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ABSTRACTThe 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 low-temperature 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. [ABSTRACT FROM AUTHOR]

AbstractThe degree of crop sensitivity to water deficit, during the different developmental stages of a plant, is an important aspect to consider in tropical semiarid regions. The effect of water deficit on flowering dynamics and fruit production of sweet pepper (Capsicum chinense) was evaluated under several water regimes, during three consecutive years. A complete randomized block design with three replicates was employed. Flowering dynamics and fruit production were measured weekly, and water potentials were obtained twice during the experiments. For all years, flowering began at approximately 70 days after transplanting. In the most favourable year (trial 1996), there were no differences in total flower and fruit production in terms of irrigation frequency. For the other less favourable years, flowering and fruit production were significantly different between irrigation frequencies. Our results show that low water availability, prior to flowering, reduces the number of flowers produced and retards the occurrence of maximum flowering. On the other hand, a water deficit during the period between flowering and fruit development reduces final fruit production. This suggests that a water deficit during this last stage affects final yield to a greater extent. [ABSTRACT FROM AUTHOR]

The effect of a mistletoe, Phthirusa maritima, on the water, nitrogen and carbon balance of two mangrove host species, Conocarpus erectus and Coccoloba uvifera, was studied. Several daily cycles of water potential and its components (pressure-volume curves); leaf nitrogen content (Kjeldahl method); leaf conductance, transpiration rates and carbon assimilation (portable gas exchange system) were measured on mistletoe, infested and uninfested plants in the Caribbean coast of Venezuela. The mistletoe on both host species showed higher transpiration rates and lower CO2 assimilation rates, and therefore lower water use efficiencies. With respect to infested and uninfested plants, C. erectus did not show large differences in the parameters measured with the exception of assimilation rates which were significantly lower in the infested plants. On the other hand, C. uvifera did show differences in all parameters and, therefore, was affected to a greater degree by the mistletoe. The behaviour of mistletoe- infested and uninfested plants, with respect to habitats with different degrees of water stress and with respect to the salinity gradient in which these mangroves grow, is discussed. [ABSTRACT FROM AUTHOR]

Frost avoidance mechanisms were studied in Espeletia spicata and Espelelia timotensis, two Andean giant rosette species. The daily courses of soil, air and tissue temperatures were measured at a site at circa 4000 m. Only the leaves were exposed to subzero temperatures; the apical bud and stem pith tissues were insulated by surrounding tissues. The leaf tissues avoided freezing by supercooling rather than by undergoing active osmotic changes. The temperatures at which ice formed m the tissues (the supercooling points) coincided with injury temperatures indicating that Espeletia tissue does not tolerate any kind of ice formation. For insulated tissue (apical bud, stem pith, roots) the supercooling point was around -5°C coinciding with the injury temperature. Supercooling points of about -13 to -16°C were observed for leaves. These results contrast with those reported for Afroalpine giant rosettes which tolerate extracellular freezing. The significance of different adaptive responses of giant rosettes to similar cold tropical environments is discussed. [ABSTRACT FROM AUTHOR]