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The relationship of LT 50 to prolonged freezing survival in winter wheat

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The relationship of LT 50 to prolonged freezing survival in winter wheat
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  The relationship of LT   to prolonged freezing survivalin winter wheat D. Z. Skinner and K. A. Garland Campbell USDA -ARS and Department of Crop and Soil Sciences Washington State University Pullman WA USA 99163. Received 9 January 2008 accepted 29 May 2008. Skinner, D. Z. and Garland-Campbell, K. A. 2008. The relationship of LT 9  to prolonged freezing survival in winter wheat. Can. J. Plant Sci. 88: 885-889. Twenty-six wheat (Trizicum aestii'wn L.) lines were tested for their ability to withstand remaining frozen for extended periods of time. Survival of acclimated seedlings was evaluated after remaining frozen at €5C for 15 or 20 wk. Survival after 15 wk ranged from 0 to 100% and after 20 wk ranged from 0 to 33%. The relationship of survival and LT,,   scores, the temperatures at which 50 of the plants were predicted to die, was examined with linear regression analysis. The linear relationship was highl y  statistically signilicant after 15 wk and after 20 wk. The cultivars Norstar and Froid survived being frozen for 20 wk nearly twice as well as the other cultivars about 33 vs. 17 for the next best cultivar. These results indicated that the LT   score, which call estimated in about 8 wk reliably predicts the ability to survive in the frozen state for as long as 20 wk. and that Norstar and Froid possess a long-term freezing tolerance mechanism that is far superior to the other cultivars tested. Key words: Winter wheat, freezing tolerance, freezing injurySkinner, D. Z. et Garland-Campbell, K. A. 2008. Lien entre la LT. 0 et la survie prolong€e au gel ehez Ic bl€ d hiver. Can. J. Plant Sci. 88: 885-589. Les auteurs ont test• vingt-six lign•es de bl• (Tuilicum aesliviwi L.) pour verifier leur capacitC demeurer longtemps gel•es. Ils ont •valu• la survic des plantules acclimat•es et gardCes gel•es i €   C Pendant IS ou 20semaines. Le taux dc survie aprCs 15 semaines vane de 0 a 100 apr‚s 20 sernaines. il  Iluctue dc 0 a 33 %. Le lien entre la survie et la LT   , temperature a laquelle la moitiC des plants dcvraient p•rir. a Ct• analyse par regression linCaire. Les relations lin•aires sont statistiquement tr‚s significatives apr‚s 15 et 20 semaines. Les cultivars Norstar et Froid survivent prCs de deux fois mieux que les autres cultivars a 20 sernaines de gel, soit environ 33 c. 17 pour Ic meilleur cultivar suivant. Ces r•sultats indiquent que la LT>, qu'on peut estimer en a peu pr‚s 8 semaines, prCdit de maniCre liable Ia capacit• de survie au gel pendant un maximum de 20 semaines et que les cultivars Norstar ci Froid possedent un m•canisme de tolerance au gel nettement sup•rieur celui des autres vari•t•s testCes. Mots des Bl• dhiver. tolerance au gel, dommages causes par Ic gel Inconsistent winter survival is a major impediment toproductive growth of winter wheat (Triticum aestivum L.). The ability of the plants to remain alive in the frozen state for extended periods of time is crucial to surviving the winter months in Canada and the Northern US. Survival in the field is the ultimate measure of winter- hardiness of a cultivar, but the use of field survival as atool in selection is unreliable because of variable winter severity with differential winter kill (Gusta et al. 2001), and microgeographic environmental variation resultingin the plants throughout a field being exposed to a widevariation in temperature stress (Fowler 1979). Typically, the freezing tolerance of winter wheat cultjvars is measured as the LT 5 , or lethal temperature 50%, the temperature at which 50% of the plants die due to freezing injury. The way in which LT 5 is measured varies among researchers. Some have tested for freezing survival by removing most of the shoots and roots from the plant and freezing the remaining crownregion in a damp sponge (Olien 1984; Livingston 1996; Herman et al. 2006). Others have packed the crown regions in moist sand in aluminum dishes and frozen the dishes on an aluminum block in a modified freezer (Gusta et al. 1978). Intact plants, grown in long, narrow plastic containers ( Cone-tainers or root-trainers ) have been used (Gusta et al. 1997, Koemmel et al. 2004). In our programs, the LT 6 is determined using intact plants grown in plastic, horticultural cell packs. Regardless of how it is determined, the LT   is a measurement of cold tolerance at the particular point in time at which it is measured. With plants grown in the field until frozen under natural conditions, then moved to constant €5 or €SC storage, the LT 5 increased (less freezing tolerance) over time as the plants remained frozen (Gusta et al. 1997). The LT   of plants grown at low, above-freezing temperatures, first declined, thenincreased (Mahfoozi et al. 2001, 2000). Nonetheless, ithas been shown that survival in the field and the LT). determined at a particular point in the cold acclimation process are often significantly correlated (Gusta et al. 1997, 2001), but not with all cultivars (Gusta et al. 2001). Fowler et al. (1981) reported that a 1 C difference in LT 5 translated to a 30% difference in field survivalindex. Survival in the field depends on many factors, amajor factor is the temperature reached in the meriste- matic region (crown) of the plants.    886 CANADIAN JOURNAL OF PLANT SCIENCE Although many factors contribute to the variation in subsurface soil temperature (Greer et al. 2006), in our observations (unpublished) and in observations made by others in the Palouse region of the US Pacific North- west, the temperature of the soil at 2-5 cm depth (approximate depth of the crowns of winter wheat plants) is rarely less than €5C (Kahraman et al. 2004; Greer et al. 2006). Hence, a requirement for winter wheat to survive in the US Pacific Northwest is an ability to remain viable while frozen to €SC formonths. The ability to survive prolonged periods offreezing and the ability to survive freezing to severe temperatures, such as is measured with LT 5 , are two separate measures of the ability to tolerate cold stress. The objective of this study was to examine the relation- ship of LT 5 with the ability to remain viable while frozen to €5 C for extended periods of time. MATERIALS AND METHODSPlant Materials Twenty-five cultivars and one germplasm line of winter wheat were studied (Table I). These wheat lines were developed for states in the United States of America or areas of Canada where winter injury is a threat every year. Determination of LT5 Plants were planted about 2 cm deep in SunShine mix LCI (Sun Gro Horticulture, Bellevue, WA) in cell pack 1020 inserts packed into 1020 flats (The Blackmore Company, Belleville, MI) with dimensions of 28 x 54 x6 cm. Ten seedlings, randomized within a test tempera- ture, represented each genotype. Immediately after planting, germination time was synchronized by placingseeded, watered trays in a cold room at 4ƒC for 3 d and then moving them to a greenhouse at 24ƒC and 16 h photoperiod. Macronutrients and micronutrients (Pe- ter's All Purpose, Scotts-Sierra, Marysville. OH) diluted to 100 ppm were applied through a watering system once per day to saturation. At about the three-leaf stage, plants were vernalized for 5 wk in a 2 .25-in 2 growth chamber (Conviron Inc., Ashville, NC) with lighting at200 jimol m 2  s' supplied by fluorescent and incan- descent bulbs, with a 16/8 h light/dark cycle and temperature maintained at a constant 4ƒC. After the vernalization period, plants were saturated with water toequalize water content throughout trays. The tops of the plants were removed to approximately 2 cm above thesoil surface, and ice chips were placed on the plantingmedium to nucleate ice formation. The artificial freeze test was conducted by reducing the growth chamber temperature from 4 to - 10ƒC over a period of 8 h. Then the temperature was decreased for 12 more hours atthe rate of - 1ƒC/h until the lowest test temperature(-20.5ƒC) was reached. Ten plants per genotype were removed at each of eight temperatures: €10, €11.5, 13. 14.5.   6. €17.5. €19 and €205C. After freezing, seedlings were placed in a growth chamber at 4C in the dark. The temperature of the growth chamber was increased over 24 h to 13 C. Plants were then placed in the greenhouse at 24 C for regrowth. Supplementallighting from 1000-W metal halide lamps was used toprovide a 16-h photoperiod with about 350 p.rnol m 2 s   irradiance at soil level. Survival was scored as the proportion of plants to regrow after 5 wk. LT 5 wasdetermined with probit analysis of the survival data using proc probit (SAS Institute, Inc. http://www.sas. corn ). The temperature at which 50% survival of acultivar was predicted by the probit equations was recorded as the LT 5 . Determinations were repeated atleast three times for each cultivar at separate times. The LT 5 values reported here are the means of those determinations. Long term Freezing Survival Tests Fifty seeds of each cultivar were planted about 2 cmdeep in soilless potting mix in 30-cm x 50-cm x 6-cm plastic flats. The order of the cultivars was randomized in each flat resulting in a randomized complete blockdesign. The flats were fertilized with a water-soluble fertilizer (20:20:20) containing chelated micronutrients. Seeds were allowed to germinate and seedlings were grown to the three-leaf stage in a greenhouse with supplemental lighting to provide a 16-h photoperiod with about 350 imol m 2  s irradiance at the soil level. The flats then were transferred to a growth chamber (Conviron Model GR48) set to 4ƒC and a 16-h photoperiod to effect cold hardening. After 5 wk of cold hardening. the number of plants were recorded. flats were covered with a layer of crushed ice to simulate snow cover and to nucleate ice formation (care was taken to maintain the plants in an upright position) and were transferred to a walk-in freezer set to - 5ƒC. Theflats were placed on wooden planks (3.8 cm thickness, 8.9 cm width, 2.4 m length) on the floor of the freezer. No light was provided. Soil temperature was recorded each hour in 16 of the flats using a portable temperaturedata logger (Hobo Model 1„18). After 15 wk, two of the flats were removed from thefreezer and placed in a growth chamber set to 6ƒC and 16-h photoperiod, with about 160 j.tmol m 2   irradiance supplied by fluorescent bulbs. After 3 d, thetemperature was raised to 20-C. Recovery was scored after 4 wk. All top growth existing at the time of freezing had died. Plants that had produced actively growing, green leaves from the crowns were scored as surviving: recovery was expressed as the proportion of plants to survive, relative to the initial number of plants. After 20 wk of incubation at €5ƒC, eight flats were removedfrom the freezer and survival was scored as above. Theentire experiment was repeated a second time, resulting in four flats scored after 15 wk of freezing and 16 flats scored after 20 wk of freezing. For statistical analysis. survival was expressed as the arcsin of the square root of the surviving proportion of  SKINNER AND GARLAND CAMPBELL FREEZING SURVIVAL IN WINTER WHEAT 887 Table 1. Origin, market class, T 5  and long-term freezing survival of 6 winter wheat cultivars Proportion surviving freezing to - S C for LT55 - 19.5 -15.7-14.6-13.5 -14.6 -13.8 -15. -15.5 -14.0 -14.6 -11.5-11.3 -11.7-14.6 -12.3-13.8-12.3 - 12.3 -14.6 -11.0 -9.5 -10.3-10.9 -11.3-11.2-13.0 Group'   ulti'ar Norstar FroidKestrel TiberWanserFinley Elta n Masarni Edwin HattonMadsen HillerLewjain TubbsBruehl Buchanan Hill 81 Rely ORCF 101 Moro ORFW' FinchStephensCoda Rod Chukar Origin Saskatchewan, CanadaMontana, USASaskatchewan, CanadaMontana, USAWashington, USAWashington, USAWashington, USAWashington, USAWashington, USAWashington, USAWashington, USAWashington, USAWashington, USA Oregon, USA Washington. USAWashington, USA Oregon, USA Washington, USA Oregon. USAOregon, USAOregon, USA Washington, USA Oregon, USA Washington, USAWashington, USAWashington, USA Market class' HRWWHRWW F-I RWW HRWW I-I RWW HRWW sWwW SWWWSWWCWHRWWSWWWSWWCWSWWWSWWWSWWCWHRWWSWWWSWWCWSWWW SWwW SWWWSWWW sWWW SWWCW SWwW SWWCW 15 wk1,00a 1.0061 0.99ab 0.96abc 0.95ahc0.95ahc 0.S7ahaI 0.76a1?cde 0.7Oahuk f 0. 73abcs/cf 0.7Ohcdefg 0.69bcdefiI, 0.65bcdc/ ghi 0. 58idet/u/ 0.49dc1glujk 0.41 e/ghi/k0.27/glujk 0.2 lghi/k 0. 2OghijJ 0.20g/zi/k1 0.l5i/kl 0.12/k 0.09/k 0.06k 0.050 0.00/ 20 wk 0.33 11.32 0.130.100.100.080.06 0.150.05 0.10 0.11 0.080.03 0.01 0.02 (1.17 0.120.050.100.100.15 0.01 (1.08 0.000.000.00 Groups include cultivars whose survival after IS wk was not significantly different from the cultivar with the greatest survival within that grolLp. 5 Cultivars are sorted in order of decreasing survival after IS wk of freezing.'HRWW =hard red winter wheat, SWWW =soft white winter wheat, SWWCW = soft white winter club wheat. Rely is a 10-parent multiline cultivar. 'ORFW = Oregon Feed Wheat #5, developed from a cross of winter and spring wheats. a-i Means followed by the same letter are not significantl y  different according to Duncan's multiple range test (P=0.05). plants, weighted by the initial number of plants. Analysis of variance and means separations were carried out with PROC GLM of SAS Institute, Inc. software.For presentation in this report, means are presented in the srcinal scale. R SU TS The LT,,,   scores determined for each of the wheat lines are presented in Table 1. The temperature in the planting medium in the flats, when covered with ice and transferred to the walk-in freezer, declined from about 3 to 5 C at an average measured rate of - 1.03'C h -and then remained at a constant - 5ƒC, varying by lessthan 0.2C during the test period. Survival of cultivars as a function of time frozen, appeared to fit into one of three categories: cultivars whose survival remained relatively high for the first IS wk of freezing, then declined rapidly over the next 5 wk; those whose survival declined at essentially the same rate over the entire 20-wk period and those whose survival declined rapidly in the first 15 wk, then less rapidly in the next 5 wk (Fig. 1). Using statistical means separation, groups were defined as the cultivars that were not statistically significantly different from the most freezing-tolerant cultivar within that group. In the First group after 15 wk of freezing, the most freezing-tolerant cultivars, Norstar and Froid, had 100% survi- val, and these means were not significantly different from the cultivars Kestrel, Tiber, Wanser. Finley, Eltan, Masami. Edwin, and Hatton (Table I). The average survival of this group was 89.7% and the average LT50 was - 15.1-C. The second group consisted of Madsen. Hiller, Lewjain. Tubbs, Bruehl, Buchanan, Hill Xl. Rely, ORCF 101. and Moro with average survival of 46.7% and LT 50 of -12.7 C (Table I). The third group, with less than 15% survival after 15 wk of freezing included ORFW. Finch, Stephens, Coda. Rod, and Chukar, with average survival of 7.8% and LT 60 of - I 1.0C (Table I). These three groups showed considerable overlap with respect to statistically significant differ-ences in survival, but from these observations, it was apparent that the LT 5 as measured in this study, was strongly related to survival after 15 wk of freezing. Linear regression analysis indicated a highly significantlinear relationship between LI   and survival (r =0.68, P <0.001, Fig. 2). The average increase in survival associated with ]ƒC lower LT 5 . determined from all possible comparisons of the 26 cultivars. was 29.3%.    888 CANADIAN JOURNAL OF PLANT SCIENCE 0.9 0.8 € 0.7 C   t 0 6   5   4 f 3 0.2 0.100   5 Weeks frozen to -S C Order from top to bottom at IS weeks€. Norstar -* 4roid  . Kestrel €fiber Wanner Finley EltanMasami €Edwin Hutton MadsenI tiller Lewjain1l.bbsBruehl Buchanan € Hill 81 €Rely € oscr tot Mon  . ORFW Finch Stephens Coda 20 €Rod €Chukar Fig. I. Survival of 26 winter wheat cultivars frozen to €5ƒC for 0, 15, or 20 wk. After remaining frozen for 20 wk at - 5•C. there were only two groups of cultivars apparent. Norstar and Froid in one group with about 33% survival, and the remaining 23 cultivars with an average survival of about 7% (Fig. 3). Norstar and Froid were significantly different from all other cultivars (P = 0.01). Nonethe- less, linear regression with LT 5 as the independentvariable and survival as the dependent variable was highly significant in the data from plants frozen for 20 wk (r =0.69, P <0.001, Fig. 3). With Norstar and Froid omitted from the data set, linear regression remained significant, although not as strongly (r =0.45, P =0.03, not shown), indicating that LT 5 continued to predictsurvival when that survival was well below 50%. The relationship of increased survival due to a 1C lower LT 5 is not nearly as obvious in these 20-wk 0.90.8 0.7 a•-6   o.50.40.1 0  20   18   16   14   12   10 IT50 Fig. 2. Survival of 26 winter wheat cultivars after remaining frozen to - 5C for 15 wk as related to LT 5 in ƒC. Each point represents one cultivar. Linear regression was significant at P =0.0001. r=0.68. survival data as in the 15-wk data. The LT 5 of Norstar and Froid differed by nearly 4ƒC, but the levels of survival after remaining frozen for 20 wk were virtually identical (Table I). Of the remaining 24 cultivars, comparing the cultivar with the lowest LT 5 (Masami)to the cultivar with the greatest LT 5 (ORFW), a 1ƒCdifference in LT 5() was associated with 2.4% greater survival (Table 1). DISCUSSION Determination of the LT 5 using the method described here applied to plants after 5 wk of cold acclimation resulted in LT 5 values that were strongly, linearly related to the ability to survive freezing to -5 C for 15 wk. The average increase in survival associated with 1ƒC lower LT 5 was about 29.3%, remarkably 0.35   0.30   m 0.10 0.05   8   6   44   1240   B IT50 Fig. 3. Survival of 26 winter wheat cultivars after remainingfrozen to -5 C for 20 wk. as related to LT   in C. Each point represents one cultivar. Linear regression was significant at P =00001 r=0.69.  SKINNER AND GARLAND CAMPBELL FREEZING SURVIVAL IN WINTER WHEAT 889 close to the 30% increase in field survival reported by Fowler ci al. (1981). After 20 wk frozen to 5   C, the relationship between LT   and the ability to survive was not as robust but was statistically significant. The frequencies of survival of all of the cultivars after 20 wk in the frozen state were much less than 50 . However, survival of Norstar and Froid, the cultivars with the lowest LT 5() scores of the cultivars studied, survived with nearly identical frequency and that frequency was nearly twice as great as the next closest cultivar. This result was consistent with the possibility ofdistinct groups of cultivars with similar ability to survive long-term freezing within the groups, but differences between the groups diminish with longer periods in the frozen state. We suggest that the LT 5 can be used toprovide evidence of these groups, but we agree with Gusta et al. (2001) that long-term freezing tests are needed to identify genotypes with superior ability to remain viable while frozen for long periods of time.  CKNOWLEDGEMENT The authors are grateful to Brian Bellinger for excellent technical assistance. This project was supported by USDA-ARS project 5348-21430-003-OUr). Mention of product names does not represent an endorsement ofany product or company but is given only to clarify the methodology; other products may he equally effective. Fowler D. B. 1979. Selection for winterhardiness in wheat. 11. Variation within field trials. Crop Sci. 19: 773-775. Fowler, D. B., Gusta, L. V. and T y ler, N. J.. 1981. Selection for winterhardiness in wheat. 111. Screening methods. Crop Sci.21: 896901. Greer R. C. Wu J. Q   Singh P. and MeCool D. K. 2006. VVEPP simulation of observed winter runoff and erosion in the U.S. Pacific Northwest. Vad. Zone J. 5: 261-272. Gusta L. V. Bo y achek M. and Fowler D. B. 1978. A system for freezing biological materials. Hortic. Sci. 13: 171-172. Gusta L. V. O Connor, B. J. and MacHutcheon, N1. C. 1997. The selection of superior winter-hardy genotypes using a prolonged freeze test. Can. J. Plant Sci. 77: 15-21. Gusta, L. V. O Connor, B. 3., Gao, Y. P. and Jana, S. 2001. A re-evaluation of controlled free7e-tests and controlled environ- ment hardening conditions to estimate the winter survival potential of hardy winter wheats. Can. J. Plant Sci. 81: 241- 246. Herman, E. M. Rotter, K. Premakumar, R., Elwinger, C., Bae, R. Ehier-King L Chen S. and Livingston D. P. III. 2006. Additional freeze hardiness in wheat acquired by exposure to -3C is associated with extensive physiological rnorpholo-gical and molecular changes. J. Exp. Bot. 14: 3601 3618. Kahraman A. Kusmenoglu 1. Aydin N. Avdogan A. Erskine W. and Muehlbauer F. J. 2004. Genetics of winter hardiness in 10 lentil recombinant inbred line populations. Crop Sci. 44: 5-12. Koemel, J. E., Jr., Guenzi, A. C., Anderson J. A. and Smith, E. L. 2004. Cold hardiness of wheat near-isogenic lines differing in vernalization alleles. Theor. Appl. Genet. 109: 839-846. Livingston D. P. 111. 1996. The second phase of cold hard- ening: freezing tolerance and fructan isomer changes in wintercereal crowns. Crop Sci. 36: 1568-1573. Mahfoozi, S. Limin, A. E. and Fowler, D. B. 2001. Influence of vernalization and pholoperiod responses on cold hardiness in winter cereals. Crop Sci. 41: 1006-1011. Mahfoozi, S. Limin, A. E., Hayes, P. M., 1-tucl, P. and Fowler, D. B. 2000. Influence of photoperiod response on the expres-sion of cold hardiness in wheat and barley. Can. J. Plant Sci. 80: 721-724. Olien C. R. 1984. An adaptive response of rye to freezing. Crop Sci. 24: 5 1-54.
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