EFFECTS OF NEAR-LETHAL STRESS ON BUD DORMANCY AND STEM COLD-HARDINESSIN RED-OSIER DOGWOOD

We studied the effects of ''near-lethal'' (NL, 47 degrees C for 1 h) h eat stress, applied to intact shoots of red-osier dogwood (Cornus seri cea L.) during early (October), deep (November) or late (December) dor mancy, on bud dormancy release and development of stem tissue cold har diness under natural conditions and at a constant temperature of 0 or 23 degrees C in the dark. The NL heat-stress treatment overcame bud do rmancy when applied during the early and late stages of dormancy. Duri ng October and December, all plants in the 23 degrees C + dark post-st ress environment broke bud within 35 and 12 days, respectively, wherea s the corresponding values for days to bud break in the control plants were more than 150 and 110 days, respectively. Application of NL heat stress during deep dormancy caused only slightly earlier bud break co mpared to the control plants. In the O degrees C + dark post-stress en vironment, all NL heat-treated plants died within 9 weeks. Under natur al post-stress conditions, bud break in plants receiving NL heat stres s during early and deep dormancy occurred at the same time as in contr ol plants, whereas bud break of plants receiving NL heat stress during late dormancy occurred 55 days earlier than in control plants. Under both natural and 23 degrees C + dark post-stress conditions, cold hard iness of plants receiving NL heat stress during early dormancy was sim ilar to that of controls. Application of NL heat stress during deep do rmancy hastened the rate of deacclimation under the 23 degrees C + dar k post-stress conditions but had no effect on deacclimation under natu ral post-stress conditions. Application of NL heat stress during late dormancy enhanced deacclimation of plants in both the 23 degrees C + d ark and natural post-stress environments.