Cold Hardy Quince Identified in the USDA Germplasm Collection: Potential Rootstock Candidates for Pear

Quince (Cydonia oblonga Mill.) clones are a preferred rootstock for pear due to their size controlling influence on the scion, but lack cold hardiness for northern latitude sites. We evaluated cold acclimation, minimum hardiness, and freeze tolerance of phloem, cambium, and xylem of 56 clonal quince accessions and two intergeneric hybrids in an in-situ collection located at the USDA clonal genebank in Corvallis, OR.  Seven Pyrus accessions ranging from sensitive to hardy, including leading commercial U.S. rootstock clones, were used as standards. Additionally, we evaluated three Amelanchier sp. clones from the Bavarian Center for Fruit Crops, Hallbergmoos, Germany.  One-year-old shoot samples were screened monthly, Sept. through Mar. (2009–12).  Samples were loaded into a programmable freeze chamber [in 4 replications (1 per day)], frozen at 4 °C per hour and removed following one hour at each of five treatment temperatures (0 °C, –10 °C, –20 °C, –30 °C, and – 40 °C).  Following a seven-day incubation period (20 °C), samples were sectioned transversely and observed under a stereomicroscope for percent oxidative browning of phloem, cambium, and xylem. In all years, peak hardiness was attained in December/January, irrespective of the accession.  At peak hardiness, tissue browning of 22 of the 56 quince accessions and both intergenerics was <50% following exposure to –30 °C (based on 3-year means).  All Amelanchier clones were hardy to –40 °C.  None of the pear accessions tested, including ‘OHxF 87’ and ‘OHxF 97’, Pyrus ussuriensis, and several scion clones were hardy to temperatures below –30 °C.  Beginning in mid-late January, de-acclimation was evident as tissue injury was observed at higher temperatures.  We observed greater browning in the cambial zone throughout the measurement period, especially in early fall, and late spring, followed by xylem.  Phloem developed the greatest hardiness in mid-winter.  Differential thermal analysis (DTA) data confirmed these observations.  In mid-winter, low-temperature exotherms were not observed for phloem tissue at temperatures to –50 °C. Cold acclimation and minimum hardiness levels will be discussed relative to ambient temperatures (minimum and mean) recorded at the genebank.  A novel grafting system to evaluate regrowth relative to oxidative browning and DTA data will be presented.