Interactions between Abscisic Acid, Gibberellins and Cytokinins in Grand Rapids Lettuce Seeds Germination

Experiments with Grand Rapids lettuce seeds (Lactuca sativa L.) maintained in darkness or irradiated with red light have shown that the inhibition of germination induced by low concentrations of ABA (2, 4, 6 μM) could be overcome by gibberellins (GA₃ or GA₄). The same results were obtained, although to a lesser extent, under the influence of two out of the four cytokinins tested (K and BAP) for seeds maintained in darkness. To suppress the block induced by higher concentrations of ABA (for example 8 μM), it was necessary to apply a cytokinin (K, BAP, Z or 2iP) and a gibberellin (GA₄ or GA₃) simultaneously, or a cytokinin following a red light treatment. Experiments conducted in darkness in which ABA (8 μM) was applied together with a cytokinin (BAP) and a gibberellin (GA₄) showed that the gibberellin and the cytokinin played similar roles towards each other and towards ABA.     

Absorption, Distribution, Metabolism and Leaching of [14C] ABA during Culture of Apple Embryos

Barthe, Ph. and Bulard, C. 1987. Absorption, distribution, metabolismand leaching of [¹⁴C] ABA during culture of apple embryos.—J.exp. Bot. 38: 1002–1011. It has been known for some time that dormant embryos, laid flaton damp filter-paper one cotyledon only being in direct contactwith it (C/2M mode of culture), exhibit unequal growth and greeningof their cotyledons. The aim of this work was to investigatewhether this particular mode of culture led to detectable differencesbetween the two cotyledons in the distribution and metabolismof [¹⁴C] abscisic acid (ABA). Two different approaches wereused, the material in both cases being dormant embryos of Pyrusmalus L. cv. Golden Delicious cultured at 23°C in darkness.In a first experiment, the embryos were cultured directly inthe C/2M mode and in the presence of 10^–2 mol m^–3[¹⁴C] ABA. In these conditions marked differences in the distributionof radioactivity between the lower (LC) and upper (UC) cotyledonappear after 24 h of culture. After 5 d, amounts of total radioactivitywere four times higher in LC than in UC, and the level of ABAwas three times higher. Metabolism was extremely active in LC,since certain metabolites were found in relative percentages(percentages with respect to the total radioactivity of thecotyledon) equivalent to (esters, glucosides), or even higher(dihydrophaseic acid, DPA) than those found in UC. It is suggestedthat this high level of metabolism in LC limits the availabilityof the transportable molecule, that is to say ABA. In a secondexperiment double culturing was carried out. The embryos werefirst cultured in the presence of 10^–2 mol m^–3 [¹⁴C]ABA in conditions which ensured equal distribution of ABA andits metabolites between the two cotyledons. After 5 d they weretransferred to an ABA-free medium and cultured in the C/2M mode.After 2 d and 5 d of culture, dissymmetry between the two cotyledonswas again noted; UC this time containing greater amounts ofradioactivity than LC. The differences, however, were less markedthan in the first experiment. This dissymmetry is due to leachingof a certain amount of radioactivity from LC, which is onlypartially compensated for by migration from UC. Key words: ¹⁴C-ABA distribution, leaching and metabolism, embryo culture, embryo dormancy     

Microclimatic buffering and resource‐based habitat in a glacial relict butterfly: significance for conservation under climate change

In contrast to several organisms that have already shown range shifts to the north as a response to climate change, southern populations of relict species are trapped in isolated altitudinal habitats. Therefore, there is a growing interest to better understand their habitat use, with particular attention to the thermal aspects and associated significance for their habitat management. We address this issue by a study of larval habitat use relative to vegetation structure and microclimate in a glacial relict butterfly of peat bog ecosystems, using a functional, resource‐based habitat approach. We analysed caterpillar presence and density relative to vegetation composition (reflecting gradients of humidity, temperature, and natural succession of the peat bog) and to the availability and quality of thermal refuges for caterpillars (i.e., structures provided by Sphagnum hummocks). We also tested caterpillar survival rates under different temperature and humidity treatments. We found that (1) Boloria aquilonaris was a specialist butterfly of early successional stages with very humid zones of peat bog, (2) the lack of Sphagnum hummocks reduced larval habitat suitability, and hence the population density, and (3) a reduction of the thermal buffering ability of Sphagnum hummocks was observed in less humid, degraded parts, or late‐successional stages of peat bog. A larval rearing experiment showed a significant impact of temperature on caterpillar survival; survival being higher at lower temperature. Our field and laboratory results support the idea that the thermal environment exploited by caterpillars should be considered as a functional resource and included in a population‐specific habitat definition. Appropriate management of the peat bog habitat of this glacial relict species should not exclusively focus on the larval and adult feeding resources, but also on the quality of thermal refuges provided by Sphagnum hummocks in humid zones of the peat bog, especially in the current critical context of climate warming.     

The stoichiometry of nitrogen and phosphorus spiralling in heterotrophic and autotrophic streams

1. Nutrient spiralling provides a conceptual framework and a whole‐system approach to investigate ecosystem responses to environmental changes. We use spiralling metrics to examine how the coupling of nitrogen and phosphorus uptake varies between streams dominated by either heterotrophic (i.e. bacteria‐dominated) or autotrophic (algal‐dominated) microbial communities. 2. Algae generally exhibit greater capacity to store nutrients than bacteria because of differences in cellular structures. These differences led us to hypothesise that the uptake of N and P in heterotrophic ecosystems should have reduced stoichiometric variation in response to changes in supply N : P compared to autotrophic ecosystems when assimilation dominates nutrient uptake. 3. To test this hypothesis, we used an array of serial nutrient additions in several streams in the South Fork Eel River watershed in Northern California. In one set of experiments, N and P were added alone and simultaneously in separate experiments to two small, heterotrophic streams to assess uptake rates and interactions between nutrient cycles. In a second set of experiments, N and P were added simultaneously at a range of N : P in one heterotrophic and one autotrophic stream to assess differences in uptake responses to changes in supply N : P. 4. Results of these experiments suggest two important conclusions. First, increased N supply significantly shortened P uptake lengths, while P addition had little impact on N uptake in both streams, indicating that uptake of non‐limiting nutrients is tightly coupled to the availability of the limiting element. Second, changes in P uptake and uptake ratios (U_N : U_P) with increased supply N : P supported our hypothesis that heterotrophic streams are more homeostatic in their responses to changes in nutrient supply than autotrophic streams, suggesting that physiological controls on nutrient use scale up to influence ecosystem‐scale patterns in nutrient cycling.     

Isolation and characterization of nuclear microsatellite loci for the common green darner dragonfly Anax junius (Odonata: Aeshnidae) to constrain patterns of phenotypic and spatial diversity

Fourteen polymorphic microsatellite loci were developed from an enriched genomic library of the widely distributed migratory North American dragonfly species, the common green darner (Anax junius). For a group of 22 larvae, these loci averaged 16 alleles, with individual loci ranging from nine to 29 alleles. Observed heterozygosity averaged 0.784 per locus.     

Thermal and hydric implications of diurnal activity by a small tropical frog during the dry season

Amphibians are typically intolerant of high temperatures and dehydrating conditions, and small species are particularly susceptible to desiccation. The rockhole frog, Litoria meiriana (Hylidae), is diurnal and is often observed on rocks in the sun near streams in tropical Australia. These hot, desiccating conditions are avoided by most frog species. We measured the microclimate in the areas used by frogs and the activity, body temperatures and hydric state of free‐ranging individuals of this small frog. We also used plaster models to further explore the dynamic nature of hydric state by combining estimates of water loss and water uptake with behavioural observations of activity and microhabitat selection. Both direct measures and estimates of dynamic hydric state indicated that free‐ranging frogs generally maintained a hydric state above 95% of full hydration, but occasionally, particularly during the afternoon, frogs allowed their hydric state to fall as low as 85%. Body temperatures of frogs remained below the critical thermal maximum (CT_max) even when the frogs were in the sun, because this species has no cutaneous resistance to evaporative water loss and so they cool by evaporation. However, during the hotter part of the day, on dry sunny substrates, the hydric state of the frogs could fall to near lethal hydration states (approximately 70% of full hydration) within a short period (approximately 20 min). Thus, the threat of desiccation appears to be more limiting than the threat of overheating. These diurnal frogs rely on frequent bouts of rehydration to support their ability to venture onto hot, dry rocks during the day.     

Degradation of pectic substances in green flax fibre by Erwinia carotovora

In this paper, we report that Erwinia carotovora can perform controlled biological pectinolysis of green flax fibre. The degradation of pectic substances is considered a key step during this process and is estimated by determining the cation exchange capacity (CEC). A CEC characteristic of a ‘good retting degree’, is reached after 20 h incubation, whatever the tested temperature and initial pectate lyase activity of the bacterial culture. Similar results are obtained after only 4 h, at a temperature of 45°C and the pectate lyase activity is greater than 2 nmol ml^-1 min^-1. The linear relationship between the CEC and the weight loss of green fibre which is established, allows us to estimate an equivalent weight of 1000 g eq^-1 for the solubilised polymers.     

Stem Cell Factor Regulates Human Melanocyte-Matrix Interactions

Stem cell factor (SCF) is hypothesized to play a critical role in the migration of melanocytes during embryogenesis because mutations in either the SCF gene, or its ligand, c-kit, result in defects in coat pigmentation in mice and in skin pigmentation in humans. In this report we directly show that SCF alters the adhesion and migration of human melanocytes to extracellular matrix (ECM) ligands and regulates integrin expression at the protein level. SCF decreased adhesion of neonatal and fetal cells to collagen IV, and increased attachment of fetal cells to laminin. Attachment of fetal cells to fibronectin was decreased, but was unchanged in neonatal cells. Flow cytometry analysis of neonatal melanocytes showed that SCF down-regulated the expression of the α2 receptor, and up-regulated the expression of the α3, α5 and β1 integrin receptors. SCF down-regulated expression of α2, α5 and β1 integrins by fetal melanocytes, and up-regulated expression of the αv and α3 integrin receptors. Analysis of melanocyte migration using time-lapse videomicroscopy showed that SCF significantly increased migration of neonatal, but not fetal, melanocytes on fibronectin (FN). We conclude that SCF regulates integrin expression at the protein level and that SCF has pleiotropic effects on melanocyte attachment and migration on ECM ligands. We suggest that this may be one mechanism by which SCF regulates melanocyte migration during development of the skin.