The purpose of this paper is twofold. Firstly, we introduce the concept of boundedκΦ-variation in the sense of Schramm-Korenblum for real functions with domain in a rectangle ofR2. Secondly, we study some properties of these functions and we prove that the space generated by these functions has a structure of Banach algebra.

Giant rosettes are ones of the most striking features of the vegetation in the high tropical Andes, with Coespeletia moritziana reaching the highest altitudes up to 4,600 m a.s.l. Different from other giant rosettes, this species grows on rock outcrops with poorly developed soils and where water availability may be limited. Two questions are addressed in this study: How does this species respond in terms of water relations to maintain favorable gas-exchange conditions? Considering that adult plants rely on a water-reserving central pith, how do early stages respond to this environment’s extreme conditions? Water relations and gas-exchange studies were carried out on juveniles, intermediate and adult C. moritziana plants during wet and dry seasons in Páramo de Piedras Blancas at 4,200 m a.s.l. Adult plants maintained higher leaf water potentials (ΨL) during the wet season, however, no differences between stages were found for the dry season. Minimum dry season ΨL were never near the turgor loss point in any of the stages. Juveniles show a more strict stomatal control during the dry season to maintain a favorable water status. Net photosynthesis significantly decreased in intermediate and juvenile stages from wet to dry seasons. Our results suggest that C. moritziana resists more extreme conditions compared to other Andean giant rosettes

In this paper we collect some properties of strongly midconvex functions. First, counterparts of the classical theorems of Bernstein-Doetsch, Ostrowski and Sierpinski are presented. A version of Rode support theorem for strongly midconvex functions and a Kuhn-type result on the relation between strongly midconvex functions and strongly t-convex functions are obtained. Finally, a connection between strong midconvexity and generalized convexity in the sense of Beckenbach is established.

In this paper we prove that if a uniformly continuous Nemytskii operator maps a space of bounded variation with weight functions in the sense of Riesz into another space of the same type, its generator function is an affine function.

The seasonal effect of soil water availability on leaf gas exchange of plantain plants cv. Hartón growing on two different texture soils (loamy and clayey) were evaluated. Soil water deficits corresponded to 48, 24 and 4 days without precipitation. Daily measurements of leaf gas exchange and microclimatic conditions were carried out at 2h intervals in a humid tropical environment south of Maracaibo Lake, Venezuela. The results show that cv. Hartón is sensitive to conditions of low water deficit on loamy and to a much greater degree on clayey soils. A marked reduction in leaf conductance (gs) was observed under severe as well as moderate deficit (below 50mmolm−2s−1) on clayey soils. Under low deficit gs increases to values between 60 and 100mmolm−2s−1. The same trend was observed in plants on loamy soils but higher gs for all conditions were obtained compared with plants on clayey soil. Stomatal closure produced a reduction of 85 and 55% of total assimilation (Atot) for severe and moderate deficit in plants on clayey soils, respectively. While plants on loamy soil exhibited a 65 and 35% reduction, respectively. Water use efficiency (WUE) consistently decreased as available soil water decreased on both soil types. Independently of soil water conditions, higher WUE were always obtained for loamy soils. This suggests that cv. Hartón does not have the ability to adjust the CO2 assimilation to transpiration ratio in order to optimize gas exchange. This evidences the importance of maintaining high conditions of available soil water in order to avoid lower assimilation rates that probably influence negatively on yield and fruit quality.

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 (PN). 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 PN (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.

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.

Results of a 3-y study on the populations of three evergreen woody species (Byrsonima crassifolia, Palicourea rigida and Bowdichia virgilioides), from a regularly burnt savanna, in Venezuela, are reported. Tree density, size structure, fruit-seed production per tree of different size classes and soil seed bank were estimated. Viable seed production was one order of magnitude higher in B. crassifolia, which was also the only species with a permanent soil seed bank. Seeds of B. virgilioides germinated readily after dispersal, whilst seeds of P. rigida showed an innate dormancy synchronously released after a year. Both of the latter species seemed to maximize the first growth season by germinating at the beginning of the favourable season. In B. crassifolia, germination proceeds with an early small burst of germination, possibly followed by a second peak at the beginning of the next rainy season and little or no germination thereafter. The results showed that despite the high fire frequency, the studied species produce a substantial number of viable propagules, and have the capacity to germinate in field conditions (readiness depending on the species). Sexual reproduction is expected, therefore, to play an important role, as suggested by their reproductive investment, in longterm population maintenance.

The dispersal system of Palicourea rigida (Rubiaceae), a common woody species of neotropical savannas, was studied in a seasonal Venezuelan savanna. Production and consumption of fruits, dispersal agents, feeding behaviour, transport and effects on seed viability, were investigated by field observations, cage experiments and viability tests. To compare different dispersers, quantitative and qualitative dispersal parameters were calculated. About 59% of the fruit crop was dispersed successfully by birds. Although 10 different bird species consumed the fruits of Palicourea rigida, three bird species dispersed 79% of the fruits (Tyrannus melancholicus, Molothrus bonariensis and Elaenia chiriquensis). T. melancholicus (Tyrannidae) was the most important dispersal agent, with more than 39% of dispersed seeds. In contrast to the predictions of the specialization vs. generalization paradigm, this system achieves high effectiveness by means of predominantly unspecialized traits in both the plant and the dispersers. Se estudió el sistema de dispersión de Palicourea rigida (Rubiaceae), una leñosa común de las sabanas neotropicales, en una sabana estacional de Venezuela. Mediante observaciones de campo y experimentos en jaulas y en el laboratorio, se estudiaron paramétros tales como producción, transporte y consumo de frutos, los agentes dispersores y su comportamiento y los efectos sobre la viabilidad de las semillas. Para comparar los agentes de dispersión se estimaron paramétros cualitativos y cuantitativos. Cerca del 59% de la cosecha total de frutos fue dispersada existosamente por pájaros. Aunque los frutos fueron consumidos por diez especies de aves, el 79% de los frutos fueron dispersados por solamente tres especies (Tyrannus melancholicus, Molothrus bonariensis y Elaenia chiriquensis). T. melancholicus (Tyrannidae) fue el más importante agente dispersor con más del 39% del total de semillas dispersadas. En contraste con el paradigma de especialización vs. generalización, este sistema alcanza alta efectividad pero se caracteriza por rasgos poco especializados, tanto en la planta como en los dispersores.

Temperature may determine altitudinal tree distribution in different ways: affecting survival through freezing temperatures or by a negative carbon balance produced by lower photosynthetic rates. We studied gas exchange and supercooling capacity in a timberline and a treeline species (Podocarpus oleifolius and Espeletia,neriifolia, respectively) in order to determine if their altitudinal limits are related to carbon balance, freezing temperature damage, or both. Leaf gas exchange, leaf temperature-net photosynthesis curves and leaf temperature at which ice formation occurred were measured at two sites along an altitudinal gradient. Mean CO2 assimilation rates for E. neriifolia were 3.4 and 1.3 mu mol . m(-2)s(-1), at 2 400 and 3 200 m, respectively Mean night respiration was 2.2 and 0.9 mu mol . m(-2)s(-1) for this species at 2 400 and 3 200 m, respectively. Mean assimilation rates for P. oleifolius were 3.8 and 2.2 mu mol . m(-2)s(-1) at 2 550 and 3 200 m, respectively. Night respiration was 0.8 mu mol . m(-2). s(-1) for both altitudes. E. neriifolia showed similar optimum temperatures for photosynthesis at both altitudes, while a decrease was observed in P. oleifolius. E. neriifolia and P. oleifolius presented supercooling capacities of -6.5 and -3.0 degrees C, respectively. For E. neriifolia, freezing resistance mechanisms are sufficient to reach higher altitudes: however, other environmental factors such as cloudiness may be affecting its carbon balance. P. oleifolius does not reach higher elevations because it does not have the freezing resistance mechanisms. (C) 2000 Editions scientifiques et medicales Elsevier SAS.

The high north-central Chilean Andes present strong daily and seasonal temperature variations together with an extremely dry growing season, typical of mediterranean climates. Adesmia is highly diverse in terms of life-forms, and the most dominant plant genus in this region; therefore, it represents an opportunity to study plant responses to constraints imposed by water stress during the growing season. This paper characterizes variations in water relation parameters, which occur under low water availability conditions, and their effects on gas exchange characteristics in four Adesmia species with different life-forms and inhabiting different vegetation belts at different altitudes. Daily courses of gas exchange and water relations were performed on A. hystrix (3300 m), A. aegiceras (3300 and 3750 m), A. echinus (4200 m) and A. subterranea (3750 and 4200 m) in the growing season. Osmotic potential at full turgor and turgor loss and relative water content at turgor loss were also evaluated for each species. All Adesmia species show important stomatal control during most of the day as a response to law soil water availability. A. aegiceras, at both altitudes, exhibits maximum assimilation rates (6-8 mu mol m(-2)s(-1)) compared to the other three species (2-6 mu mol m(-2)s(-1)). Only A. hystrix at 3300 m and A. aegiceras at 3750 m lost turgor during the daily courses. Maximum CO2 assimilation rates are low compared to other species from mid-latitude high mountains.