Soil organic C and nutrient contents under trees with different functional characteristics in seasonally dry tropical silvopastures

Aims

The selection of tree characteristics is critical for the outcome of the tree effects on soil fertility in silvopastoral pastures. This study aims to quantify the effects of trees on soil nutrient and C stocks, as well as assessing differences on the effects between legume (Albizia saman; Enterolobium cyclocarpum) and non-legume tree species (Tabebuia rosea; Guazuma ulmifolia).

Methods

In Central Nicaragua, soil was sampled (0–10 cm deep) in paired plots, under both a canopy and in open grassland, in 12 sites per tree species and analysed for organic C, total N stocks, available P and extractable K⁺, Ca²⁺ and Mg²⁺. To assess the effects of herbaceous composition and cattle to soil proprieties, we recorded the cover of plant groups and assessed the mass of dung in each plot.

Results

Soil organic C and N, available P and extractable K⁺ and Ca²⁺ were higher under the tree canopy than under paired open grassland. The basal area of trees was positively related with the canopy effect on soil variables, thus suggesting that the age or sizes of the trees are relevant factors associated with the content of soil C and nutrients. No specific effects related to the legume species group were detected.

Conclusions

Our results indicate that in fertile seasonally dry subtropical pastures, scattered trees have an overall effect on soil fertility, and that the magnitude of the effect depends more on the tree characteristics (i.e. basal area, crown area) than on whether the species is a legume or not.     

Revisiting the use of Iprodione and Trichoderma in the integrated management of onion white rot

The biocontrol properties of Trichoderma species are well documented, but their effectiveness in antagonism of the problematic Sclerotium cepivorum, the causal agent of white rot in Allium species, appears limited with reports of significant control only relating to deliberately-mutated strains of Trichoderma. Our previous studies have indicated the possibility of using selected naturally-occurring strains of the antagonist in the suppression of other diseases; now in vitro and controlled environment in vivo studies have indicated that a degree of control of Onion White Rot is possible, and that the selected antagonist strains can be used in integrated treatments with Iprodione to good effect. The possible value of such treatments is considered in light of other approaches to the suppression of this continuing problem.     

8-Bromo-cyclic GMP inhibits the calcium channel current in embryonic chick ventricular myocytes

Superfusion with 8-bromo-cyclic GMP or intracellular injection of cyclic GMP inhibits calcium-dependent slow action potentials in embryonic chick or guinea pig ventricular cells, suggesting that cyclic GMP inhibits calcium currents. Recently, cyclic GMP has been shown to reduce cyclic AMP-stimulated calcium currents in voltage-clamped ventricular myocytes. Since earlier results in intact cells had suggested that cyclic GMP might inhibit basal (i.e., unstimulated by cyclic AMP) calcium currents, we directly investigated the effect of 8-bromo-cyclic GMP on basal calcium channel currents (using barium as the charge carrier) in voltage-clamped ventricular myocytes isolated from embryonic chick hearts. Superfusion with 1 mM 8-bromo-cyclic GMP (without prior cyclic AMP elevation) progressively decreased peak calcium channel currents (-68% at 15 min after the onset of drug exposure). In contrast, the currents were unchanged during 15 min superfusion with control solution, or 1 mM 8-bromo-GMP (the noncyclic inactive analog of 8-bromo-cyclic GMP). The present results in voltage-clamped embryonic chick heart cells indicate that cyclic GMP can inhibit basal calcium channel currents, apparently through a cyclic AMP-independent mechanism.     

Local and landscape determinants of pollen beetle abundance in overwintering habitats

• 1 The development of integrated pest management strategies requires that the semi‐natural habitats scattered across the landscape are taken into account. Particular determinants of insect pest abundance in overwintering habitats just before they migrate onto crops appear to be poorly known and of crucial importance for understanding patterns of crop colonization and pest population dynamics at the landscape scale.
• 2 The emergence of pollen beetle Meligethes aeneus F. was studied in grassland, woodland edge and woodland interior over a 3‐year survey in France using macro‐emergence traps. A suite of variables at the local and the landscape scale was assessed for each trap, aiming to identify potential relevant habitat indicators. The effects of habitat characteristics were evaluated using partial least square regressions.
• 3 It was found that M. aeneus can overwinter in all types of habitat but that particular habitat characteristics at the local and landscape scales may explain their abundance in overwintering sites more than the types of habitat: relative altitude, litter thickness, soil moisture and proximity to the previous year's oilseed rape fields appear to be positively correlated with abundance of adults over the 3 years.
• 4 Hence, the abundance of emerged pollen beetles depends on both the landscape configuration of the previous year's oilseed rape fields around overwintering sites and local habitat characteristics. Landscape configuration may determine population flow towards overwintering sites in the late summer, and local habitat characteristics may influence survival rates during the winter. The findings of the present study provide valuable insight into the role of semi‐natural habitats as a source of pests, patterns of crop colonization in the spring, and the influence of landscape on pollen beetle abundance.

     

Homogeneous detection of cyanobacterial DNA via polymerase chain reaction

Aims: To design a primer set enabling the identification through PCR of high‐quality DNA for routine and high‐throughput genomic screening of a diverse range of cyanobacteria. Methods and Results: A codon‐equivalent multiple alignment of the phycocyanin alpha‐subunit coding sequence (cpcA) of 22 cyanobacteria was generated and analysed to produce a single degeneracy primer set with virtually uniform product size. Also, an 18S ribosomal RNA detection set is proposed for rejecting false positives. The primer sets were tested against five diverse cyanobacteria, Chlorella vulgaris, Saccharomyces cerevisiae, and Escherichia coli. All five cyanobacteria showed positive amplification of cpcA product with homogeneous fragment length, and no products were observed for any other organism. Additionally, the only product formation observed for the 18S rRNA set was in C. vulgaris and S. cerevisiae. Conclusions: The newly proposed primer set served as effective check primers for cyanobacteria. Cyanobacteria gDNA had a positive, homogenous result, while other bacteria, eukaryotes and alga tested were negative. Significance and Impact of the Study: These novel, broad‐spectrum primers will greatly increase the utility of PCR on newly discovered cyanobacterial species.     

The microtubule plus end tracking protein Orbit/MAST/CLASP acts downstream of the tyrosine kinase Abl in mediating axon guidance

Axon guidance requires coordinated remodeling of actin and microtubule polymers. Using a genetic screen, we identified the microtubule-associated protein Orbit/MAST as a partner of the Abelson (Abl) tyrosine kinase. We find identical axon guidance phenotypes in orbit/MAST and Abl mutants at the midline, where the repellent Slit restricts axon crossing. Genetic interaction and epistasis assays indicate that Orbit/MAST mediates the action of Slit and its receptors, acting downstream of Abl. We find that Orbit/MAST protein localizes to Drosophila growth cones. Higher-resolution imaging of the Orbit/MAST ortholog CLASP in Xenopus growth cones suggests that this family of microtubule plus end tracking proteins identifies a subset of microtubules that probe the actin-rich peripheral growth cone domain, where guidance signals exert their initial influence on cytoskeletal organization. These and other data suggest a model where Abl acts as a central signaling node to coordinate actin and microtubule dynamics downstream of guidance receptors.     

Isolation and Characterization of a Galactosamine Wall from Spores and Spherules of Physarum polycephalum

The myxomycete, Physarum polycephalum, can be induced under laboratory conditions to form two different hard-walled forms, spores and spherules. Characterization of both types of walls revealed only a single sugar, galactosamine. It was identified after acid hydrolysis of the isolated walls by chromatography in three solvent systems, by its positive reaction with ammoniacal silver nitrate, ninhydrin, Galactostat, and the Elson-Morgan test, and by ninhydrin degradation to lyxose. Galactosamine was present as a polymer with solubility characteristics the same as the β1-4–linked glucosamine polymer (chitosan). The walls were also found to contain about 2% protein. Spherule walls revealed a single glycoprotein on gel electrophoresis. Spore walls contained a similar protein component. The phosphate content of isolated spherule walls was 9.8%, and that of spore walls was 1.4%. Spore walls also contained about 15% melanin which was shown to be similar to fungal melanin. A novel method was used to measure the rate of mature spherule formation based on the loss of extractability of P. polycephalum natural pigment. The presence of a rare galactosamine polymer in P. polycephalum spore and spherule walls as the only carbohydrate suggests that the myxomycetes are not closely related to the fungi or the protozoa.