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Lisa J. Rowland, Elizabeth L. Ogden, Fumiomi Takeda, David Michael Glenn, Mark K. Ehlenfeldt, and Bryan T. Vinyard

, 2008 ). Only a few studies have been carried out on frost tolerance of open flowers of blueberry, primarily on the southern rabbiteye species, V. virgatum Aiton (syn. V. ashei Reade) ( Gupton, 1983 ; NeSmith et al., 1999 ; Spiers, 1978 ). Spiers

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Diego Barranco, Natividad Ruiz, and María Gómez-del Campo

This study aims to determine the relationship between laboratory frost-resistance data for the leaves of eight olive cultivars and observed field resistance in the same genotypes undergoing natural frost damage. The lethal freezing temperature (LT50) for each cultivar was established by measuring the electrical conductivity (EC) of the medium into which solutes from damaged leaf tissue were leaked. The value obtained was then correlated with percentage frost shoot for the same eight cultivars damaged by natural frosts in a field test. A negative correlation was observed between the percentage frost shoot and leaf LT50 for all the cultivars under study. The most frost-hardy cultivars (`Cornicabra', `Arbequina', and `Picual') were those presenting the lowest percentage frost shoot and lowest LT50. Conversely, the most frost-susceptible cultivar (`Empeltre') displayed 100% frost shoot, together with one of the highest LT50 values (–9.5 °C). According to these results, lethal freezing temperature (LT50) calculated from leaf ion leakage at a range of freezing temperatures, seem to be a valid parameter for evaluating frost tolerance in olive cultivars.

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László Szalay, Béla Timon, Szilvia Németh, János Papp, and Magdolna Tóth

frost resistance, which also vary greatly over time. The experiments carried out in apricot and peach chiefly examined changes in the frost resistance of the flower buds, but there is information on the frost tolerance of the vegetative buds and the

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Francine Bigras

Spring frosts frequently cause significant damage to conifer seedlings during bud flushing and shoot elongation in forestry nurseries. To ensure adequate protection, levels of frost sensitivity must be known during these stages of development. Eight-month-old, containerized, black spruce seedlings were submitted to freezing temperatures of 0, –4, –6, –8, and –10C for 1, 2, 3, 4, 5, and 6 h at the following stages: 1) nonswollen buds; 2) swollen buds; 3) bud scales bursting, needle tips emerging; and 4) shoot elongation, 1 to 5 cm. After the treatments, seedlings were grown for 90 days in a greenhouse. Seedling survival then was estimated; dead seedlings discarded; and damage to buds, needles, and roots and shoot increment and diameter were measured on the remaining seedlings. Results show that frost sensitivity increases with the developing bud and shoot. A decrease in seedling and bud survival was noted with an increase in time of exposure (stages 2, 3, 4); otherwise, time exposure has no effect. Damage to needles and roots increases and diameter decreases with decreasing temperatures at all stages. Shoot increment was influenced by decreasing temperatures at stages 2 and 3 only.

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E. W. Neuendorff and K. D. Patten

A late spring frost, -2°C on 10 Mar 1989, destroyed all blossoms on `Delite' rabbiteye blueberries. To determine the effect of hedging as a rejuvenation method, six-year-old `Delite' plants were pruned on 26 April 1989. All branches were removed at 46 cm from ground level. Unpruned control plants were approximately 184 cm tall. On 21 Mar 1990 a frost of -2°C occurred. Two days later bud damage was assessed on three wood types: spring-old (SO), spring growth on old, weak wood; spring-new (SN), spring growth on vigorous 1-year-old shoots; and fall (F), postharvest late summer/fall growth. Buds were identified as to their stage of development. Buds formed on both types of spring wood were further developed than those on fall wood. As flower stage advanced frost damage increased. Blossoms on fall growth were most frost tolerant and SN was more hardy than SO. Subsequent yields will be determined and reported.

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Timothy P. Hartmann, Justin J. Scheiner, Larry A. Stein, Andrew R. King, and Sam E. Feagely

PBDD (where plant size is considered), as noted by Chat [1995 (data not shown)]. Improved frost tolerance within the fuzzy kiwifruit species is reportedly associated with a smaller amount of larger-diameter xylem vessels ( Massai et al., 1991

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E. W. Neuendorff and K. P. Patten

Rabbiteye blueberry flower buds are initiated and differentiated on three distinct wood types - spring growth on old weak growth, spring growth on vigorous 1-year-old shoots, or postharvest late summer/fall growth. Flower buds on spring growth are usually formed and visible by July, while buds formed on postharvest growth flushes appear in late summer and early fall. To evaluate the influence of wood type on cold damage, shoots of `Tifblue' and `Delite' were tagged by season of growth. Following a -10°C freeze in Feb. flower buds on shoots from each growth flush were examined for dead ovaries. Flower buds surviving the freeze were evaluated following a -2° late frost in Mar. Influence of wood type on floral bud and fruit development was determined. All fruit were removed from 5 shoots of each wood type on 2 harvest dates corresponding to early and midseason harvests. Floral buds formed on fall growth were more freeze and frost tolerant than those initiated on spring growth at similar stages of bud development. `Tifblue' was more cold tolerant than `Delite'. Floral buds formed on both spring wood types were earlier to develop than buds formed on fall wood. There were no differences in ripening patterns and quality of fruit removed from spring - new and fall wood. Fruit formed on spring - old wood were later maturing and smaller sized for both harvests than spring-new or fall wood. Postharvest pruning to encourage fall growth may be a cultural means of frost avoidance.

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Valtcho D. Zheljazkov, Charles L. Cantrell, Tess Astatkie, and Ekaterina Jeliazkova

‘Native’ spearmint (Mentha spicata L.) is a widely grown essential oil crop worldwide and in the midwest in the United States. There is interest in expanding spearmint production to Wyoming and other states. However, there is no information to determine if spearmint would perform well under the Wyoming high-altitude and short-growing season and if its productivity and oil quality would be affected by fall frosts. The objective of this study was to evaluate the effect of fall frosts at the end of the cropping season on ‘Native’ spearmint productivity and oil profile. Spearmint plants were harvested at the following harvest dates (HDs): 14 Sept., 21 Sept., 28 Sept., 5 Oct., 12 Oct., 24 Oct., and 1 Nov. 2011. The HDs were selected to coincide with the fall frosts in northern Wyoming. Indeed, during that time, frost occurred on the following dates: 21 Sept., 10 Oct., 13 Oct., 15 Oct., 19 Oct., 20 Oct., 24 Oct., 25 Oct., 26 Oct., 27 Oct., 28 Oct., 29 Oct., 30 Oct., 31 Oct., and 1 Nov. 2011. The first heavy snow occurred on 3 Nov. Fresh herbage yields were higher at the 1 Nov. HD relative to the 14 Sept. HD, whereas the yields at the other HD were not significantly different. Generally, the oil content was high at 14 Sept., 21 Sept., and 5 Oct. HD and low at the 24 Oct. and 1 Nov. HD. Carvone concentration (42% to 75% range) in the oil reached a maximum at the 12 Oct. HD. The concentration of limonene was low at the first HD (14 Sept.) and higher at the other HDs. The yield of carvone (a function of the fresh herbage yields, oil content, and the concentration of carvone in the oil) was high at the 12 Oct. HD and low at the other HDs. In the spring of 2012, spearmint emerged in late April and was unaffected by the Wyoming winter or by the early spring frosts. This preliminary study suggests ‘Native’ spearmint may be a viable crop for northern Wyoming at elevation of ≈1170 m.

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Kim Patten, Elizabeth Neuendorff, Gary Nimr, John R. Clark, and Gina Fernandez

The relative tolerance of flower buds and flowers of southern highbush blueberry (Vaccinium spp.) to cold damage was compared to rabbiteye (Vaccinium ashei Reade) and highbush blueberry (Vaccinium corymbosum L.). For similar stages of floral bud development, southern highbush and highbush cultivars had less winter freeze and spring frost damage than rabbiteye cultivars. Cold damage increased linearly with stage of flower bud development. Small fruit were more sensitive to frost damage than open flowers. Rabbiteye blueberry flower buds formed during the fall growth flush were more hardy than buds formed during the spring growth flush, regardless of cultivar or stage of development.

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C.L. Haynes, O.M. Lindstrom, and M.A. Dirr

Cooling treatments of 2, 4, and 6C/hour or warming at 25, 4, or 0C influenced the cold hardiness estimates of x Cupressocyparis leylandii (A.B. Jacks. and Dallim.) Dallim. and A.B. Jacks. (Leyland cypress), Lagerstroemia indica L. (crape myrtle), and Photinia ×fraseri Dress `Birmingham' (redtip photinia) at four times during the year. New growth from all taxa, especially spring growth, was injured or killed at higher temperatures by the fastest cooling rate and/or by warming at 25C. Cold hardiness of Leyland cypress was unaffected by the cooling and warming treatments. Crape myrtle had a significantly higher lowest survival temperature (LST) when warmed at 25C than at 4 or 0C. Photinia leaves and stems cooled at 6C/hour or warmed at 25C generally resulted in a higher LST than those cooled more slowly or warmed at lower temperatures. Cooling rates of 14C/hour and warming at 0 to 4C should be used in freeze tests with Leyland cypress and crape myrtle. For leaves and stems of photinia, 2C/hour cooling and warming at 0 to 4C should be used.