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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Stress-induced restrictions to carbon balance, growth, and reproduction are the causes of tree-line formation at a global scale. We studied gas exchange and water relations of Polylepis tarapacana in the field, considering the possible effects of water stress limitations imposed on net photosynthetic rate (P N ). Daily courses of microclimatic variables, gas exchange, and leaf water potential were measured in both dry-cold and wet-warm seasons at an altitude of 4 300 m. Marked differences in environmental conditions between seasons resulted in differences for the dry-cold and wet-warm seasons in mean leaf water potentials (-1.67 and -1.02 MPa, respectively) and mean leaf conductances (33.5 and 58.9 mmol m -2 s -1 , respectively), while differences in mean P N (2.5 and 2.8 µmol m -2 s -1 , respectively) were not as evident. This may be related to limitations imposed by water deficit and lower photon flux densities during dry and wet seasons, respectively. Hence P. tarapacana has coupled its gas exchange characteristics to the extreme daily and seasonal variations in temperature and water availability of high elevations.

A simple and sensitive method for the colorimetric determination of reducing sugars in plant materials is proposed. The procedure is based on reduction under alkaline conditions of potassium ferricyanide by the reducing groups of the carbohydrates, followed by colour development as the o-phenanthroline complex. All sugars tested produced an equal colour yield. The method proved to be reproducible and precise with a high degree of sensitivity which makes it possible to use with small sample quantities. The method is quick, easy and cheap, hence permiting its use in routine analysis. #

A simple and sensitive method for the colorimetric determination of reducing sugars in plant materials is proposed. The procedure is based on reduction under alkaline conditions of potassium ferricyanide by the reducing groups of the carbohydrates, followed by colour development as the o‐phenanthroline complex. All sugars tested produced an equal colour yield. The method proved to be reproducible and precise with a high degree of sensitivity which makes it possible to use with small sample quantities. The method is quick, easy and cheap, hence permiting its use in routine analysis. © 1998 John Wiley & Sons, Ltd.