Variation in Low-temperature Exotherms of Pecan Cultivar Dormant Twigs

Pecan (Caryaillinoinensis(Wangenh.)K.Koch)trees nativetonorthern regions are more cold-tolerant than those native to and grown in the southern United States. To identify a possible assay for cold hardiness, dormant winter twigs from 112 diverse pecan cultivars grown in Texas were surveyed using differential thermal analyses (DTA). The low temperature exotherm (LTE) from DTA was identifiable when twigs were stored at - 3 8C for up to 120 d after harvest. Thirty-nine percent of the southern pecan cultivars lacked an obvious LTE, and the remaining southern cultivars had an average LTE of - 32.9 8C. In contrast, only 11% of the northern pecan cultivars lacked the LTE and the remaining cultivarshadasignificantly lowerLTEof-35.4 8C.Because twigsampleswere collected from trees grown in the same Texas orchard, it is suggested that there is a genetic component that affects the temperature of the LTE. Budbreak generally occurred earlier in southern cultivars than those that originated in the north. Both budbreak and LTE data can be correlated with regional origin; timing of budbreak may be preferred over DTA to predict relative cold hardiness in pecan.