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.
Christopher L. Owens
Christopher L. Owens and Ed Stover
Christopher L. Owens and Eddie W. Stover
Early fruit production and control of tree size are important factors in the economic viability of high-density apple orchards. A horticultural tool permitting growers to induce terminal budset should provide greater control over the balance between vegetative growth and reproduction, increasing orchard production and profitability. With this goal, the experimental GA-biosynthesis inhibitor, BAS-125W, is being evaluated for effects on enhancing floral initiation and controlling tree size in young orchards. In nursery stock, the effect of inducing earlier terminal budset is also being studied for influence on storage carbohydrates and performance after planting. Studies in 1996 showed that 250 ppm BAS-125W induced terminal bud set on actively growing second-leaf `Macoun', `Delicious', and `Fuji' trees. Seven application dates from 17 June to 9 Sept. were compared to determine how time of treatment would effect degree and distribution of flowering the following year. Terminal budset typically occurred 2 weeks after application, with shoot growth resuming in 4 to 5 weeks. At two dates, treatment of growing tips only was compared with entire tree application to distinguish the direct effect of GA-inhibition on floral initiation from the effect of redistributing photosynthate. Treatment from 17 June to 29 July significantly reduced total annual shoot growth compared to the untreated controls, while later treatments had no significant effect on shoot length. Treatments of nursery stock with BAS-125W on 1 Sept. accelerated terminal bud set by at least 7 days compared to untreated controls of both `Fuji' and `Golden Delicious'. Effects of treatments on flowering and tree growth in 1997 will be discussed.
Christopher L. Owens, J.F. Hancock and A.F. Iezzoni
Sour cherry and strawberry are examples of two Rosaceous species that often suffer crop reductions due to spring freezes. Breeding for improved floral freezing tolerance has the potential to mitigate the susceptibility of these plants to spring frosts. In model plant systems, researchers have been able to identify genes that play a role in freezing tolerance by initially searching for mRNAs regulated in response to cold temperatures. To search for cold-responsive freezing-tolerance genes in strawberry and sour cherry, it is necessary to first define their cold acclimation response. To test the hypothesis that sour cherry and strawberry flowers have the ability to cold acclimate, blooming plants were exposed to 4 °C and 16 h light for 14 days. Sour cherry styles and strawberry receptacles from open, fully developed flowers were excised, and electrolyte leakage curves were generated over a range of subzero temperatures. The temperature at which 50% electrolyte leakage (EL50) occurred was used to compare treatments. The flowers of two strawberry cultivars were tested for the ability to cold acclimate. Non-acclimated `Chandler' receptacles had an EL50 of -2.9 °C, while non-acclimated `Honeoye' had an EL50 of -3.4 °C. Conversely, acclimated `Chandler' receptacles had an EL50 of -7.7 and acclimated `Honeoye' receptacles had an EL50 of -8.7 °C, both are significantly different from non-acclimated values (P ≤ 0.01). Additionally, sour cherry styles were collected from the field at full bloom from a mapping population of 86 individuals from the cross `Rheinische Schattenmorelle' × `Erdi Botermo' and acclimated as previously described. The EL50 of the 86 progeny ranged from approximately -2.0 to -6.0 °C.
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.