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Chon C. Lim, Rajeev Arora and Edwin C. Townsend

Seasonal patterns in freezing tolerance of five Rhododendron cultivars that vary in feezing tolerance were estimated. Electrolyte leakage was used, and raw leakage data were transformed to percent leakage, percent injury, and percent adjusted injury. These data were compared with visual estimates of injury. Percent adjusted injury was highly correlated (0.753) to visual estimates. Two asymmetric sigmoid functions—Richards and Gompertz—were fitted to the seasonal percent adjusted injury data for all cultivars. Two quantitative measures of leaf freezing tolerance—Lt50 and Tmax (temperature at maximum rate of injury)—were estimated from the fitted sigmoidal curves. When compared to the General Linear Model, the Gompertz function had a better fit (lower mean error sum of squares) than Richards function. Correlation analysis of all freezing tolerance estimates made by Gompertz and Richards functions with visual LT50 revealed similar closeness (0.77 to 0.79). However, the Gompertz function and Tmax were selected as the criteria for comparing relative freezing tolerance among cultivars due to the better data fitting of Gompertz function (than Richards) and more descriptive physiological representation of Tmax (than LT50). Based on the Tmax (°C) values at maximum cold acclimation of respective cultivars, we ranked `Autumn Gold' and `Grumpy Yellow' in the relatively tender group, `Vulcan's Flame' in intermediate group, and `Chionoides' and `Roseum Elegans' in the hardy group. These relative rankings are consistent with midwinter bud hardiness values reported by nurseries.

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Christopher L. Owens, Michael F. Thomashow, James F. Hancock and Amy F. Iezzoni

Orthologs of CBF1, a cold-induced transcription factor important in the cold acclimation response in Arabidopsis thaliana were cloned from strawberry (Fragaria × ananassa Duchesne) and sour cherry (Prunus cerasus L.) with degenerate PCR primers. The putative orthologs [Fragaria ×ananassa CBF1 (FaCBF1) and Prunus cerasus CBF1 (PcCBF1)] have 48% amino acid identity to CBF1 and mRNA levels were up-regulated in leaves of both crops following exposure to 4 °C from 15 minutes to 24 hours. However, mRNA of FaCBF1 and PcCBF1 was not detected in pistils of strawberry and sour cherry following 4 °C exposure. Agrobacterium-mediated transformation of a CaMV35S-CBF1 construct was conducted on Fragaria ×ananassa `Honeoye' crown discs. Two transgenic lines were regenerated that expressed the transgene at low levels in both leaves and receptacles. Receptacles of the transgenic lines showed no significant change in freezing tolerance when compared to wild type plants, although the temperature at which 50 % electrolyte leakage occurred in detached leaf discs from the two transgenic lines was -8.2 °C and -10.3 °C, respectively. These freezing tolerance values were significantly greater than the value for the wild-type `Honeoye' leaf discs of -6.4 °C.

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Calin O. Marian, Atilla Eris, Stephen L. Krebs and Rajeev Arora

The influence of photoperiod and temperature on the seasonal (fall to winter) cold acclimation and accumulation of a 25 kDa dehydrin in Rhododendron `Chionoides' was studied by exposing two groups of plants each in the greenhouse or outdoors to either a natural photoperiod (or short days) or an extended photoperiod (or long days) regime. Results suggest that the shortening daylength alone is sufficient to trigger both the first stage of cold acclimation and concomitant 25 kDa dehydrin induction. Exposure of the plants to natural photoperiod and temperatures induced the greatest cold hardiness and 25 kDa accumulation, while exposure to extended photoperiods (long days) and warmer temperatures (in the greenhouse) failed to induce any significant freezing tolerance in leaves. Whereas short days trigger the cold acclimation process initially, low inductive temperatures can eventually replace the photoperiod stimulus. Seasonal accumulation of 25 kDa dehydrin, on the other hand, appears to be predominantly effected by short photoperiods. Data indicated that the leaf water content of outdoor plants maintained under natural photoperiod was lower than that of plants grown under extended photoperiod. This was also true for the greenhouse plants at the first (September) and the last (January) sampling. It is hypothesized that early 25 kDa dehydrin accumulation may be due to short-day-induced cellular dehydration. Accumulation of two other dehydrins of 26 kDa and 32 kDa molecular masses does not appear to be associated with short day (SD)-induced first stage of cold acclimation. Results show that their accumulation may be regulated by low, subfreezing temperatures and may be associated with the second and/or third stage of cold acclimation of `Chionoides' rhododendron leaves.

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Milton E. Tignor Jr., Frederick S. Davies and Wayne B. Sherman

Citrus hybrids USDA 17-11 [Citrus grandis L. × (C. paradisi Macf. `Duncan' × Poncirus trifoliata (L.) Raf. `Gotha Road')] and 119 [(C. paradisi Macf. `Duncan' × P. trifoliata (L.) Raf. `Gotha Road') × C. sinensis (L.) Osb. `Succory'], `Hamlin' orange [C. sinensis (L.) Osb.], and satsuma mandarin (C. unshiu Marc.) were planted March 1993 and 1994. Trees were irrigated and fertilized in an identical manner. In 1993, electrolyte leakage readings were taken monthly using 17-11, 119, and satsuma leaf discs. Leaf killing point (LKP) LT50 averaged from –8 to – 9C by mid-November for all selections. In 1994, leaf discs from 17-11, 119, and `Hamlin' orange were sampled weekly to determine LKP. USDA 119 had the lowest LKP and acclimated the fastest during the fall. By the end of November, there was no significant difference in LKP (–6.5C) between USDA 119 and 17-11, although both selections were significantly more freeze-tolerant than `Hamlin' orange (LKP–40C), which showed no significant decrease in LKP until the 6 weeks after the hybrid selections began acclimating. Citrus hybrids 17-11 and 119 can survive in freeze-susceptible areas that are marginal for other commercial citrus.

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Anne Fennell and Emily Hoover

The grape species Vitis labruscana Bailey and V. riparia Michx. were subjected to a decreasing photoperiod at constant moderate temperatures to determine whether acclimation occurred in response to a shortening photoperiod. Cane growth, periderm development, killing temperature of the primary bud, and bud dormancy were measured in vines receiving a natural photoperiod (ND), a simulated long photoperiod of 15 hours (LD), and shorter photoperiods of 14, 13, or 12 hours (SD). The LD treatment was effective at maintaining growth and inhibiting periderm development and the onset of bud dormancy in V. labruscana. Cane growth rate with all SD treatments decreased as compared to the LD regime. A significant increase in periderm development occurred with the 12-hour SD treatment. Similarly, the onset of bud dormancy was promoted by the 12-hour SD in V. labruscana. The primary bud killing temperature was 1C lower in V. labruscana under the 12-hour SD than under the LD treatment. In contrast, the LD treatment neither maintained growth nor fully inhibited periderm development and the onset of dormancy in V. riparia. The decrease in the cane growth rate upon exposure to SD was significantly greater in V. riparia than V. labruscana. Periderm development was observed in both the SD and its respective LD-treated V. riparia vines. However, the rate of periderm development was significantly greater in the SD vines than in the LD vines. The onset of bud dormancy was promoted by 13-hour SD in V. riparia. Similarly, the primary bud killing temperature was 2 to 3C lower in V. riparia upon exposure to SD. Vitis riparia has a longer critical photoperiod than V. labruscana and appears to be more sensitive to changes in light intensity or light quality. While the change in freezing tolerance in response to short photoperiods is small, the photoperiod response at a longer critical photoperiod, when combined with lower temperatures, will promote an earlier and possibly more rapid cold acclimation in V. riparia than in V. labruscana.

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

, J.S. Lim, C.C. Panta, G.R. Vorsa, N. 2000 Genetic analysis of freezing tolerance in blueberry ( Vaccinium section Cyanococcus) Theor. Appl. Genet. 100 690 696 Arora, R. Rowland, L

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

can be used and selected to produce desirable acclimation and deacclimation processes. Literature Cited Arora, R. Rowland, L.J. Lehman, J.S. Lim, C.C. Panta, G.R. Vorsa, N. 2000 Genetic analysis of freezing tolerance in blueberry ( Vaccinium section

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Rajeev Arora, Michael Wisniewski and Lisa J. Rowland

Seasonal changes in cold tolerance and proteins were studied in the leaves of sibling deciduous and evergreen peach [Prunus persica (L.) Batsch]. Freezing tolerance [defined as the subzero temperature at which 50% injury occurred (LT50)] was assessed using electrolyte leakage. Proteins were separated by sodium dodecyl sulfate polyacrylamide-gel electrophoresis. Electroblots were probed with anti-dehydrin and anti-19-kD peach bark storage protein (BSP) antibodies. Leaf LT50 decreased successively from -5.8 °C on 18 Aug. to -10.3 °C in the evergreen genotype and from -7.0 °C to -15.0 °C in the deciduous genotype by 14 Oct. Protein profiles and immunoblots indicated the accumulation of a 60- and 30-kD protein during cold acclimation in the leaves of deciduous trees; however, levels of these proteins did not change significantly in the evergreen trees. Immunoblots indicate that the 60-kD protein is a dehydrin-like protein. Gel-electrophoresis and immunoblots also indicated that the 19-kD BSP progressively disappeared from summer through fall in leaves of deciduous peach but accumulated to a high level in bark tissues. A similar inverse relationship was not evident in evergreen peach.

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Majken Pagter and Michelle Williams

January or February, when the plants were held outside. Cold-hardiness evaluations. Freezing tolerance of stems and buds was determined at seven temperatures: one control (4 °C) and six subfreezing temperatures. Temperatures chosen depended on the

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Lisa J. Rowland, Anik L. Dhanaraj, Dhananjay Naik, Nadim Alkharouf, Ben Matthews and Rajeev Arora

of more cold-hardy cultivars. To survive the winter, woody perennials of the temperate zone increase their freezing tolerance seasonally by a process known as cold acclimation. Cold acclimation in woody perennials is generally considered a two