Seasonal Changes in Cold Hardiness of Rhododendron L. `Catawbiense Boursault' Grown under Continuous and Periodic Water Stress

Abstract

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.