Abstract
The flora of the South American pa'ramo comprises plants of tropical and temperate origin, which either avoid or tolerate freezing. We tested the hypothesis that the frost-resistance mechanism in pa'ramo plantsisdetermined by their geographic origin, i.e. that plantsof tropical origin avoid freezing by supercooling, whereas temperate plants tolerate formation of the extracellular ice.
We used original along with published data and analysed a total of 47 species from 14 genera for which the origin in the pa'ramo isinferred from molecular phylogenies. The resistance mechanism was determined by comparing the ice-nucleation temperature in the tissues with the temperature at which 50% injury to the tissue occurred.
We found that genera of tropical and south temperate origin mostly avoid freezing, whereas genera of north temperate origin mostly tolerate freezing. Significant correlation between the frost-resistance mechanism and origin of the pa'ramo plants suggests that the geographic origin of these plants determines, to a certain degree, their frost-resistance mechanism.
However, in several genera both mechanisms are present which indicates that the frost-resistance mechanism also evolved in response to the selective pressure of the pa'ramo environment.