| Summary: | Cold hardiness in actively growing plants of Saxifraga caespitosa L., an arctic and subarctic cushion plant, was examined. Plants collected from subarctic and arctic sites were cultivated in a phytotron at temperatures of 3, 9, 12 and 21 °C under a 24-h photoperiod, and examined for freezing tolerance using controlled freezing at a cooling rate of 3–4 °C either in air or in moist sand. Post-freezing injury was assessed by visual inspection and with chlorophyll fluorescence, which appeared to be well suited for the evaluation of injury in Saxifraga leaves. Freezing of excised leaves in moist sand distinguished well among the various treatments, but the differences were partly masked by significant supercooling when the tissue was frozen in air. Excised leaves, meristems, stem tissue and flowers supercooled to −9 to −15 °C, but in rosettes and in intact plants ice nucleation was initiated at −4 to −7 °C. The arctic plants tended to be more cold hardy than the subarctic plants, but in plants from both locations cold hardiness increased significantly with decreasing growth temperature. Plants grown at 12 °C or less developed resistance to freezing, and excised leaves of arctic Saxifraga grown at 3 °C survived temperatures down to about −20 °C. Exposure to −3 °C temperature for up to 5 d did not significantly enhance the hardiness obtained at 3 °C. When whole plants of arctic Saxifraga were frozen, with roots protected from freezing, they survived −15 °C and −25 °C when cultivated at 12 and 3 °C, respectively, although a high percentage of the leaves were killed. The basal level of freezing tolerance maintained in these plants throughout periods of active growth may have adaptive significance in subarctic and arctic environments.
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