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Orville M. Lindstrom and Glen W. Kent

Magnolia has graced southern landscapes for many years. However, its northern distribution is limited due to injury at low, freezing temperatures. Laboratory methods are available to assess the cold hardiness of many plants, but specific methods for Southern magnolia have not been established. Effects of exposure time, temperature at which plants were frozen, rate of warming, sample size and methods of injury evaluation were investigated. With exposure to -1.5 and -4C the leaves and stems were not injured when frozen for up to 7h. Stems and leaves that were nucleated with ice at -4C underestimated the cold hardiness as compared to similar plants that were nucleated at -1.5 and -3C. Samples warmed as taken from the temperature bath at 4C or at 4C/hr in the bath exhibited less injury than those taken directly out of the bath and exposed to room temperature. Similar cold hardiness determinations were obtained using whole and half leaf samples, while a quarter of a leaf or a leaf disk exhibited high variability and resulted in unreliable cold hardiness determinations. Visual analysis for injury was compared to electrolyte leakage and similar cold hardiness levels were obtained using the two methods.

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Mark K. Ehlenfeldt, Lisa J. Rowland, Elizabeth L. Ogden, and Bryan T. Vinyard

The observed floral bud cold hardiness (or lack of) in blueberry ( Vaccinium sp.) plants containing southern-adapted germplasm is a result of both midwinter cold hardiness and deacclimation rates ( Rowland et al., 2005 ). Southern U.S. blueberry

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Timothy F. Bourne and James N. Moore

Cold hardiness was evaluated in twelve seedling populations of tetraploid blackberry, in 1988 and 1989. Seedling populations resulted from crosses made between nine parents of three different categories of cold hardiness. Viability testing of xylem, phloem, and bud tissues were conducted following exposure of tissues to a low temperature estimated to kill one-half of all tissues. Tissues were rated as alive if green and dead if any browning of tissue was seen.

Significant population effects (P<.05) were seen for xylem and bud survival in 1988 and for xylem, phloem, and bud survival in 1989. Results were similar for the two years, although there was a greater discrimination between populations for xylem and phloem survival in the second year. The four populations having `Darrow' as a parent consistently showed greater survival than the other eight lines. The six populations which had `Brison' as one parent showed consistently poor survival with the exception of one resulting from a cross of `Brison' × `Darrow'. This population showed consistently good hardiness, indicating that dominance effects may play a role in cold hardiness of blackberries.

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Timothy F. Bourne and James N. Moore

Cold hardiness was evaluated in twelve seedling populations of tetraploid blackberry, in 1988 and 1989. Seedling populations resulted from crosses made between nine parents of three different categories of cold hardiness. Viability testing of xylem, phloem, and bud tissues were conducted following exposure of tissues to a low temperature estimated to kill one-half of all tissues. Tissues were rated as alive if green and dead if any browning of tissue was seen.

Significant population effects (P<.05) were seen for xylem and bud survival in 1988 and for xylem, phloem, and bud survival in 1989. Results were similar for the two years, although there was a greater discrimination between populations for xylem and phloem survival in the second year. The four populations having `Darrow' as a parent consistently showed greater survival than the other eight lines. The six populations which had `Brison' as one parent showed consistently poor survival with the exception of one resulting from a cross of `Brison' × `Darrow'. This population showed consistently good hardiness, indicating that dominance effects may play a role in cold hardiness of blackberries.

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J. Ryan Stewart, William R. Graves, and Reid D. Landes

Can Carolina buckthorn (Rhamnuscaroliniana) persist north of its native habitat without becoming invasive? Its distribution (USDA zones 5b to 9b) suggests that genotypes vary in cold hardiness, and invasiveness of other Rhamnus sp. has been linked to unusually early budbreak each spring. Therefore, we investigated depth of cold hardiness and vernal budbreak of Carolina buckthorns from multiple provenances and made comparisons to the invasive common buckthorn (Rhamnus cathartica). Budbreak was recorded in Ames, Iowa, from 9 Apr. to 10 May 2002. Buds of common buckthorn broke earlier than those of Carolina buckthorn, and mulching plants of Carolina buckthorn hastened budbreak. Stem samples were collected in October, January, and April from a plot in Ames, Iowa (USDA zone 5a), of Carolina buckthorns from three provenances (Missouri, Ohio, and Texas) and of naturalized common buckthorns. A similar schedule was followed during the next winter, when two plot locations [Ames, Iowa, and New Franklin, Mo. (USDA zone 5b)], were compared, but Carolina buckthorns from only Missouri and Texas were sampled. Carolina buckthorn and common buckthorn survived midwinter temperatures as low as –21 °C and –24 °C, respectively. Provenance differences were minimal; Carolina buckthorns from Missouri were more hardy than those from Ohio and Texas only in April of the first winter. We conclude that its cold hardiness will permit use of Carolina buckthorn beyond where it is distributed in the southeastern United States. Delayed budbreak of Carolina buckthorn relative to that of common buckthorn may underscore the potential for Carolina buckthorn in regions with harsh winters and may lessen its potential to be as invasive as common buckthorn.

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Allen V. Barker

Plant Cold Hardiness: From the Laboratory to the Field. 2009. Lawrence V. Gusta, Michael E. Wisniewski, and Karen K. Tanino (eds.). CABI, 875 Massachusetts Avenue, 7 th Floor, Cambridge, MA 02139. 317 p. $170, hardcover, ISBN-13: 978

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Girish K. Panicker and Frank B. Matta

Growth regulators ABA and paclobutrazol were used at different concentrations to induce hardiness in blueberry flower buds and floral parts. Critical freezing temperatures and the effectiveness of the treatments were determined by differential thermal analysis (DTA), electrolyte leakage, visual browning, and tetrazolium staining. Treatment effects of growth regulators were nonsignificant on whole flower buds, but treatments induced hardiness in floral parts on the second flush of flowers at stage six produced in April. Induction of cold hardiness by ABA and paclobutrazol was concentration dependent. The higher the concentration, the greater the response. Viability test results on each floral part showed a close relationship with the critical freezing temperatures recorded by DTA. Control treatments showed that floral parts at stage six developed in April were more prone to freezing injury compared to floral parts at stage six developed in early March.

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Annette M. Zatylny, J.T.A. Proctor, and J.A. Sullivan

Two selections and two cultivars of red raspberry (Rubus idaeus L.) were evaluated for cold hardiness in vitro. Tissue-cultured shoots were exposed to temperatures from 0 to –18C and samples were removed at 2C intervals. Injury was assessed by a visual rating of tissue browning after freezing. Only shoots subjected to step-wise acclimation at low temperatures before freezing revealed significant differences among the four types in the lowest shoot survival temperature. Acclimation treatments increased the lowest survival temperatures of in vitro shoots by a mean of 3.1C. The hardiness obtained from this screening method agreed with that of winter survival in the field. Ranking, from the most to least cold hardy, was `Boyne', Gu 72, Gu 63, and `Comox'.

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Annie M. Chozinski and Shawn A. Mehlenbacher

Laboratory freezing tests were conducted for two consecutive winters to determine the cold hardiness (CH) of hazelnut trees (Corylus).Thirty-six different genotypes were evaluated. LT50s were calculated for female inflorescences, catkins and vegetative buds. Most (> 70%) female flowers achieved their maximum hardiness in January. Nearly half (45 %) of all female flowers had LT50s between -38.0 C and -21.4 C. Catkins were most hardy in December after which they began to elongate and lose their CH. In December, catkin CH ranged from -33.0 to -13.4 C. Vegetative buds were more hardy than both female flowers and catkins. LT50s ranged from, -40.0 to -26.8 C with 95% achieving maximum CH in January. More than half (54%) had LT50s between -30.0 and -20.0 C. In summary, vegetative buds are more CH than female buds which in turn are more hardy than catkins.

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Mark K. Ehlenfeldt, Elizabeth L. Ogden, Lisa J. Rowland, and Bryan Vinyard

The midwinter cold hardiness of 25 rabbiteye (V. ashei) blueberry cultivars was assayed across 2 years using a shoot freezing assay. LT50values (i.e. temperature at which 50% of buds are damaged) for the cultivars ranged from –24.9 °C for `Pearl River' (a 50% V. ashei derivative) to –13.7 °C for `Chaucer'. Under New Jersey conditions, numerous cultivars were observed to exhibit dimorphism for dormant floral bud size. Comparisons of bud dimorphism with LT50 values, found dimorphism more common in cultivars with lower floral bud hardiness. LT50 values generally supported empirical observations of winter hardiness, but exceptions suggest that additional factors contribute to observed winter hardiness under field conditions.