Because evergreen plants in a temperate zone experience a wide range of temperature conditions, the foliar cold tolerance is required for keeping leaves in cold season. When evergreens extend their distribution to colder habitats (ex. high altitude or latitude), they must reinforce their cold tolerance. In this research, we focused on the altitudinal divergence of two types of cold tolerance traits, photoprotection and freezing avoidance, which function in different temperature zones.
At Mt. Ibuki in central Japan, Arabidopsis halleri Ssp. gemmifera, a evergreen herb, shows distinct morphological and physiological differentiations (ecotypes) between high and low altitudes. Comparing leaf traits between the altitudinal ecotypes, we found that highland ecotype maintains higher freezing avoidance and photoprotection than lowland ecotype even when they are grown at warm temperature. Whereas highland ecotype had a constitutive high photoprotection, lowland ecotype had inducible photoprotection and improved its photoprotective ability after it experienced chilling temperature. On the other hand, freezing avoidance did not show plasticity in both ecotypes and was constantly higher in highland ecotype than in lowland ecotype irrespective of their growth temperature.
These results imply that, in the process of altitudinal adaptation in this plant, selective pressures which act on the two types of cold tolerant traits were different from each other. Whereas the difference in the frequency of chilling temperature may drive the evolution of the plasticity in photoprotection, the difference in the harshness of cold (minimum temperature) may change the maximum freezing avoidance between two altitudinal ecotypes.