The effect of water stress imposed at three dates in late summer and early fall on cold hardiness was examined in Rhododendron L. `Coral Bell', `Hinodegiri', and `Red Ruffle'. The persistence of the water stress-induced cold hardiness was also examined following plant recovery from the stress. Container-grown plants were exposed to three weeks of reduced water supply starting 8 Aug., 29 Aug., or 19 Sept., while control plants were well watered. Cold hardiness of leaves, lower, middle, and upper stems was evaluated with laboratory freeze tests. Reduced water supply independent of time initiated increased cold hardiness by 1 to 4C in the majority of the tested plant parts in the three cultivars. Cold hardiness of all plant parts tested strongly depended on the current water status of the plants as indicated by the stem water potential. In most cases, 3 weeks after rewatering, the cold hardiness of previously water stressed plants did not differ from that of nonstressed plants.
Water status is known to have an impact on cold hardiness of plants. Cold hardiness of `Catawbiense Boursault' rhododendron was examined under continuous and periodic water stress. Under continuous stress, water content of growing medium was maintained at 0.6 to 0.75, 0.45 to 0.6, or 0.3 to 0.45 m3·m-3. Under periodic stress, water content was either maintained between 0.6 to 0.8 m3·m-3 or plants were subjected to drought episodes at various times in late summer, autumn, and early winter. During a drought episode, watering was delayed until water content was below 0.4 m3·m-3. Watering then resumed and water content was maintained between 0.3 to 0.4 m3·m-3. Cold hardiness was evaluated on detached leaves and stem sections. The effect of continuous water stress depended on its severity and duration. Moderate stress did not increase cold hardiness compared to well watered plants during the first winter, but it did so when continued into the second winter. More severe stress increased cold hardiness during the first winter, but it decreased cold hardiness during the subsequent winter. The effect of periodic water stress depended on the timing of application. During initial and final stages of acclimation, cold hardiness increased in response to water stress less than during the intermediate stages. Water-stress-induced cold hardiness gradually decreased after rewatering.
The primary cause of losses in evergreen azaleas injured by early freeze is bark split on lower stems. Delayed acclimation in the fall is thought to permit this injury. We examined whether reduced water supply affects acclimation of Rhododendron L. `Coral Bell', `Hinodegiri', and `Red Ruffle'. Containerized plants were grown under four watering regimes and placed outdoors or in the greenhouse. The water content of the growing medium was maintained at either 0.3 to 0.4 or 0.5 to 0.6 m3·m-3 from 16 June to 30 Aug. 1993, when half of the plants under each of these regimes was switched to the other watering regime. Freeze tests were conducted on 30 Aug. and 9 (let. Injury to leaves, and lower, middle, and upper stems was evaluated visually. Acclimation of leaves and upper stems before the August test, in most cases, was not stimulated by reduced water content, while the response of lower and middle stems was cultivar- and location-specific. The lower water content treatment after 30 Aug. generally increased freeze tolerance of all plant parts regardless of the previous watering regime. The higher water content treatment after 30 Aug. either prevented or delayed acclimation. This study demonstrated that the reduced water supply provided a feasible means of promoting acclimation of evergreen azaleas in late summer.
Although differential thermal analysis has been routinely used to evaluate cold hardiness, the relationship of deep supercooling ability and plant survival are not well understood. In this study, we compared the seasonal profiles of changes in low-temperature exotherm (LTE) occurrence with visually determined cold hardiness of Acer rubrum L. `Armstrong', Fraxinus americana L. `Autumn Purple' and Zelkova serrata (Thunh.) Mak. `Village Green' growing in three locations representing plant cold hardiness zones 8b, 7b, and 5a. Between December and February, LTEs in Acer rubrum `Armstrong' and Fraxinus americana `Autumn Purple' occurred at temperatures around 10 to 25C lower than the lowest survival temperatures. The mean difference between LTEs and lowest survival temperature was not significant for Zelkova serrata `Village Green' from January to April and for Acer rubrum `Armstrong' and Fraxinus americana `Autumn Purple' in March. Data indicated that LTEs could be used as an estimate of lowest survival temperature in Zelkova serrata `Green Village' but not in Acer rubrum `Armstrong' and Fraxinus americana `Autumn Purple'. This study demonstrated that LTEs may not reliably estimate cold hardiness in all species that deep supercool. Factors other than freeze avoidance ability of xylem may limit stem survival at temperatures above the LTE.