Ma Sobrado

Simon Bolívar University · Departamento de Biología de Organismos

In tropical montane areas, water limitation is a common occurrence, and both pioneer and forests species experience water stress during the dry season. Adjustments of leaf area during periods of drought allow for the maintenance of the water supply and physiological functions of the remaining leaves. Here, we compared leaf blade water relations bet...

The objective of this study was to examine the water relations and hydraulic architecture and vulnerability to cavitation in Virola surinamensis and V. michelii and to compare to similar measurements in Eperua falcata. In several previous reports Virola was seen to have a rather narrow range of xylem pressure potentials (Ψ x) near zero in the cours...

The landscape of the upper Rio Negro basin (North Amazon) exhibits distinctive habitats that are associated with differential soil characteristics and topographical conditions as well as species composition (Herrera et al. 1978). The mixed forests thrive on well-structured oxisols on slightly more elevated areas. The valleys with sandy podzols are...

Vegetation of the upper Rio Negro (Amazon region, Venezuela) includes a patchy arrangement of forests of distinct structure and species related to topography, soil fertility and water table fluctuation. The region receives high and non-seasonal rainfall. In all habitats, top canopy leaves are exposed to high light levels year round which saturate p...

Leaf blade physical and chemical characteristics, wood composition and anatomy, as well as long-term water-use efficiency and hydraulic characteristics of leaf-bearing terminal branches were assessed in tree species growing in contrasting forests of the Venezuelan Amazonas: mixed forest on oxisol soil and caatinga on podzol soil. Two upper-canopy t...

The objective of this Study was to explore the leaf tissue water relations in terminal branches. as well as the relations between xylem structure and function of live selerophyllus species coexisting on white sands within the Amazon region. In these species, which possess costly leaves and thrive in art extremely nutrient-poor habitat, the preserva...

This study estimated the construction const (CC) and maintenance cost (MC) of leaf tissue on the basis of dry mass (CCMass, MCMass) and leaf area (CCArea, MCArea), as well as the maximum leaf gas exchange capacity, so as to examine leaf cost:benefit relationship in six dominant species of the ‘Bana’ vegetation. Minimum and maximum CCMass averaged 1...

Gas exchange and chlorophyll a fluorescence measurements of expanding and adult leaves of four plant species were compared under field conditions. The pioneer species (PS) tended to have thinner leaves with lower nitrogen content and higher stomatal density compared to forest species (FS). Expanding leaves featured lower photosynthetic pigment cont...

In six dominant species of the Amazonian ‘Bana’ vegetation, leaf blade characteristics, pigment composition, and chlorophyll (Chl) fluorescence parameters were measured in young and mature leaves under field conditions. Leaf δ13C was comparable in the six species, which suggested that both expanding and expanded leaves contained organic matter fixe...

The purpose of this study was to investigate the xylem anatomy and hydraulic characteristics of the mangrove Laguncularia racemosa grown under contrasting salinities. The study addressed the hypothesis that, at high salinity, water transport capacity may decrease in association with higher water use efficiency. Plants were grown in media to which 0...

The purpose of this work is to increase ecological understanding of Avicennia germinans L. and Laguncularia racemosa (L.) Gaertn. F. growing in hypersaline habitats with a seasonal climate. The area has a dry season (DS) with low temperature and vapour pressure deficit (vpd), and a wet season (WS) with high temperature and slightly higher vpd. Seas...

Leaf gas exchange was assessed in Avicennia germinans L. grown under different NaCl concentrations (0-40 per thousand), after salt-relief, and then during drought. Stomatal conductance (gs) and net photosynthetic rate (Pn) decreased with increasing NaCl concentration, and intrinsic water use efficiency (Pn / gs) increased. Under desalinization Pn /...

Abstract This study reports on the effect of water deficit on the tissue water relations and leaf growth of six corn cultivars, growing in glasshouse conditions, in order to understand growth responses to drought of tropical corn. A mild water-stress treatment was imposed slowly; plants reached a minimum pre-dawn leaf water potential of about –1.5...

This study aimed to determine if two species of sunflower, Helianthus annus L. cv. Hysun 31 (cultivated, single-stemmed genotype) and Helianthus petiolaris Nuttall ssp. fallax (wild, many-hranched genotype) differed in the response of leaf growth to water deficits. Earlier published studies, concerned only with H. annuus, failed to reveal differenc...

This study examines the effect of leaf age on photosynthesis, transpiration and nitrogen concentration in four deciduous (DC) and two evergreen (EG) species coexisting in a tropical dry forest of Venezuela. Leaf age was characterized on the basis of leaf chorophyll, nitrogen content, and construction and maintenance costs. The mean leaf area-based...

Under constant salinity we analysed the leaf characteristics of Laguncularia racemosa (L.) Gaertn. in combination with gas exchange and carbon isotopic composition to estimate leaf water-use efficiency (WUE) and potential nitrogen-use efficiency (NUE). NaCl was not added to the control plants and the others were maintained at salinities of 15 and 3...

This study assessed if mature leaves of Laguncularia racemosa were able to demonstrate salt secretion, and if the magnitude of secretion was a function of soil salinity. Thus, salinity influence on the osmolality of leaf tissue, xylem sap and leaf secretion was assessed in field and glasshouse experiments. As salinity increased, solutes were accumu...

This study was carried out in pioneer and successional forest tree species in a lower montane tropical forest with seasonal rains. We tested whether pioneer species feature high hydraulic conductance allowing them to use water profusely at leaf level. Conversely, forest species may have relatively low hydraulic conductance accompanied with better c...

This study assessed the effects of salinity changes over space and time upon leaf gland secretion in Avicennia germinans trees growing naturally in an area featuring markedly seasonal rainfall. Soil ? (, soil N MPa) during the wet season was -0.95&#450.05 and -2.12&#450.08 at low and high salinity sites, respectively. During the dry season, these v...

The relative changes in osmolality of leaf tissue, xylem sap and leaf secretion, as well as leaf gas exchange characteristics of the mangrove A. germinans, cultivated under moderate salinity (0 to 428 mol NaCl m-3) and hypersaline conditions (856 mol NaCl m-3), were examined. Water content and net amount of solutes per unit leaf area increased at m...

Water transport was assessed in seedlings of the mangrove Avicennia germinans L. grown at 171 and 684 mol m–3 NaCl. Leaf specific conductivity declined by 25 % at high salinity. This was related to low specific conductivity, because Huber values remained similar. Leaves of A. germinans featured low internal conductance to water transport. This was...

The aim here was to use total-reflection X-ray fluorescence (TXRF) to determine the elemental composition (besides Na), and the relative contribution of each element in leaf secretion of the mangrove species Avicennia germinans, grown under contrasting salinities (0-865 mol m(-3) NaCl). Na was determined by conventional atomic absorption spectromet...

We used pressure–volume analysis and dewpoint hygrometry to determine leaf water relation parameters of mangrove (Avicennia germinans L.) seedlings grown at salinities of 0, 8, 20 and 32 ‰. Seedlings responded to an increase in salinity from 0 to 32 ‰ by an increase in leaf succulence as reflected in an increase in leaf water content per unit area...

Mangrove species more tolerant to salinity may function with less efficient water transport, which may be related to more conservative water use. To test the hypothesis, we investigate the gas exchange and hydraulic properties of three mangrove species: Rhizophora mangle L., Laguncularia racemosa Gaert and Avicennia germinans (L.)L. Experiments wer...

In leaves of the mangrove species Avicennia germinans (L.) L. grown in salinities from 0 to 40 ‰, fluorescence, gas exchange, and δ13C analyses were done. Predawn values of Fv/Fm were about 0.75 in all the treatments suggesting that leaves did not suffer chronic photoinhibition. Conversely, midday Fv/Fm values decreased to about 0.55-0.60 which ind...

 Drought effects on leaf photosynthesis of A. germinans growing under two contrasting salinities were studied in a Venezuelan fringe mangrove. During both wet and dry seasons, severe chronic-photoinhibition at predawn was not observed but strong down regulation occurred at midday during both seasons. Carbon assimilation rates (A, μmol CO2 m−2 s−1)...

Photosynthesis was studied in relation to light use in the mangrove, Avicennia marina (Forsk.) Vierh. var. australasica (Walp.) Moldenke, growing under soil salinities equivalent to one and two times seawater (i.e. 35 and 60‰). Midday CO2 assimilation rates averaged 7.6 0.7 and 4.3 0.3 µmol m–2 s–1 at the seawater and hypersaline sites, respectivel...

Liquid-flow-pathway hydraulic conductances (KL) were measured in leaves of 24 species of temperate and tropical plants. The leaves had specific leaf areas (SLA) ranging from 0.05 to 0.5 m2 g1. Leaves with low SLA were typically sclerophylls and leaves with high SLA were typically malacophylls. No correlation was found between KL and SLA. Whole shoo...

Physiological traits involved in leaf water relations were evaluated in Avicennia germinans (L.) L. seedlings growing at different salinities in the field. Analysis of pressure-volume (P-V) curves and sap osmometry were combined to evaluate osmotic adjustment and cell elasticity, and the contribution of accumulated inorganic ions to osmotic potenti...

Plant water status plays an important role in determining phenological patterns of leaf expansion and abscission of drought-deciduous (DC) and evergreen (EG) species co-occurring in tropical dry forests. If vulnerability to embolism is related to the phenological pattern, we should find that drought-deciduous species are intrinsically more suscepti...

Several pieces of research suggest that leaf liquid-path conductance (K1) may be smaller in evergreen species (EG) than in deciduous species (DC), coexisting in tropical dry forests. This was investigated in four DC and two EG in a tropical dry forest in Venezuela. The K1 (kg m-2 MPa-1 s-1) were consistently lower in EG (8.4 10-5) than in DC (12 to...

Leaf carbon isotope ratios were measured on dominant deciduous-leaved and evergreen-leaved species in a seasonally dry tropical forest in Venezuela. Carbon isotope ratios were higher and the intercellular to ambient CO2 concentrations were lower in the deciduous-leaved species. Associated with this are predicted lower water-use efficiencies in ever...

Leaf bearing stems ofCuratella americana L. were very susceptible to induced cavitation: embolisms began at a pressure of 0.5 MPa (15 %) and at 2.0 MPa most of the conductivity was lost (85 %). Nevertheless, in nature similar leaf specific conductivities, of about 90 x 10-5 kg m-2 s-1 MPa-1 during both wet and dry seasons indicated absence of droug...

In five leaf age classes of an evergreen tree Curatella americana L. widespread in savanna, fluorescence parameters, water loss and carbon assimilation were measured during both wet and dry seasons, and afterwards their nitrogen and water use efficiencies were analyzed. Variable to maximum fluorescence ratios (F(v)/F(m) ≃ 0.74) were higher in matur...

Reserves of twig nitrogen of drought-deciduous species changed seasonally and provided some of the nitrogen required for canopy reconstruction after drought relief. By contrast, in evergreen species, nitrogen mobilization from old leaves and use in expanding leaves occurred simultaneously at the time of leaf exchange and twig nitrogen remained cons...

Drought-deciduous and evergreen species coexist in tropical dry forests. Drought-deciduous species must cope with greater seasonal leaf water-potential fluctuations than evergreen species and this may increase their susceptibility to drought-induced xylem embolism. The relationship between water transport efficiency and leaf life-span were determin...

Evidence from previous studies relates the significance of evergreeness and deciduousness to differences in leaf drought resistance and nutrient-use characteristics. Considering possible differences in leaf construction costs and lifespans in both plant types, I hypothesized that they should have contrasting strategies underlying their nitrogen-use...

This study compared the tissue water relations and seasonal changes in leaf water potential components of an evergreen tree,Morisonia americana, and two evergreen shrubs,Capparis verrucosa andC. aristiquetae, with two deciduous trees,Humboltiella arborea andLonchocarpus dipteroneurus, and the deciduous vineMansoa verrucifera. All these species coex...

This study reports on the effects of water deficits on photosynthesis, plant growth and carbon allocation in the wild sunflower Helianthus petiolaris and in the cultivated sunflower Helianthus annuus grown under controlled conditions in the glasshouse. Water deficits reduced the rate of net photosynthesis and the dry weight of leaves, stems, roots...

The portable instrument described by Heathcote, Etherington, and Woodward (1979) for the non-destructive measurement of turgor pressure was evaluated in Helianthus annuus and Helianthus paradoxus. A good correlation was obtained between turgor pressure measured with the instrument and turgor pressure estimated by the pressure-volume technique for i...

The effect of water deficits on the water relations and stomatal responses of Helianthus annuus and Helianthus petiolaris were compared in plants growing in the glasshouse under controlled conditions. Unirrigated plants of both genotypes were subjected to two different stress rates in which predawn leaf water potentials declined steadily at either...

The effects of water deficits on the water relations characteristics and yield of two cultivated Helianthus annuus L. lines, Hysun 31 and Seneca, and two wild Helianthus species, H. nuttallii and H. petiolaris, were compared. The study was conducted on frequently irrigated and unirrigated plants growing in the field during the summer. Predawn measu...

Species of the ‘bana’ vegetation in the Amazonas region of equatorial South America have scleromorphic leaves. This leaf type, which characterizes the vegetation of Mediterranean climates, among others, has apparently evolved in this community in response to the oligotrophic soils and widely fluctuating water table. Anatomically, the leaves have se...

The influence of leaf orientation on leaf temperature has been studied in an sclerophyll vegetation of the Amazon basin, which grows on white sandy soils of very low water retention capacity and variable depth of the water table. Leaf size of the species studied is mainly mesophyllous (sensu Raunkiaer). The high degree of leaf inclination in all sp...