In temperate zone, early freeze, particularly if preceded by a period of warm and wet weather, can severely injure many woody plants. The young plants are particularly sensitive to frost, especially those grown in containers. The timing and capacity of cold acclimation are modified by environmental cues. We examined whether regulated deficit irrigation (RDI) imposed in the fall affects cold acclimation of containerized Lawson cypress (Chamaecyparis lawsoniana)‘Columnaris' and Knaphill-Exbury azalea (Rhododendron) ‘Oxydol'. Three-year-old plants were grown in containers placed in the open field and were cultivated according to standard nursery practice. In the end of growing season (from 5th October to 2nd November 2015, half of the plants were expose to moderated water stress by ceasing irrigation and protecting them from the rain (RDI treatment). The volumetric water content of the growing medium gradually decreases in this time from 0.45 to 0.2 m3/m3. Irrigation of the second part of the plants were continued in October and water content was maintained at 0.45 m3/m3 (control treatment). Cold hardiness of stem tissues was assessed two times (2nd November and 7th December) in the laboratory using the ion-leakage test. Stem tissue were exposed to 6 test temperatures, ranging from +4 to -26˚C. Additionally, plant quality after overwintering was evaluated. Freeze tolerance of plant stems of both woody plants was significantly higher in December than November. Regulated deficit irrigation (RDI) generally increased cold hardiness of Lawson cypress in November as well as in December. At RDI treatment, ion-leakage from plant tissue frozen to -26˚C was lower by 20% in the first time of assessment, and by 8% in the second time. Regulated deficit irrigation (RDI) increased cold hardiness of azalea stems only in November, but did not affect frost hardiness in December. At water deficit treatment, ion-leakage from plant tissue frozen to -26˚C was lower by 12% than at control treatment. This study demonstrated that reduced water supply early autumn promoting acclimating to low winter temperatures.
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