Abstract
Alpine herbaceous plants employ two mechanisms to prevent freezing injury to leaves during the growing season, i.e., they avoid freezing by transient supercooling or tolerate the occurrence of extracellular ice. I examined seasonal changes in the two freezing resistance mechanisms in leaves of alpine plants from two temperate mountain regions, Niwot Ridge (3500 m) in the Front Range of the Rocky Mts. (USA) and Modr, sedlo (1510 m) in the Giant Mts. (Czech Republic).
Although plants from Niwot Ridge were on average ca. 2.6 K more resistant to freezing than plants from the Giant Mts., patterns of freezing resistance changes during the growing season were consistent between the two regions. Both freezing resistance mechanisms, i.e., avoidance by supercooling and tolerance of extracellular ice were encountered.
Plants predominantly avoided freezing injury during early summer and tolerated extracellular freezing at the end of the growing season, and the seasonal change from avoidance to tolerance mechanisms was significant for both regions. The avoidance and tolerance mechanisms provided comparable freezing resistance to the plants during early summer but, unlike in avoidant plants, in tolerant plants resistance increased over the course of the season.
The species formed three groups with regard to resistance mechanisms employed during the season: (1) species that avoided injury by supercooling during early summer and employed freezing tolerance towards autumn; (2) species that were freezing tolerant during the entire growing season; and (3) species with ephemeral shoots that solely avoided freezing by supercooling. Although freezing tolerance is a common feature in north-temperate alpine plants, avoidance by supercooling as a freezing resistance strategy might be selected for in species with particular life histories such as ephemeral hemiparasites.