-changing environmental conditions. In this environmentally responsive system, there is a need for a plant to record the progress of seasons so that it can properly anticipate the normal periods of cold stress and commit fully to growth and reproduction once the weather is favorable. In regions with cold winters, vernalization requirement is an important adaptive feature that delays heading by postponing the transition from the vegetative to the reproductive phase. Similarly, photoperiod requirement is an adaptation that positions the plant to flower at the optimum time. Cool-season plants also have the ability to cold acclimate.

Although there is an overlap among species, cultivars of rye (Secale cereale L.) have the best low-temperature tolerance of the cereals, followed by common wheat (Triticum aestivum L.) and triticale (xTriticosecale Wittmack), durum wheat (Triticum turgidum L. var. durum), barley (Hordeum vulgare L.), and then oat (Avena sativa L.) (Fowler and Carles, 1979). Phenotypic studies have shown that the cold-induced protective mechanisms in cereals are developmentally regulated and involve acclimation process that can be stopped, reversed, and restarted (Fowler et al., 1999). Once the acclimation process starts, cold-hardiness expression, maintenance of freezing tolerance, and degree of cold injury are directly related to the sequence of temperature changes to which the plant is exposed. In wheat and its relatives, the threshold temperature for the initiation of cold acclimation has been generally been accepted as approximately 10°C (Olien, 1967; Alden and Hermann, 1971). However, there are recognized differences in the temperatures at which cereal genotypes start to acclimate under field conditions (Fowler and Carles, 1979), and recent studies indicate that there are associated differences in the induction temperatures of some cold-regulated genes (Vagujfalvi et al., 2000).

In most controlled experiment studies, plants are moved from conditions favorable for active growth and establishment (e.g., near 20°C) immediately into temperatures well into the acclimation range (e.g., 2–6°C). Differences in genetic potential are quickly magnified under these conditions, and the inverse relationship observed between exposure temperature and freezing tolerance indicates that cereals are able to monitor temperature with a high level of precision (Fowler and Limin, 2004). However, while temperatures in nature are constantly changing, these extreme shifts are unlikely, and plants normally have a much broader range of environmental cues and a longer time to prepare for extremes in cold stress.

Cold acclimation is a cumulative process, and at temperatures colder than the threshold, there is an inverse relationship between temperature and acclimation rate (Fowler et al., 1999). The largest differences among genotypes are normally observed when plant freezing tolerance is at its maximum (Dantuma and Andrews, 1960; Marshall, 1969; Brule-Babel and Fowler, 1989). But the most rapid changes in freezing tolerance occur during the initial stages of acclimation producing a curvilinear relationship between rate and stage of acclimation when acclimating temperatures are held constant (Fowler et al., 1999). Cold acclimation gene expression has been reported in plants grown at temperatures warmer than those normally considered in the induction range; however, differences in genetic potential are poorly expressed under these conditions and difficult to quantify accurately (Fowler and Carles, 1979; Brule-Babel and Fowler, 1989). As a result, we have a limited understanding of how and at what temperatures acclimation mechanisms are activated. This study was initiated with the objectives of (i) determining the threshold induction temperatures for genotypes representative of the freezing tolerance range found in wheat, barley, and rye, and (ii) determining the relationship between the activation of cold acclimation mechanisms and the genetic potential at full acclimation.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Experimental Design and Data Analyses
Three separate experiments were conducted to determine the threshold induction temperatures of the genotypes considered in this study. The design for each experiment was a 9 genotype x 6 acclimation temperature x 3 to 5 acclimation period factorial in three replicate randomized complete blocks. Each experiment was replicated three times in both time and space. Temperatures in the growth cabinets (Controlled Environments of Winnipeg, MB, Canada) were set at 2°C increments, starting at 8 and 5°C in Exp. 1 and 2, respectively. The growth cabinet temperatures in Exp. 3 were set at 3°C increments, starting at 5°C. This level of precision is difficult to achieve, and the actual temperatures recorded at plant crown position are reported for each experiment.

The recorded acclimation temperatures in Exp. 1 were 8, 10, 11, 13, 14, and 16°C, and the treatments were sampled after 0, 2, 7, 14, and 21 d of acclimation. The acclimation temperatures in Exp. 2 were 4, 6, 8, 10, 12, and 14°C, and the treatments were sampled after 0, 2, 7, and 14 d of acclimation. The acclimation temperatures in Exp. 3 were 5, 8, 11, 14, 16, and 18°C, and the treatments were sampled after 0, 2, and 7 d of acclimation. Analyses of variance were conducted to determine the level of significance of differences due to genotype, acclimation temperature, acclimation period, and their interactions in these experiments.

The threshold induction temperature was recorded as the warmest temperature at which each genotype achieved an LT50 (low-temperature tolerance; temperature at which 50% of the plants are killed by cold stress) of –3°C after 2 (as shown in Fig. 1 ) or 7 d acclimation in Exp. 1, 2, and 3. Data derived from the three experiments was combined in an analysis of variance to determine the level of significance of differences among the threshold induction temperatures of the genotypes after 2 and 7 d acclimation. Freezing tolerance after 28 d acclimation at 6°C was determined in a separate experiment using the procedure outlined by Fowler and Limin (2004). The experimental design for this experiment was a 9 genotype x 3 replicate randomized complete block.


Figure 1
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  		Figure 1. Low-temperature tolerance (LT50, SE = 0.30) of ‘Sisler’, ‘Dicktoo’, and ‘Kold’ barley, ‘Manitou’, winter ‘Manitou’, spring ‘Norstar’, and ‘Norstar’ wheat, and ‘Gazelle’ and ‘Puma’ rye established for 13 d at 20 to 22°C and then acclimated at 5, 8, 11, 14, 16, or 18°C in for 2 d in Exp. 3. The experiment was replicated three times in time and space.

 
Plant Material
Common wheat, barley, and rye cultivars and research lines that represented different reproductive strategies and cold-hardiness potentials were selected for detailed evaluations and comparisons in these studies. ‘Norstar’ is a winter-habit wheat cultivar (Grant, 1980) with a long vernalization requirement that has been widely used as a source of freezing-tolerance genes in western Canadian winter wheat breeding programs. ‘Manitou’ (Campbell, 1967) was a widely adapted Canadian hard red spring wheat cultivar. The common wheat reciprocal near-isogenic lines (NILs) spring Norstar and winter Manitou were developed to determine the interactions between low-temperature tolerance and vernalization requirement. ‘Dicktoo’ is an extremely short-day sensitive barley cultivar without a vernalization requirement that was included because it has been used extensively to study the effect of photoperiod requirements on the freezing tolerance gene expression in controlled environments (Mahfoozi et al., 2000; Fowler et al., 2001) and in the molecular mapping of winter-hardiness genes (Hayes et al., 1997). ‘CDC Sisler’ is a six-row Canadian malt barley with little or no photoperiod sensitivity (CFIA registration number 4964; B.L. Harvey, personal communication, 2007). ‘Kold’ (Hayes et al., 1995) is a hardy winter habit barley cultivar. ‘Gazelle’ is a spring-habit cultivar (Sosulski and Curran, 1975), and ‘Puma’ (Shebeski et al., 1973) is a highly cold tolerant winter-habit rye cultivar. Both have been grown extensively in western Canada.

The reciprocal NILs (Limin and Fowler, 2002) were produced using the nonhardy spring-habit (Vrn-A1) cultivar Manitou and the very cold-hardy winter habit (vrn-A1) cultivar Norstar (Brule-Babel and Fowler, 1988) to determine the effect of spring and winter habit-determining alleles in each genetic background. The parent cultivars were crossed to produce an initial hybrid that was then backcrossed to each parental cultivar. In subsequent generations, each cultivar was crossed to the BCF1 of the previous generation based on selection for heterozygosity (Vrn1/vrn1) at the Vrn-A1 locus. When Norstar was the recurrent parent, heterozygosity at the Vrn-A1 locus was based on the spring habit, which would be Vrn1/vrn1, due to the dominance of the spring habit allele; all other progeny would be winter habit. When Manitou was the recurrent parent, heterozygosity (Vrn1/vrn1) at the Vrn-A1 locus was based on the heterozygote's flowering time, which was several weeks later than the homozygous (Vrn1/Vrn1) spring habit. This phenotype-based selection ensured that the donor parent allele was incorporated into the genetic background of the recurrent parent. Ten backcrosses were made to each recurrent parent, heterozygous plants (BC10F1) of each reciprocal line were self-pollinated, and the progeny were grown out. Homozygous winter and spring growth-habit plants were selected from the self-pollinated progeny of each reciprocal line. This procedure resulted in reciprocal NILs in which theoretically 99.95% of the recurrent parent DNA is recovered. These reciprocal NILs, based on the Vrn-A1 locus, produced in essence a winter habit type of nonhardy Manitou (Manitou with the vrn1 allele of Norstar = winter Manitou) and a spring habit type of the hardy winter Norstar (Norstar with Vrn1 from Manitou = spring Norstar). The nonhardy spring habit Manitou, the very cold-hardy winter habit Norstar, and two reciprocal NILs that differed in vernalization requirement were used in these studies.

Plant Growth, Cold Acclimation, and Freeze-Testing Conditions
Plants for LT50 determinations were grown hydroponically as described in detail previously (Limin and Fowler, 2002). Imbibed seeds were held in the dark for 2 d at 4°C to break dormancy and ensure even germination and were then transferred to an incubator and held for 1 d at 22°C. Actively germinating seeds were then transferred with the embryos down into white light–blocking plastic trays with holes backed by a 1.6-mm mesh screen. The trays were returned to the incubator for 2 d to allow for further root growth. The seedlings were then grown for 10 d in hydroponics tanks filled with continuously aerated one-half strength modified Hoagland's solution under a 16-h day at 20°C with a photosynthetic photon flux density (PPFD) of 300 µmol m–2 s–1, at which time they had developed two to three fully expanded leaves and visible crowns. They were then transferred to the cold acclimation–vernalization chambers set at the designated temperatures and a 16-h day at 250 µmol m–2 s–1 PPFD.

The procedure outlined by Mahfoozi et al. (2001) was used to determine the LT50 of each genotype at the end of each cold acclimation period. Five crowns were frozen at 2°C intervals for each of five test temperatures that were predetermined for each genotype in each treatment using data from preliminary experiments. The crowns were placed in aluminum weighing cans, covered in moist sand and then loaded into a programmable freezer that was held at –3°C for 12 h. After 12 h (at 8:00 a.m.), they were cooled at a rate of 2°C h–1 down to –17°C. Rate of freezing comparisons (unpublished data) conducted to identify ways to complete freeze tests in an eight-hour work day established that increasing the cooling rate from 2 to 8°C h–1 once the test temperature reached –17°C did not change the temperature at which 50% of the plants were killed (LT50). Therefore, once the freezing temperatures reached –17°C, the cooling rate in these studies was increased to 8°C h–1 until the predetermined temperature for each genotype in each treatment was reached. The aluminum weighing cans with the crowns in the frozen sand were removed from the freezer once the predetermined temperature for each sample was reached. The sand and crowns were thawed overnight at 3°C. Thawed crowns were then transplanted into 52 by 26 by 6 cm black plastic trays (Kord Products, Bramalea, ON, Canada) containing Sunshine artificial soil medium (Sungro Horticulture, Bellevue, WA) for regrowth. The trays were placed in a growth room maintained at 20°C with a 16-h day and 8-h night. Plant recovery was rated (alive vs. dead) after 3 wk, and the LT50 (determined by extrapolation within the selected temperature ranges) was calculated for each treatment within each replicate. Threshold induction temperature for each genotype was defined as the warmest temperature at which plants achieved an LT50 of –3°C after 2 (Fig. 1) or 7 d acclimation at the test temperatures selected in each experiment.

Phenological observations were made to determine double ridge formation. In the "double ridge" method (Kirby and Appleyard, 1987), stage of shoot apex development was determined on crown samples of plants grown under the conditions for cold acclimation described above. A minimum of two plants from each genotype were sampled for dissection at each of the acclimation periods at each acclimation temperature, and number of days to double ridge formation was recorded to establish the influence of low-temperature growth on rate of phenological development.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Analyses of variance indicated significant differences in freezing tolerance due to genotypes (P < 0.001), acclimation temperatures (P < 0.001), acclimation time (P < 0.01), and their interactions (P < 0.01) in all three experiments conducted in this study. This variation was associated with major differences in threshold induction temperatures and acclimation patterns (Fig. 1 and 2 ). In wheat and its relatives, the threshold temperature for the initiation of cold acclimation has been generally accepted as approximately 10°C (Olien, 1967; Alden and Hermann, 1971), but the results of this study indicate that there is a wide range within and among species and that the cold protective mechanisms of some genotypes are activated after exposure to temperatures as warm as 15 to 17°C for 2 d.


Figure 2
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  		Figure 2. Low-temperature tolerance (LT50, SE = 0.35) of ‘Manitou’, winter ‘Manitou’, spring ‘Norstar’, and ‘Norstar’ wheat established for 13 d at 20 to 22°C and then acclimated for 0, 2, 7 14, or 21 d at 8, 10, 11, 13, 14, or 16°C in Exp. 1. The experiment was replicated three times in time and space.

 
Low-temperature acclimation was not necessarily an immediate response following exposure to temperatures in the threshold range. For example, more than a week at 13 and 14°C was required before winter Manitou achieved an LT50 of –3°C in Exp. 1 (Fig. 2). Similarly, acclimation did not start immediately when spring Norstar was exposed to temperatures warmer than 11°C. This delayed response resulted in an average threshold induction temperature that was almost 2°C warmer after 7 d compared with 2 d (Table 1 ), indicating that subtle differences in time and temperature, or some other environmental factor, can eventually trigger the acclimation process when plants are exposed to temperatures approaching the threshold induction level.


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  		Table 1. Average cold acclimation threshold induction temperatures for selected spring, facultative and winter habit genotypes of barley, wheat, and rye after 2 and 7 d of acclimation in Exp. 1, 2, and 3 and low-temperature tolerance (LT50) after 28 d acclimation at 6°C.

 
Once initiated, cold acclimation was cumulative and an inverse relationship between temperature and degree of acclimation was immediately apparent. The most rapid changes in LT50 occurred during the initial stages of acclimation, and plants were not able to fully acclimate until temperatures dropped well below the threshold induction level (Fig. 2; Table 1). These observations are consistent with the developmental model of cold acclimation gene regulation (Fowler et al., 1999) where the genes that determine the length of the vegetative stage act as switches controlling the duration of expression of freezing tolerance genes (Fowler et al., 1996a,b; Limin and Fowler, 2006) and a rate component determines the degree that these genes are up regulated (Fowler and Limin, 2004). Results of the present study agree with these concepts and provide for more in-depth analyses of the complicated interactions that determine freezing tolerance and regional adaptation of cereals.

The large differences in threshold induction temperatures of genotypes revealed after 2 d acclimation (Table 1) support the notion that the cold sensing mechanism and responses in the early stages of acclimation play a critical role in determining plant cold acclimation potential. Once acclimation started, the differences in genetic potential were quickly magnified with the result that genotypes with warmer threshold temperatures had the most rapid responses to decreases in temperature. Highly significant correlation coefficients (P < 0.001) demonstrated that there were close relationships among freezing tolerance after 28 d acclimation at 6°C and threshold induction temperatures after 2 and 7 d acclimation (2d vs. 7d, r = 0.96; 2d vs. 28d, r = –0.916; 7d vs. 28d, r = –0.967), suggesting that threshold induction temperatures are primarily responsible for differences in freezing tolerance of fully acclimated plants. However, these observations are based on the responses of genotypes selected to represent the extremes within and among species, and there was evidence of small, but important, differences in cold response that warrant further investigation.

The wheat Vrn-A1 NILs used in this study provided the opportunity for independent assessment of differences in freezing tolerance genetic potential and rate of phenological development on low-temperature responses. Earlier studies using these lines have shown that vernalization and photoperiod requirements allow cold acclimation genes to be expressed for a longer duration, while a more rapid rate of acclimation has been associated with the Norstar genetic background (Limin and Fowler, 2002; Fowler and Limin, 2004). In agreement with these observations, differences in the initial rates of acclimation were primarily responsible for the differences in freezing tolerance observed between Norstar and winter Manitou (Fig. 2) and the importance of differences in threshold temperatures were reinforced when all the genotypes were compared (Fig. 1). Earlier studies with the Norstar–Manitou NILs for the Vrn-A1 locus in wheat have also demonstrated that the superior freezing tolerance of Norstar genetic background was due to a faster rate of initial acclimation (Fowler and Limin, 2004). An average 5.7°C warmer activation temperature after 2 d acclimation for Norstar and spring Norstar compared with Manitou and winter Manitou (Table 1) also established that the Norstar genetic background has a warmer threshold induction temperature and demonstrates the range of activation temperatures that can be expected when the influence of the vernalization requirement due to vrn-A1 has been neutralized in wheat.

Among the genotypes without a vernalization requirement, Sisler, Dicktoo, and Gazelle all reached the double ridge stage during the 13-d preacclimation establishment period at 20 to 22°C, and Manitou and spring Norstar entered the double ridge stage shortly after 9 and 13 d, respectively, when acclimating temperatures were warmer than 13°C (Fig. 3 ). With the exception of Dicktoo, the influence of early commitment to the vegetative–reproductive transition was also evident as the threshold induction temperature only decreased an average of 0.8 compared with 2.5°C after 2- compared with 7-d acclimation (P < 0.05) for the spring- vs. winter-habit genotypes (Table 1). Dicktoo is a facultative barley cultivar that is recessive for vrn-H2 but carries the winter habit Vrn-H1 allele (Karsai et al., 2005), and it has been shown to have a unique system of cold response (Limin et al., 2007), suggesting that its winter habit allele may be responsible for its variant behavior. The delay in the vegetative–reproductive transition associated with a vernalization requirement meant that winter-habit genotypes were more responsive to extended periods of acclimation at temperatures in the threshold range. Consequently, while differences in the threshold induction temperatures were closely related to the differences in freezing tolerance after 28-d acclimation at 6°C, a vernalization requirement meant that winter-habit genotypes had a significant advantage when preparing for the low-temperature extremes associated with production areas that experience high-stress winters.


Figure 3
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  		Figure 3. Days to the double ridge stage of phenological development (SE = 0.46) for ‘Manitou’ and spring ‘Norstar’ wheat established for 13 d at 20 to 22°C and then acclimated at 8, 10, 11, 13, 14, or 16°C in Exp. 1. The experiment was replicated three times in time and space.

 
Several additional factors have been shown to influence the rate of phenological development and the time to vegetative–reproductive transition. Among these, low temperatures delay reproductive transition in plants that have reached the stage of competence to flower (Fowler and Limin 2004), and a high minimum final leaf number (Limin and Fowler, 2002) prolongs the vegetative phase in cereals. The effect of temperatures at or below the induction threshold in delaying phenological development of plants without a vernalization requirement was evident in both Manitou and spring Norstar (Fig. 3). The Norstar genetic background also has a significant influence on final leaf number that is independent of the vernalization response with the result that spring Norstar (containing the Manitou Vrn-A1 allele) normally produces two more leaves than Manitou before flowering (Fowler and Limin, 2004). As seen in the present study, a greater final leaf number meant that spring Norstar required a longer time to reach the double ridge stage (Fig. 3), although these differences did not appear directly related to degree day accumulation as would normally be expected (Limin and Fowler, 2002). Obviously, the minimum final leaf number option for fine tuning the length of the vegetative stage requires more study before firm conclusions can be drawn concerning its effect on acclimation rate and threshold induction temperatures.

Highly significant (<0.001) differences among the threshold temperatures indicate that there is important variability in the mechanisms by which the genotypes considered in this study monitor and respond to temperature. Molecular studies in Arabidopsis have shown that the cold signaling system requires a cascade of transcriptional regulators; however, the results of mapping studies suggests that a single quantitative trait locus (QTL), designated as Fr2, determines a large part of the phenotypic variation for low-temperature tolerance in cereals. Fr2 has been mapped to chromosome 5A of diploid (T. monococcum; Vágújfalvi et al., 2003) and hexaploid (T. aestivum; Båga et al., 2007) wheat and an orthologous locus in barley (Francia et al., 2004). Differences in the initial rate of acclimation have also been mapped to the Fr-A2 QTL (Båga et al., 2007), establishing that this region is directly involved in the temperature-sensing mechanism of wheat. In all three species, clusters of C-repeat binding factors (CBFs) have been located in the Fr-2 QTL (Vágújfalvi et al., 2003; Francia et al., 2004; Miller et al., 2006; Skinner et al., 2005; Båga et al., 2007) and Cbf genes have been shown to encode transcriptional factors that play an important role in the activation of downstream cold-regulated (COR) genes and cold responses in plants (Thomashow et al., 2001; Jaglo et al., 2001).

While considerable attention has been given to the role of CBFs in determining phenotypic variation in freezing tolerance, QTLs associated with upstream activators have not been identified in cereals, and the cold-sensing mechanism itself remains very much a mystery. The ICE1 genes (Chinnusamy et al., 2003), which are constitutively expressed in Arabidopsis, and cold shock proteins (Nakaminami et al., 2006) have been implicated in the regulation of cold response transcriptional activators. The genotypic specific differences in threshold induction temperatures and an initial rapid response that was inversely related to the new exposure temperature when plants were moved from 20°C directly into temperatures lower than the threshold (Fig. 1) indicate that cereals are able to monitor and respond to temperature changes with a high level of precision. These observations support the notion that the temperature monitoring mechanism may involve post-transcriptional modification of constitutively expressed gene products whose efficiency in regulating the expression of downstream activators is directly related to temperature.

From an applied standpoint, the results of this study indicate that there is a wide range of threshold induction temperatures both within and among wheat, barley, and rye genotypes and, as expected, the genetic mechanisms responsible for activation are upstream from those determining freezing tolerance at full acclimation. Within wheat, after 2 d acclimation, the cold-hardy cultivar Norstar had a 5.4°C warmer activation temperature than the tender Manitou genetic background when the vernalization gene had been neutralized in the winter Manitou NIL (Table 1). A 5.9°C warmer induction temperature for spring Norstar compared with spring Manitou further highlights the advantage of the Norstar background. A greater than 12°C difference in threshold induction temperature between Puma rye and Sisler barley after 2 d acclimation emphasizes the superior low-temperature adaptation potential offered by the rye gene pool. This early response to decreasing temperatures provides the hardy rye genotypes with a much longer time to prepare for the extremes of winter and puts them in a much better position to cope with unexpected frosts that occur early in the growing season. Plants with a warm threshold temperature would also be expected to have a large advantage when subjected to the rapid changes in temperature associated frost events that occur later in the growing season.


   ACKNOWLEDGMENTS
 
Financial support from Genome Canada/Genome Prairie and Ducks Unlimited Canada is gratefully acknowledged. The excellent technical assistance of Garcia Schellhorn and Twyla Chastain is greatly appreciated.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
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Received for publication December 20, 2007.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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