ISHS Acta Horticulturae 69: II International Symposium on Pear Growing
THE RELATIONSHIP OF DEEP SUPERCOOLING TO THE NORTHERN LIMIT OF PEAR SURVIVAL AND THE POTENTIAL FOR BREEDING COLD RESISTANT CULTIVARS IN NORTH AMERICA |
| Author: | H.A. Quamme |
| Abstract:
Twig pieces from six pear cultivars and limb pieces from Bartlett were taken during the mid-winter and subjected to a preconditioning treatment to induce maximal cold hardiness. The xylem parenchyma was the most susceptible tissue of the twigs and limbs and the temperature at which it was injured coincided with the temperature at which a low temperature exotherm was detected by the differential thermal analysis. Pear production of 4 commercial cultivars, Bartlett, Bosc, Anjou and Flemish Beauty, is not found beyond the average annual minimum temperature isotherm which ranges from -23.3 to -28.9C (Zone 5) and this temperature range was just above the freezing resistance of the xylem (-30 to -35) and the temperature at which the low temperature exotherm occurred (-33 to -38C). The low temperature exotherm was previously shown to be produced by freezing of deep supercooled water in the xylem. In this study, the freezing point of the deep supercooled water appeared to be related to the northern limit of pear production in North America. The xylem of the non-commercial cultivar Moe was injured from -35 to -40C and this cultivar survives in Zone 4 (average annual minimum temperatures from -28.9C to -34.4C). There appears to be a potential for extending pear production into Zone 4 if cultivars can be bred which have the cold hardiness of Moe and quality of the commercial cultivars. However, there may be a physiological barrier to improving cold hardiness imposed by the physical limits of deep supercooling. | |
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