The cushion plant, Silena Acaulis, is an arctic tundra species of circumpolar distribution. It also occurs in alpine environments. This photograph was taken at Ny Ålesund, Svalbard, Norway (78° 55' N lat.) in mid-August. Episodic frost can occur throughout the short growing season these high latitude environments. Internal temperatures of the cushion can exceed 10°C above ambient temperatures. Note that this individual has been colonized by Salix polaris (rounded yellow-green leaves).

With:
Olavi Junttila, Professor, Department of Plant Physiology and Microbiology
Institute of Biology and Geology, University of Tromsø, Tromsø, Norway

Funded by Norwegian Research Council and the University of Tromsø Phytotron, Jarle Nilsen, Director.

Selected References

Junttila, O. and R. Robberecht. 1993.The Influence of Season and Phenology on Freezing Tolerance in Silene acaulis L., a Subarctic and Arctic Cushion Plant of Circumpolar Distribution. Annals of Botany 71:423-426.

Abstract Changes in the freezing tolerance for Silene acaulis L., a subarctic and arctic species of circumpolar distribution, were examined to understand the extent of cold hardening and dehardening that occurs seasonally and with changes in plant phenology. Shoots of whole plants collected on a mountain ridge near Tromsø, Norway (69° N, 700 m above sea level) were frozen under controlled conditions at cooling rates of 3 to 4°C h^-1. The extent of freezing-induced injury was examined both by chlorophyll fluorescence and by visual inspection with a microscope. A freezing tolerance level of -30°C was observed in mid-winter, based on a 50% lethal point for freezing injury. Loss of cold hardiness was substantial in mid-summer, with freezing tolerances of -8.5 to -9°C observed in mid-July. Plants still covered by snow in mid-July had a freezing tolerance of -12.5 to -13°C. The maintenance of a basic level of freezing tolerance throughout the summer may be adaptive in the northern latitude-regions because of the occurrence of episodic frosts during the growing season.

Abstract 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.

Warren Wilson, J. 1957. Observations on the temperatures of arctic plants and their environmnent. Journal of Ecology 45:499-531.