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.
AbstractmPolylepis sericea trees grow well above the continuous forest line in the Venezuelan Andes. In these environments, extreme daily temperature ranges can occur at any time of the year and trees experience a 4 month dry period. The purpose of this work was to study carbon and water relations of this species in the field during wet and dry seasons in order to understand this species' success at such high altitudes. Leaf gas exchange (portable system in open mode) and leaf water potential (pressure chamber) were measured at 1 -2 h intervals during several daily courses at 4000 m elevation in the Páramo de Piedras Blancas. CO2 assimilation versus leaf temperature curves were also obtained for this species in the laboratory. Clear differences in the measured parameters were observed between seasons. For a wet season day, maximum CO2 assimilation rate was 7.4 µmol m -2 s -1 and leaf conductance was relatively constant (approximately 100 mmol m -2 s -1 ). In the dry season day, maximum CO2 assimilation rate was 5.8 µmolm -2 s -1 and leaf conductance was close to 60 mmolm -2 s -1 . Minimum leaf water potentials measured were -1.3 MPa for the wet and -2.2 MPa for the dry season. The CO2 assimilation-leaf temperature relationship showed a 13.4°C leaf temperature optimum for photosynthesis with maximum and minimum compensation points of 29.5 and -2.8°C, respectively. Maximum night-time respiration was relatively high (2.7 µmol m -2 s -1 ). Our results show that P. sericea maintains a highly positive carbon balance through all daily courses, even though there is a slight water stress effect during the dry season; this suggests that its carbon assimilation machinery is well adapted to the low temperatures and seasonal water stress found in the high tropical mountains.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.