Supercooling and Cold-hardiness of Acclimated and Deacclimated Buds and Stems of Persian Walnut Cultivars and Selections

To study the cold-hardiness of Persian walnut cultivars and selections, three methods were compared: 1) thermal analysis; 2) evaluation of tissue health after controlled freezing; and 3) field observations after a severe midwinter freeze. Stem segments and buds were collected from eight Persian walnut genotypes (four commercial cultivars and four promising Iranian selections). Thermal analysis was conducted using thermoelectric modules (TEM) to measure the high (HTE) and low (LTE) temperature exotherms produced when water and tissues freeze. TEM signals were recorded as the temperature of the samples was decreased at a rate of 2 °C/h. Tissue injury under controlled temperatures was evaluated using pre-chilled stem segments cooled at 2 °C/h to set temperatures ranging from –5 to –30 °C and then held at these temperatures for 16 h. Frozen samples were thawed and visually evaluated for severity of injury. Cold damage under field conditions was evaluated after an unusually severe winter freeze. Twigs from affected trees were removed in mid-February and in April and visually rated for extent of injury and ability to recover. The occurrence of LTEs was correlated with death of the tissues as assessed by tissue browning. Both the capacity to supercool and the cold-hardiness of cultivars and selections tested increased with accumulated seasonal chilling and decreased as they approached spring budbreak. Thermal analysis showed a tendency for buds and stems to exhibit multiple LTEs at peak dormancy. The cultivars and selections were classified into three groups based on their cold-hardiness: sensitive (‘Z 30 ’ and ‘Serr’), semihardy (‘Z 53 ’ and ‘Z 67 ’), and hardy (‘Lara’, ‘Hartley’, ‘Z 63 ’, and ‘Pedro’).