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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The water relations of three mangrove ecosystem species, Rhizophora mangle, Conocarpus erectus and Coccoloba uvijera were investigated in an intertidal zone of the Venezuelan coast. The influence of a salinity gradient (from seashore to inland) and seasonal fluctuations in salinity were studied. Daily courses of leaf water and osmotic potentials and other water relations characteristics were estimated from pressure-volume curves. All species exhibited a decrease in osmotic potential of leaf tissues during the dry season when salinity levels increased in ground water. This osmotic adjustment was due to changes in either the symplasmic water fraction, the osmotically active solutes in leaf cells, or both.Changes in cell wall elasticity were also observed. The cell walls were more rigid during the dry season in all three species. Rhizophora mangle, the species that grows closest to the sea, had much more rigid cell walls than the other two species. The adaptive significance of these mechanisms for turgor maintenance and water uptake in mangrove habitats is discussed.

Freezing tolerance as a cold resistance mechanism is described for the first time in a plant growing in the tropical range of the Andean high mountains. Draba chionophila, the plant in which freezing tolerance was found, is the vascular plant which reaches the highest altitudes in the Venezuelan Andes (approximately 4700m). Night cycles of air and leaf temperature were studied in the field to determine the temperature at which leaf freezing began. In the laboratory, thermal analysis and freezing injury determinations were also carried out. From both field and laboratory experiments, it was determined that freezing of the leaf tissue, as well as root and pith tissue, initiated at a temperature of approximately-5.0°C, while freezing injury occurred at approximately-12.0°C for the pith, and below-14.0°C for roots and leaves. This difference in temperature suggests that the plant still survives freezing in the-5.0 to-14.0°C range. Daily cycles of leaf osmotic potential and soluble carbohydrate concentration were also determined in an attempt to explain some of the changes occurring in this species during the nighttime temperature period. A comparison between Andean and African high mountain plants from the point of view of cold resistance mechanisms is made.

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