Freezing tolerance and avoidance were studied in several different sized species of the tropical high Andes (4200 m) to determine whether there was a relationship between plant height and cold resistance mechanisms. Freezing injury and supercooling capacity were determined in ground level plants (i.e. cushions, small rosettes and a perennial herb), intermediate height plants (shrubs and perennial herbs) and arborescent forms (i.e. giant rosettes and small trees). All ground-level plants showed tolerance as the main mechanism of resistance to cold temperatures. Arborescent forms showed avoidance mechanisms mainly through supercooling, while intermediate plants exhibited both. Insulation mechanisms to avoid low temperatures were present in the two extreme life-forms. We suggest that a combination of freezing tolerance and avoidance by insulation is least expensive and is a more secure mechanism for high tropical mountain plants than supercooling alone.
Factors affecting supercooling capacity and cold hardiness were investigated in leaves of ten giant rosette species of the genus Espeletia (Compositae). These species grow along a 2,800-4,200 m elevation gradient in the Venezuelan Andes. In this high tropical environment, freezing frequently occurs every night, particularly above 3,300 m, but lasts for only a few hours. Supercooling capacty is linearly related to leaf water potential (Ψ ) in all species; however supercooling is more responsive to Ψ changes in Espeletia species from high paramos. The rate of change in the species-specific supercooling point and the rate of change of average annual minimum temperature along the elevation and climatic gradient follow the same trend (approximately -0.6 K per 100 m elevation). At a given elevation, the expanded leaves of the different species tend to supercool 8-10 K below minimum air temperatures. Experimentally-induced freezing was accompanied by the formation of intracellular ice and tissue damage. The relative apoplastic water content (RAWC) of the leaves, which may influence the ice nucleation rate or the facility by which ice propagates, was determined by pressure-volume methods. Species from higher sites tend to exhibit lower RAWC (2%-7%) than species from lower sites (7%-36%). A causal relationship between supercooling capacity and RAWC is suggested. In the high tropical Andes, the temperature oxotherm plateau of Espeletia leaves seems to be sufficiently fow to avoid freezing.
The Polylepis tarapacana forests found in Bolivia are unique with respect to their altitudinal distribution (4200-5200 m). Given the extreme environmental conditions that characterize these altitudes, this species has to rely on distinct mechanisms to survive stressful temperatures. The purpose of this study was to determine lowtemperature resistance mechanisms in P. tarapacana. Tissue was sampled for carbohydrate and proline contents and micro-climatic measurements were made at two altitudes, 4300 and 4850 m, during both the dry cold and wet warm seasons. Supercooling capacity (-3 to -6°C for the cold dry and -7 to -9°C for the wet warm season) and injury temperatures (-18 to -23°C for both seasons), determined in the laboratory, indicate that P. tarapacana is a frost-tolerant species. On the other hand, an increase in supercooling capacity, as the result of significant increase in total soluble sugar and proline contents, occurs during the wet warm season as a consequence of higher metabolic activity. Hence, P. tarapacana, a frost-tolerant species during the colder unfavourable season, is able to avoid freezing during the more favourable season when minimum night-time temperatures are not as extreme.
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