The cold resistance of Macaronesian Sempervivoideae.

Cold resistance of twenty-seven species of Macaronesian Sempervivoideae uniformly cultivated under cool moderate but not hardening conditions was measured. The resistance limits of all the tested species ranged between-4 and-10°C. Cold stress response was principially different: Cold resistance of about half of the tested species was due to freezing point lowering. This response type, avoidance of freezing, in which any ice formation in the leaves leads to injury, was found in the most cold resistant species (Aeonium spathulatum, several Aichryson species). The other species developed tolerance to freezing, thus resembling the behaviour of the hardy Eurasian Sempervivum species. Several Aeonium and Monanthes species resist to reasonable lower temperatures than normally occur in their natural habitats. The species-specific differences in resisting cold stress may originate from different abilities to tolerate cellular freeze dehydration. The Sempervivum alliance illustrates well the two evolutionary strategies of cold tolerance: Avoidance mechanisms, like lowering the osmotic potential, are typical for species colonizing higher altitudes with moderate frosts. For species extending their distribution area into higher latitudes with more severe frosts, however, freezing tolerance is necessary.