Abundance and habitat segregation in Mediterranean grassland species: the importance of seed weight

Abstract. We analysed the relationship between seed traits (weight, shape and dispersal structures) and the abundance and habitat segregation of Mediterranean grassland species. To take into account possible correlations with other plant traits, the study also includes 5 vegetative traits (growth form, plant longevity, clonality, onset of flowering and plant size) of commonly accepted functional importance. Data were recorded for 85 species from dehesa grasslands in central Spain. Species abundance was measured in upper (dry and less productive, high stress) and lower (moist and more productive, low stress) slope zones in the same area. Habitat segregation was estimated using an index based on the relative frequencies of species in upper and lower slope zones. Multiple regression models were fitted using species, as well as phylogenetically independent contrasts, as data points. Annual small‐seeded species without specialised dispersal structures are over‐represented in dehesa grasslands. Abundance was negatively related to seed weight in upper slope zones. None of the recorded plant traits were related to abundance in the lower slope zones. Habitat segregation was mainly related to seed weight, but also to some vegetative traits. Annual, early flowering and small‐seeded species were relatively more abundant in the upper than the lower slope zones. This pattern is independent of phylogeny. Our results suggest that in dry Mediterranean grasslands, abundance of many species is determined by dispersal (production of numerous small seeds) rather than by competitive ability.     

Ontogenetic changes in the carapace shape of the non-marine ostracod Eucypris virens (Jurine)

Developmental changes in carapace form (size+shape) during ontogeny have been explored in Eucypris virens (Crustacea, Ostracoda) using elliptic Fourier analysis. Clones from different geographic localities raised under controlled constant conditions (temperature and photoperiod) were used to characterize developmental pathways in the species. A larger data set including field populations and laboratory populations cultured under a range of environmental conditions were used to infer influence of environmental factors on carapace shape changes during ontogeny. Size changes between consecutive juvenile stages support empirical laws describing the doubling of ostracod volume at each moult. Ontogenetic changes point out the remarkable influence of environmental conditions on carapace shape.     

Manganese as agonist and antagonist of calcium ions: Dual effect upon catecholamine release from adrenal medulla

Manganese (2.2mM) blocked catecholamine (CA) secretion evoked by acetylcholine (ACh) in perfused bovine adrenals. This effect was reverted when the concentration of Mn²⁺ was increased to 6.6mM. Similar results were observed when higher concentrations (11 and 22mM respectively) were used. Mn²⁺ substituted for Ca²⁺ in the ACh evoked CA secretion, and this response was concentration dependent. The removal of Mn²⁺ from the perfusion medium potentiated the secretory response with respect to the first ACh stimulation. The subcellular distribution of Mn²⁺ in perfused adrenals showed a poor association with storage granules. It is concluded that Mn²⁺ inhibits Ca²⁺ entry during secretion and also substitutes for Ca²⁺ in the excitation-secretion coupling.     

l-Glutamic acid,a neuromodulator of dopaminergic transmission in the rat corpus striatum

A superfusion system was used to study the effects of neuroexcitatory amino acids upon spontaneous and depolarization-evoked release of exogenously taken up and newly synthesized [³H]dopamine by rat striatal slices. Neither l-glutamate nor other aminoacids such as l-aspartate and d-glutamate (5 × 10^?5 M) modified the spontaneous release of exogenous [³H]dopamine from rat striatal slices. In contrast, these neuroexcitatory aminoacids did potentiate spontaneous release of striatal [³H]dopamine newly synthesized from [³H]tyrosine. A different pattern of effects emerged when depolarization-evoked release of dopamine was studied. Only l-glutamate (5 × 10^?6-1 × 10^?4 M) potentiated dopamine release under these experimental conditions in a rather specific and stereoselective manner. In addition, similar results were obtained regardless of whether depolarization-induced release of exogenous or newly synthesized [³H]dopamine was studied. The effect of l-glutamate on depolarization-induced release depended both upon the degree of neuronal depolarization and upon the presence of external Ca²⁺ in the superfusion medium and it was blocked by l-glutamate diethylester. Furthermore, this effect of l-glutamate seemed quite specific with regard to regional localization within the brain as it was only demonstrated in slices from striatum and not in slices from olfactory tubercle or hippocampus. It is suggested that during depolarization a Ca²⁺-dependent event occurs at the striatal membrane level which changes the sensitivity of the dopamine release process to neuroexcitatory aminoacids in such a way as to render it relatively more specific and stereoselective towards l-glutamate stimulation. The findings reported have led us to propose that l-glutamic acid could play a role as a neuromodulator of dopaminergic transmission in the rat corpus striatum.     

Analysis of the inhibitory effect of verapamil on adrenal medullary secretion

Verapamil blocked catecholamine (CA) secretion evoked by acetylcholine (ACh), Ba²⁺ or Ca²⁺ in isolated perfused bovine adrenals. This inhibitory effect was irreversible and not modified by increasing the Ca²⁺ concentration of the perfusion fluid. Tetracaine also inhibited CA secretion, although no additive effect was found when both verapamil and tetracaine were present simultaneously in the perfusion medium. It is concluded that verapamil and tetracaine inhibit CA secretion presumably at the same site, but verapamil effect cannot be reverted by excess of calcium ions.     

Intraneuronal Site of Action for Imipramine in Rat Striatal Slices

: The uptake of ³H-labeled imipramine ([³H]IMI) in rat corpus striatum slices was found to be strongly temperature-dependent. The accumulation was shown to be saturable and two apparent K_m's were found: 2.2 × 10^?8 and 3.5 × 10^?7m . Once incorporated, the labeled drug was released from superfused slices by K⁺ (55 mm ) depolarization in the presence of calcium ions. Imipramine was also studied for its ability to induce the release of [³H]dopamine ([³H]DA) which had been previously accumulated by striatal slices. It was found that striatal slices superfused during 1 or 6 min with imipramine (10^?6-10^?4m ) release substantial amounts of radioactive dopamine, independently of the presence of Ca²⁺ in the medium. This release is completely abolished after reserpine pretreatment. It is proposed that imipramine enters the dopaminergic storage vesicles and displaces dopamine. An intraneuronal mechanism of action for imipramine is discussed.