Pear trees are inherently inefficient at partitioning growth resources to fruit. Modern-day pear rootstock/scion combinations typically used in the United States are vigorous, resulting in trees that require ample space for their structural development.
Low tree densities are slow to attain maximum production levels; consequently, the time required to recoup establishment costs is postponed. Moreover, complex tree architectures are not amenable to mechanization. Large pear canopies are reliant upon tall ladders that preclude orchard efficiencies in pruning, thinning, and harvest operations. A common thread connecting relatively recent and marked improvements in production efficiency of apple and sweet cherry is the use of dwarfing rootstocks.
In the United States, dwarfing rootstocks have not yet been developed for pear. In contrast, several regions of the world have developed high-intensive pear systems. Quince rootstocks are the cornerstone of these systems, used in combination with an array of horticultural techniques to hasten scion precocity, balance crop loads, and maintain consistent production. Pear and quince are distant relatives in the family Roseaceae.
The essential traits associated with quince are precocity and dwarfing; however, commercially available quince selections have been reported to possess insufficient cold hardiness for northern sites. We are currently characterizing seasonal changes in cold hardiness of 50 quince accessions with diverse origins located at the U.S. Department of Agriculture’s National Clonal Germplasm Repository (NCGR) in Corvallis, Oregon, which houses a representation of the world’s genetic diversity of quince.
Over the past three years, wood from each of the 50 quince accessions has been screened once per month (September through March) to identify their capacity for cold acclimation and deacclimation, determine the minimum hardiness level, and identify tissue-specific sensitivity limits when exposed to subzero temperatures.
Wood is harvested from trees growing at the NCGR and sent to Oregon State University’s Mid-Columbia Agricultural Research and Extension Center in Hood River, where it is loaded into a programmable freeze chamber, and subjected to a moderate freezing rate.
Samples are removed following one hour at each of five test temperatures (32°F, 14°F, -4°F, -22°F, and -40° F). Following a one-week incubation period, the samples are freshly cut and observed under a stereomicroscope. Individual tissue zones (phloem, cambium, and xylem) are rated according to the degree of oxidative browning observed. The lowest exposure temperature sustained with minimum observable tissue injury (less than 25% browning) was used to report minimum hardiness level.
Increased exposure to low temperatures in early fall, termed cold acclimation, is a prerequisite for a plant’s ability to survive subfreezing temperatures without injury. The ambient temperatures recorded at the NCGR gradually declined throughout early fall in 2009, providing adequate conditions for development of hardiness.
Extremely cold hardy
Following cold acclimation, we have identified 25 quince accessions capable of withstanding -22° F without detectable levels of freeze damage. Thirteen of those were categorized as extremely cold hardy, showing low levels of tissue browning (likely survivability) following exposure to -40°F.
Despite mild acclimation temperatures in early-mid autumn 2010, cold hardiness levels were in good agreement with those of 2009. During the experiment, temperatures at the repository rarely fell below 10°F for more than a few days. Interestingly, previous reports have suggested that full expression of hardiness is associated with exposure to temperatures around 10°F for a period of several weeks, indicating that the quince identified as cold hardy in our study may be capable of developing greater hardiness if planted in colder regions.
Xylem, phloem, and cambial tissue responded similarly to subzero temperatures, though the phloem tended to be slightly more hardy during December (peak hardiness period), and the cambium exhibited greater sensitivity during early fall.
Several quince clones exhibited freeze tolerance equal to or greater than the current Old Home x Farmingdale pears widely used as rootstocks in the United States today. We are currently testing the graft compatibility among these candidates and the major commercial pear cultivars produced in the Pacific Northwest. Future work will evaluate the horticultural performance of these rootstocks in pear production. In addition, phenotyping quince germ plasm for their ability to tolerate low temperatures can greatly accelerate future breeding/genetic efforts in the area of cold hardiness.
The Pear Bureau Northwest provided funding for this project.
Todd Einhorn and Janet Turner, Oregon State University, Mid-Columbia Agricultural Research and Extension Center, Hood River, and Joseph Postman, USDA National Clonal Germplasm Respository, Corvallis, Oregon