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M.A. McKellar, D.W. Buchanan, Dewayne L. Ingram and C.W. Campbell

Freezing tolerance and the lethal freezing temperature were determined for detached leaves of avocado (Persea americana Mill.) by either electrolyte leakage or visual appearance of browning. Leaves from field-grown trees of `Gainesville', `Booth8', and `Winter Mexican' in both Gainesville and Homestead, Fla., were evaluated. All cultivars in both locations survived ice formation in their tissue. Leaf tissue had a temperature limit (lethal freeze temperature) at and below which the tissue died. The lethal freezing temperature varied from -5.1 to -9.3C, depending on time of year and location. The lethal freeze temperature for a cultivar decreased over the fall and winter as temperatures decreased. Leaves of `Booth-8' and Winter Mexican' decreased 2.5 and 1.5C, respectively, in Homestead from 13 Nov. 1982 to 5 Feb. 1983. The plants growing at the lower temperature location (Gainesville) had lower lethal freeze temperatures. Leaves of `Gainesville' had a lethal freeze temperature of - 9.3C from trees at Gainesville and - 7.8C from trees at Homestead.

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Hrvoje Rukavina, Harrison Hughes and Yaling Qian

Efforts are ongoing at Colorado State University to develop turf-type saltgrass cultivars. Prior freezing studies have indicated variation in freezing tolerance in saltgrass lines. Therefore, this study was made to examine relative freezing tolerance of 27 saltgrass clones as related to collection sites in three zones of cold hardiness. Furthermore, these lines were evaluated for fall color retention with the intent to determine if there is a correlation with fall color and freezing tolerance. Saltgrass rhizomes were sampled in mid-winter 2004 from lines established in Fort Collins, Colo., and then subjected to a laboratory-freezing test. Saltgrass freezing tolerance was highly influenced by climate zones of clones' origin (P < 0.01) and genotypes within zones (P < 0.01). There was a high negative correlation between color retention in the fall and freezing tolerance (P < 0.01). Average freezing tolerance of saltgrass clones within zones of origin significantly differed among zones. Ranking of zones for least square mean LT50 (OC) was: zone 4 (–17.2) < zone 5 (-14.4) < zone 6 (–11.1). LT50 values in zone 4 ranged from –17.8 (accession 72) to –17.0 (accession 87). Clones in zone 5 showed LT50 values from –17.8 (accession A29) to –11.9 (accession A137). Zone 6 clones had LT50 values that ranged from –9.5 (accession C92) to –12.6 (accession C12). Large intraspecific variation in freezing tolerance may be effectively used in new cold hardy cultivar development. Environmental adaptation inherited by origin of clone is useful in defining clones' adaptation range and may along with fall color retention serve as a selection criterion in saltgrass cold hardiness improvement.

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Imed E. Dami, Shouxin Li, Patricia A. Bowen, Carl P. Bogdanoff, Krista C. Shellie and Jim Willwerth

both sides of the vine canopy of ‘Chardonnay’ winegrape either at véraison (V), postvéraison (PV), or postharvest (PH) at field trial sites in the United States and Canada. Bud freezing tolerance. The FT of buds was measured at each location in each

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James J. Polashock, Rajeev Arora, Yanhui Peng, Dhananjay Naik and Lisa J. Rowland

.), cold acclimation is considered a two-step process ( Weiser, 1970 ). The first stage is thought to be induced by a short photoperiod, while the second stage, characterized by a more pronounced increase in freezing tolerance, is induced by low

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Y.-K. Chen, J.P. Palta and J.B. Bamberg

Wild potato species provide a valuable source of genetic variability for the improvement of freezing tolerance in cultivated potato, Solanum tuberosum (tbr). However, breeding for freezing tolerance by using wild genetic resources has been hampered by contradictory results regarding the genetic control of this trait. Both dominance and recessiveness for this trait have been reported. To explore the genetic control of freezing tolerance, the expression of freezing tolerance was investigated in various interspecific F1 and somatic hybrids between hardy and sensitive species. In addition to 2 years of field evaluation, freezing tolerance before and after acclimation was characterized separately under controlled environments to dissect the two independent genetic components of freezing tolerance, namely nonacclimated freezing tolerance (NA) and acclimation capacity (ACC). The expression of freezing tolerance, including NA and ACC, was closer to that of hardy parent, sensitive parent, or approximate parental mean, depending on species combination. However, the expression of freezing tolerance tended to be greater when the hybrids contained more sets of chromosomes from the hardy parent than from the sensitive parent. The significance of hardy: sensitive genomic ratio was further supported by using sexual and somatic hybrids between tbr and S. commersonii (cmm) to achieve different genomic ratios without the confounding effect of species. Therefore, we propose that the hardy: sensitive genomic ratio is an important determinant for the expression level of freezing tolerance before and after cold acclimation.

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Christopher L. Owens

Low temperature is one of the most important environmental factors limiting crop plant growth, distribution, and productivity. New cultivars with improved freezing tolerance are a common breeding objective of many temperate fruit breeding programs. Improved freezing tolerance would prevent crop loss due to low temperature and reduce yearly fluctuations in crop quantity and quality. Breeding temperate fruit cultivars for improved freezing tolerance is made difficult by several factors, including complexity of the phenotype, difficulty in accurate measurement of the phenotype, and lack of fundamental knowledge concerning the inheritance and genetic control of this trait. Results from inheritance studies of freezing tolerance in temperate fruit crops as well as recent research in forestry genetics highlight some of the challenges and opportunities for further elucidating the inheritance of freezing tolerance in temperate fruit crops. A tremendous amount of research has been conducted describing the molecular biology and signal transduction of the cold acclimation response in the model plant, Arabidopsis thaliana. These findings have begun the transfer to research in agriculturally important crops and hold great promise for elucidating novel methods for generating new fruit cultivars with improved freezing tolerance.

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Reeser C. Manley and Rita L. Hummel

Mefluidide, a synthetic plant growth regulator, has been reported to protect chilling-sensitive plants from chilling damage and enhance the freezing tolerance of certain winter-hardy herbaceous plants. The potential of mefluidide to enhance the freezing tolerance of nonhardened and dehardening cabbage (Brassica oleracea L. Capitata Group) leaf tissue was investigated. Mefluidide at 0 to 60 mg·L–1 was tested on `Brunswick' and `Golden Acre' cabbage in five experiments. Leaf tissue freezing tolerance was measured 3 to 9 days postapplication by electrolyte leakage assay. The interval between application and freeze testing had no effect on leaf freeze tolerance. The effect of mefluidide at low rates on leaf freeze tolerance was small and inconsistent. At 30 and 60 mg·L–1, leaf freeze tolerance was decreased consistently. Chemical name used: N-{2,4-dimethyl-5-[[trifluromethyl)sulfonyl]amino]phenyl}acetamide (mefluidide).

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S. Ball, Y.L. Qian and C. Stushnoff

No information is available regarding endogenous soluble carbohydrate accumulation in buffalograss [Buchloe dactyloides (Nutt.) Engelm.] during cold acclimation. The objective of this study was to determine composition of soluble carbohydrates and their relationship to freezing tolerance in two buffalograss cultivars, 609 and NE 91-118, with different freezing tolerances. The experiment was conducted under natural cold acclimation conditions in two consecutive years in Fort Collins, Colo. Based upon average LT50 (subfreezing temperature resulting in 50% mortality) from seven sampling intervals in 1998-99 and six sampling intervals in 1999-2000, `NE 91-118' survived 4.5 °C and 4.9 °C colder temperatures than `609', during the 1998-1999 and 1999-2000 winter seasons, respectively. Glucose, fructose, sucrose, and raffinose were found in both cultivars in both years, and were generally higher in acclimated than pre- and post-acclimated stolons. Stachyose was not present in sufficient quantities for quantification. Cultivar NE 91-118 contained 63% to 77% more glucose and 41% to 51% more raffinose than `609' in the 1998-99 and 1999-2000 winter seasons, respectively. In 1999-2000, fructose content in `NE 91-118' was significantly higher than that of `609'. A significant negative correlation was found between LT50 vs. all carbohydrates in 1999-2000, and LT50 vs. sucrose and raffinose in 1998-99. Results suggest that soluble carbohydrates are important in freezing tolerance of buffalograss.

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Hrvoje Rukavina, Harrison Hughes and Yaling Qian

Freezing is the major abiotic stress that limits geographical distribution of warm-season turfgrasses. Prior studies have indicated variation in freezing tolerance in saltgrass clones. Therefore, this 2-year study examined the freezing tolerance of 27 saltgrass clones as related to collection sites in three zones of cold hardiness. Furthermore, these clones were evaluated for time of leaf browning in the fall with the intent to determine if there was a correlation between this trait and freezing tolerance. Rhizomes were sampled during 2004 and 2005 midwinters from clones established in Fort Collins, Colo., and then subjected to a freezing test. Saltgrass freezing tolerance was highly influenced by the climatic zone of clone origin in both years of the experiment. Clones with greater freezing tolerance turned brown earlier in fall in both seasons. Ranking of zones for the average LT50 was: zone 4 (–17.2 °C) < zone 5 (–14.4 °C) < zone 6 (–11.1 °C) in 2004 and zone 4 (–18.3 °C) < zone 5 (–15.7 °C) < zone 6 (–13.1 °C) in 2005. Clones from northern areas tolerated lower freezing temperatures better overall. This confirmed that freezing tolerance is inherited. Large intraspecific variation in freezing tolerance may be effectively used in developing cold-hardy cultivars.

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F.P. Maier, N.S. Lang and J.D. Fry

Little is known about intraspecific variability in St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] freezing tolerance and the physiological factors that may influence survival. Stolons of field-grown `Raleigh', `Floratam', and FX-332 St. Augustinegrass were sampled between October and March in 1990 to 1991 and 1991 to 1992 to measure freezing tolerance, nonstructural carbohydrates, and water content. Stolons were exposed to temperatures between 1 and -8C in a freezer, and regrowth was evaluated in the greenhouse. Generally, freezing tolerance of `Raleigh' > `Floratam' = FX-332. `Raleigh' exhibited >60% survival in December and January, while survival of `Floratam' and FX-332 was <20%. `Raleigh' was the only cultivar that acclimated, as indicated by a 75% increase in survival between October and December 1990. Starch and sucrose were the primary storage carbohydrates extracted from stolons, but neither was correlated with freezing tolerance. A negative (r = -0.80) correlation was observed between `Raleigh' survival and stolon water content between January and March 1991. Reduced water content in `Raleigh' stolons during winter months may contribute to acclimation.