Survival of Water-stressed Rhododendron Subjected to Freezing at Fast or Slow Cooling Rates

Abstract

The purpose of the present study was to determine whether water stress affects tolerance of Rhododendron L. `Catawbiense Boursault' to rapid freezing. Tolerance to freezing at cooling rates of 2 or 6C/hour in stems and leaves of plants subjected to continuous and periodic water deficit stresses was examined. Under continuous stress treatments, water content of the growing medium was maintained in a range of 0.60 to 0.75, 0.45 to 0.60, or 0.30 to 0.45 m3·m–3 between 24 Aug. 1992 and 11 Feb. 1994. Under periodic stress treatments, water content of the growing medium was maintained near field capacity, i.e., 0.6 to 0.8 m3·m–3, for the duration of the study or plants were subjected to the periodic stress at various times between 15 July and 19 Feb. during 2 years. Watering of water-stressed plants was delayed until water content reached below 0.4 m3·m–3, and then was resumed to maintain water content in the range of 0.3 to 0.4 m3·m–3. Cold hardiness was evaluated in the laboratory with freeze tolerance tests on detached leaves and stem sections. In most cases, cooling at 6C/hour caused injury at higher temperature than cooling at 2C/hour. The difference in lethal temperature between the two cooling rates depended on the level of the plant's cold hardiness. In plants cold hardy to about –25C, freezing at 6C/hour caused injury at a temperature ≈3C higher than freezing at 2C/hour. The effect of cooling rate was not evident in plants cold hardy to about –18C. Subjecting plants to continuous or periodic water stress did not have an effect on this relationship.