If freezing tolerance and freezing avoidance are such successful adaptations of giant-rosette plants to the Frostwechsel climate in the African and South-American tropical high mountains, the question arises why the plants in the two continents evolved two such different modes of frost resistance. As compared to freezing and its tolerance, freezing avoidance by supercooling has one important advantage and one important disadvantage:
• the advantage is that the cells always remain metabolically competent,
• the disadvantage is the very high risk inherent in the supercooling strategy, since supercooling is a thermodynamically labile state; at the lowest supercooling point the water freezes instantaneously, there is no time for water-export into the apoplast, and cell death becomes unavoidable.
Continuous metabolic competence should allow higher productivity. In the Andean páramos, where night temperatures go down to 0 0C and rarely below —5 0C, the risk of reaching the crystallization point of water is small (Fig. 12.15) as compared to the benefit of higher productivity. Conversely, in the Afro-alpine zone temperatures are frequently below —10 0 C and the risk inherent in supercooling becomes too high to be a reasonable choice.
Table 12.2 Productivity of an African and two Andean giant-rosette plants. (After Rada et al.
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