Vegetation heterogeneity and diversity in flat and mountain landscapes of Patagonia (Argentina)

Abstract. We studied floristic and diversity patterns and their environmental controls in two landscapes of contrasting topography in the Patagonian steppe. The analyses were focused on the effects of water availability gradients and landscape configuration on plant species distribution and coexistence. Floristic variation was investigated using Correspondence Analysis. The relationship between floristic and environmental variation was analyzed using Canonical Correspondence Analysis and correlation tests. We explored diversity patterns by relating spatial distance to floristic dissimilarities. The floristic gradient was determined by shrub and grass species and was related to precipitation in the flat area, and to precipitation, elevation and potential radiation in the mountain area. Site species richness increased with water availability in both areas. Mean site species richness and species turnover in space was higher in the mountain than in the flat area. Landscape species richness and floristic gradients were more concentrated in the mountain than in the flat area. In contrast to shrubs and grasses, forb species distributions were uncoordinated and probably independent of any environmental gradient. Our results suggest (1) that landscape configuration affects species composition and diversity through its direct effect on abiotic environmental heterogeneity, and (2) that the environmental controls of the community composition vary depending on the plant functional type considered.     

Ecosystem responses to changes in plant functional type composition: An example from the Patagonian steppe

Abstract. Grass cover along a grazing intensity gradient in Patagonia decreases, whereas bare soil and shrub cover increases. Our objective was to study the effect of a change in the dominant plant functional type on soil water balance, primary production, herbivore biomass, roughness, and albedo. Using a soil water balance model, we found increases in evaporation and deep drainage, and a decrease in total transpiration along the grazing intensity gradient. Above-ground primary production, estimated from transpiration, decreased along the grazing intensity gradient because shrubs did not fully compensate for the decrease in grass production. Using a statistical model, we calculated herbivore biomass from estimates of above-ground primary production. Estimated herbivore biomass was lowest in the shrub-dominated extreme of the grazing gradient. Roughness increased from the grass-dominated to the shrub-dominated community. Albedo had a maximum at an intermediate position along the gradient. Our results suggest that changes in plant functional type composition, independent of changes in biomass, affect ecosystem functioning and the exchange of energy and material with the atmosphere. Grasses and shrubs proved to be appropriate plant functional types to link structure and function of ecosystems.     

Effect of glutamine on glutamine-synthetase regulation in the green alga Monoraphidium braunii

Glutamine-synthetase (GS; EC 6.3.1.2) activity and protein levels were measured in crude extracts from Monoraphidium braunii Näegeli, strain 202-7d, cultures grown under different nitrogen sources. Only ammonium and l-glutamine promoted a partial enzyme inactivation, which, in the case of l-glutamine, was accompanied by a significant repression of GS. Methionine sulfoximine (MSX), a strong inhibitor of GS, produced a drastic inactivation of GS which was concomitant with a marked increase in GS protein as measured by rocket immunoelectrophoresis. Such an increase was prevented in the presence of cycloheximide. The effect of the l-glutamine analog on GS activity and protein was partially inhibited if l-glutamine was also added to cell cultures, possibly indicating competition in the transport of these two substances. In addition, the effects of MSX were reversed after longer times when cultures were treated with smaller concentrations of inhibitor. Treatment of cell cultures with azaserine, a specific inhibitor of glutamate synthase, the second enzyme acting in the ammonium assimilation pathway, promoted a strong GS inactivation and a partial repression of this enzyme, which paralleled a specific increase in the intracellular pools of glutamine High-performance liquid chromatography measurements of intracellular amino-acid concentrations showed that glutamine levels correlated negatively with GS concentration. A role for glutamine as a negative effector of GS synthesis is proposed.Abbreviations GS l-glutamine synthetase - GOGAT l-glu-tamine:2-oxoglutarate amidotransferase - MSX methionine sulfoximine During the course of this work, J.A. was supported by a fellowship from Junta de Andalucía, and J.M. G-F. by a fellowship from the Spanish Ministerio de Educatión y Ciencia. This work was supported by the Junta de Andalucía.     

Satellite Spitzenkörper in growing hyphal tips

Summary Growing hyphal tips of higher fungi contain an organized assemblage of secretory vesicles and other cell components collectively known as the Spitzenkörper. Until now, the Spitzenkörper has been portrayed as a single spheroid complex located near the apical cell wall. This study demonstrates the occurrence of multiple Spitzenkörper in growing hyphal apices imaged by video-enhanced phase-contrast microscopy. In addition to the main Spitzenkörper, smaller satellite Spitzenkörper arise a few micrometers behind the apical pole. Four developmental stages were identified: (a) the satellites first appeared as faint phase-dark plaques next to the plasma membrane, (b) gradually increased in size and assumed an ovoid profile, (c) they migrated to the hyphal apex, and (d) finally they merged with the main Spitzenkörper. After the merger, the main Spitzenkörper temporarily increased in size. Satellites were observed in 14 fungi, most of which had relatively large (5–10 m diam.), fast-growing hyphae (2–33 m/min elongation rate). The average frequency of in-focus satellites was 7+/min forFusarium culmorum and 11+/min forTrichoderma viride. As with the main Spitzenkörper, satellites were present only in growing cells. They were transient and remained visible for 3–8 s before merging with the main Spitzenkörper. Within the hyphae, satellites travelled up to six times faster than the average cell elongation rate. Multiple satellites sometimes occurred simultaneously; up to three were seen within a hyphal apex at the same time. Localized cell enlargement occurred next to stationary satellites, suggesting that satellite Spitzenkörper are functional as sources of new cell surface before they reach the main Spitzenkörper; therefore, they account for some variations in the profiles of the growing hyphae. By electron microscopy, satellites consisted of small clusters of apical vesicles surrounding a group of microvesicles located next to the plasma membrane. The identification and behavior of the satellites represent clear evidence of directional mass transport of vesicles toward the hyphal apex. Our observations indicate that satellites are a common phenomenon in growing hyphal apices of septate fungi and that they contribute to growth of the hyphal apex.Abbreviations VSC vesicle supply center     

Vitamin C stabilization as a consequence of the plasma membrane redox system

Summary Ascorbate is stabilized in the presence of HL-60 cells. Our results showed that cAMP derivatives and agents that increase cAMP stimulate the ability of HL-60 cells to stabilize ascorbate. On the other hand, tunicamycin, a glycosilation-interfering agent, inhibited this ability. The ascorbate stabilization in the presence of HL-60 cells has been questioned as a simple chemical effect. Further properties and controls about the enzymatic nature of this stabilization are described and discussed. This data, together with hormonal regulation, support the hypothesis that an enzymatic redox system located at the plasma membrane is responsible of the extracellular ascorbate stabilization by HL-60 cells.Abbreviations AFR ascorbate free radicals - FCS fetal calf serum - Sp-cAMPS Sp-cyclic adenosine monophosphothionate - Rp-cAMPS Rp-cyclic adenosine monophosphothionate     

Xanthine accumulation and vacuolization inChlamydomonas reinhardtii cells

Summary Utilization of xanthine as the sole nitrogen source for growth byChlamydomonas reinhardtii cells involved the formation of a transient, intracellular pool of xanthine. Up to 20% of the total xanthine supplied to the medium was not assimilated after uptake but stored in the cells at concentrations that exceeded xanthine solubility in water. At the subcellular level, a massive accumulation of starch grains in the chloroplast and the appearance of many vacuoles in the cytoplasm distinguished xanthine-grown from ammonium-grown cells. Starch accumulation, but not development of vacuoles, was also observed in N-starved cells. Uptake experiments with radio-labelled xanthine showed that this accumulates only in the cytoplasm, most probably inside vacuoles. The electron-dense material observed in vacuoles of xanthine-grown cells suggests that the intracellular xanthine is in part solid xanthine.     

Maximum-likelihood models for mapping genetic markers showing segregation distortion. 2. F2 populations

In F₂ populations, gametic and zygotic selection may affect the analysis of linkage in different ways. Therefore, specific likelihood equations have to be developed for each case, including dominant and codominant markers. The asymptotic bias of the classical estimates are derived for each case, in order to compare them with the standard errors of the suggested estimates. We discuss the utility and the efficiency of a previous model developed for dominant markers. We show that dominant markers provide very poor information in the case of segregation distortion and, therefore, should be used with circumspection. On the other hand, the estimation of recombination fractions between codominant markers is less affected by selection than is that for dominant markers. We also discuss the analysis of linkage between dominant and codominant markers.     

Changes in the urea concentration during controlled wine aging by two “flor” veil-forming yeasts

Summary Contents of arginine, ornithine, urea and ammonia in wine were monitored during aging process by two flor veil-forming yeast strains. The patterns of yeast growth were different for the two strains studied. Urea excretion and uptake were dependent on yeast strain, so that finished wines showed different concentrations of urea. The effect of limited aeration was also investigated for Saccharomyces cerevisiae var. capensis. The results indicated that aeration promoted yeast growth in veil, thus reducing urea concentration until its exhaustion in wine.     

Construction of Novel Shuttle Vectors for Use between Moderately Halophilic Bacteria andEscherichia coli

Shuttle vectors useful for the genetic manipulation of several moderately halophilic bacteria have been constructed. These vectors are based on the minimal replicon of pCM1, a cryptic plasmid fromChromohalobacter marismortui,combined with the useful properties of pUC18 plasmid (i.e., small size, high copy number, multiple cloning sites,lacZfragment), as well as with the trimethoprim resistance gene as a selection marker for moderate halophiles. These vectors can be efficiently transferred by RP4-mediated conjugation fromEscherichia colito the moderate halophilesChromohalobacter marismortui, Deleya halophila, Halomonas elongata, Halomonas subglaciescola,andVolcaniella eurihalina.