RELATIONSHIP OF THE LOW TEMPERATURE EXOTHERM TO APPLE AND PEAR PRODUCTION IN NORTH AMERICA

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Abstract
Twig pieces from the current season’s growth of three crabapple (Malus sp.), eight apple (Malus pumila Mill.) and seven year (Pyrus communis L.)cultivars, and limb pieces from 6- to 7-yr-old branches of Starking Red Delicious apple and Bartlett pear were taken during mid-winter and-subjected to a preconditioning treatment to induce maximal cold hardiness. The apple and pear cultivars studied included many of the major commercial cultivars grown in North America. Freezing tests and differential thermal analysis (DTA) were performed on the twig and limb pieces after preconditioning. Xylem was the most susceptible in stem tissue of both preconditioned apple and preconditioned pear. The temperature at which apple and pear xylem of commercial cultivars became injured (−35 to −40 C and −30 to −35 C, respectively) was related to the initiation of the low temperature exotherm on the DTA profile (−37 to −40 C and −33 to −38 C, respectively) and in turn was related to the average annual minimum temperatures at the northern limits of commercial production (−34.4 to −40 C and −28.9 to −34.4 C, respectively). The low temperature exotherm was previously shown to be produced by freezing of supercooled water in the xylem. The spontaneous nucleation temperature of supercooled water in xylem of the cultivars studied appears to be related to the northern limit of pear and apple production in North America.

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