Phenotypic variability in different strains of Pseudomonas syringae subsp. savastanoi isolated from different hosts

One hundred and sixty strains of Pseudomonas syringae subsp. savastanoi from Olea europaea, Olea europaea var. sylvestris, Nerium oleander, Fraxinus angustifolia and Retama sphaerocarpa, and four type strains of other pathovars were studied, investigating 102 phenotypic traits, among which we include biochemical characteristics, assimilation of different carbon sources, sensitivity or resistance to antibiotics and indoleacetic acid (IAA) production. Results were analysed with an affinity dendrogram via the Jaccard coefficient. They indicate an influence of environmental factors on the formation of the 15 phenons obtained, since isolated (knot) strains from the same species but different geographical areas are segregated. Segregation, also detected in strains from different hosts within the same area, added to the pathogenicity test helps to characterise these strains as different pathovars.     

Microhabitat effects on herbaceous nutrient concentrations at the community and species level in Mediterranean open woodlands: the role of species composition

The presence of a woody canopy in open oak woodlands affects not only the nutrition but also the species composition of the herbaceous community. Yet, the contribution of both effects (changes in nutrient concentrations and species composition) to total resources that are captured by the herbaceous community is not well understood. We assessed the mineral nutrition (N, P, K and Ca) of three herbaceous species and the herbaceous community as well as the species composition in contrasting microhabitats (beneath trees, beneath a leguminous shrub and in open spaces). Both trees and shrubs increased the nutrient concentrations of the herbaceous species that were studied, except for Taraxacum officinale. Their effects were less consistent when the entire community was considered and depended upon the nutrient being analysed. Species richness and N, P and K concentrations were positively associated, suggesting that more nutrients are captured by the herbaceous community as diversity increases. Our results suggest a close relationship between species composition and nutritional value at the community level that may explain the discrepancies observed between both levels of analysis (species vs. community). Thus, farm-level strategies based on the whole herbaceous community may overlook processes operating at the species level, which can be relevant to achieve sustainable management.     

Molecular identification of arbuscular mycorrhizal fungal spores associated to the rhizosphere of Retama raetam in Tunisia

ABSTRACT

Arbuscular mycorrhizal fungi (AMF) are found in the soil of most ecosystems where they form mutualistic associations that affect plants growth. We have investigated the community structure of AMF associated to Retama raetam growing in five regions of Tunisia. The total number of spores was significantly different across sites, ranging from 633 to 1062 spores per 100 g dry soil. A dominance of small spores was revealed. The large subunit region of the rDNA of AMF spores associated to the rhizosphere of R. raetam was sequenced. Sequences clustered into 13 operational taxonomic units. Phylogenetic analysis revealed that the majority of sequences were grouped within Glomeraceae and Claroideoglomeraceae families. Only two sequences were affiliated to the Scutellospora genus. These results suggest the dominance of the genus Glomus in the soil rhizosphere of R. raetam. A correlation between phylogenetic analysis, soil chemicals properties, and AMF community richness was also detected.     

Changes in rhizosphere microbial activity mediated by native or allochthonous AM fungi in the reafforestation of a Mediterranean degraded environment

This study was carried out in a semiarid degraded area to assess the effectiveness of mycorrhizal inoculation with a mixture of native arbuscular mycorrhizal (AM) fungi or an allochthonous AM fungus (Glomus claroideum), on the establishment of Olea europaea subsp. sylvestris L. and Retama sphaerocarpa (L.) Boissier in this area. Associated changes in the soil microbiological properties and aggregate stability related to these AM inocula were also recorded. Eighteen months after planting, G. claroideum had increased available P in the rhizosphere of both shrub species. In general, both inoculation treatments increased water-soluble C and water-soluble and total carbohydrates, G. claroideum being the most effective inoculum, particularly in R. sphaerocarpa. The mixture of native AM fungi was the most effective treatment for increasing the aggregate stability of R. sphaerocarpa soil, while that of O. europaea was increased only by G. claroideum. Increased (dehydrogenase, urease, protease-BAA, acid phosphatase and -glucosidase) enzyme activities, in particular of dehydrogenase and acid phosphatase, were recorded in the rhizosphere of both mycorrhizal shrub species. The mixture of native AM fungi was the most effective treatment for stimulating the growth of O. europaea and R. sphaerocarpa (11.6-fold and 3.3-fold, respectively, greater than control plants). The establishment of mycorrhizal shrub species favoured the reactivation of soil microbial activity, which was linked to an increase in aggregate stability.     

Shrub species affect distinctively the functioning of scattered Quercus ilex trees in Mediterranean open woodlands

A low tree stand density has been showed as necessary to thrive with summer drought in semiarid Mediterranean open woodlands. Shrub encroachment of these open woodlands is currently recommended to guarantee the persistence of the system, due to the nursery effect of shrubs on tree seedling. However, the increase in abundance and cover of a shrub understory in these water limited woodlands could bring consequences to tree overstory functioning. The present study analyzes the physiological status of scattered Quercus ilex L. trees in paired adjacent plots with and without the presence of a shrubby understory in CW Spain. Two contrasting shrub strategies were addressed in order to take into account possible species-specific effects: a dense-shallow rooting shrub (Cistus landanifer L.) and a sparse-deep rooting shrub (Retama sphaerocarpa (L.) Boiss). Leaf water potential (at predawn and midday), leaf gas exchange parameters (net photosynthetic rate and stomatal conductance), leaf nitrogen content and chlorophyll fluorescence transients (maximum photochemical efficiency and performance index, sensuStrasser et al., 2004) were measured during three consecutive summers. Trees growing with Cistus as understory showed significant lower leaf water potential, leaf gas exchange parameters, leaf nitrogen content and chlorophyll photochemical efficiency than trees growing without shrub competence. However, the presence of the legume Retama did not affect significantly the physiological state of Q. ilex. Thus, we conclude that the presence of a shrubby understory has the potential to modify the functioning of scattered trees, but these effects are species-specific.     

Contributions of indigenous arbuscular mycorrhizal fungi to growth of retama monosperma and acacia gummifera under water stress (case study: essaouira sand dunes forest)

ABSTRACT

Two leguminous plant species: Retama monosperma and Acacia gummifera were selected for water deficit tolerance test for future use in reforestation programs of the Atlantic sand dunes forest of Essaouira region. Mycorrhizal fungal colonization was used to help plants amend the water deficit. To assess R. monosperma and A. gummifera response to water stress and the effectiveness of mycorrhizal formation alleviating it, two categories for each plant species were made, one inoculated with mycorrhizal fungi and the other not, were submitted to a hydric deficit treatment of four levels for 6 months. This treatment impact on the plants was assessed by comparing root/shoot dry weight ratios, levels of nitrogen and phosphorus, and by measuring mycorrhizal colonization frequencies and intensities. Results showed that these parameters were significantly affected by water deficit. The presence of the mycorrhizal fungi had significantly enhanced plants phosphorus uptake by 72–124% for Acacia gummifera and 87–161% for Retama monosperma. Nitrogen uptake was increased by 134–388% for Retama monosperma and 510–901% for Acacia gummifera. The colonization frequencies changed significantly only for the treatment extreme level and stayed between 80%-90% for the other levels. The mycorrhization intensities showed significant lessening between each treatment level decreasing from 35% to 10%.     

Plant type differently promote the arbuscular mycorrhizal fungi biodiversity in the rhizosphere after revegetation of a degraded, semiarid land

It is suggested that the diversity of arbuscular mycorrhizal fungi (AMF) and their association with distinct plants species are crucial in the early stages of revegetation procedures since the AMF roots colonisation plays an important role improving plant establishment and growth. We carried out a study where we analyse the AMF community composition in the roots of Ephedra fragilis, Rhamnus lycioides, Pistacia lentiscus and Retama sphaerocarpa fourteen months after revegetation in a Mediterranean semiarid degraded area of southeast Spain in order to verify whether different plant species can variably promote the diversity of AM fungi in their rhizospheres after planted. We analysed a portion of approximately 795 bases pairs of the small-subunit ribosomal DNA by means of nested PCR, cloning, sequencing and phylogenetic analyses. Eight fungal sequence types belonging to Glomus group A and B and to the genus Paraglomus were identified. The different plant species had different AM fungal community composition. Thus, R. lycioides harboured the highest number of four fungal sequence types while from E. fragilis only two types could be characterized that were specific for this plant species. P. lentiscus and R. sphaerocarpa harboured each one three sequence types and two of them were shared. All AMF sequence types were found in the natural soil. These results show that one effective way of restoring degraded lands is to increase the number of plant species used, which would increase the AMF diversity in the soil and thus the below-ground, positive interactions.     

Soil Microbial Biomass Under the Canopy of Coastal Sand Dune Shrubs

Coastal sand dunes are affected by seasonal fluctuations in salinity due to seawater spray and precipitation. Salinity was found to be very high in summer soil samples and was about 15-20 times lower in the preceding rainy winter. High microbial biomass values of 234.6 and 173.1 mugC g-1 dry soil were found under Artemisia monosperma and Retama raetam, respectively, even at elevated salinity. The metabolic quotient for CO2, which may serve as an indicator for changes in microbial diversity or environmental stress, was unexpectedly high (58.5 and 48.8 mug mg-1 d-1 under A. monosperma and R. raetam, respectively) in low-saline samples from the winter when compared to the summer (a maximal value of 20 mug mg-1 d-1), probably indicating a shift in community structure. It may be concluded that even under hostile conditions in poor sandy soils and with high salinity the presence of perennial shrubs creates favorable niches for microorganisms and affects their activity.     

Seasonality of mycorrhizal attributes,soil phosphorus and nitrogen of Juniperus phoenicea and Retama monosperma boiss. in an Atlantic sand dunes forest

Seasonal variation of mycorrhization attributes in a degraded ecosystem seems to be key information in conceiving rehabilitation programs that use mycorrhized plantations specific to that ecosystem. In order to record seasonality of arbuscular mycorrhizal fungi in an Atlantic sand dunes forest, root colonization and spores abundance alongside rhizospheric phosphorus and nitrogen of two plant species: Juniperus phoenicea and Retama monosperma, endemic to Essaouira sand dunes forest, were followed through three years. The findings were tested for statistical significance and correlated among each and to some meteorological variables: temperatures, precipitations, and humidity. Results showed that mycorrhization frequencies and intensities and spores densities changed seasonally. They had their peak at late winter-early spring and their low at summer. Phosphorus and nitrogen evolved significantly following seasons. Significant negative correlation was noted between phosphorus and mycorrhizal attributes evolution. Nitrogen correlated positively and significantly though weakly with R. monosperma mycorrhizal attributes. The correlations between mycorrhizal attributes and meteorological variables were not all evident except for temperatures and temperatures-precipitations combination, which had a direct negative impact on mycorrhizal attributes.     

Two new flavonoids from Retama raetam

Two new flavones were isolated from the aerial parts of Retama raetam subsp. raetam. Their structures were established as luteolin 4′-O-neohesperidoside (1) and 5,4′-dihydroxy-(3″,4″-dihydro-3″,4″-dihydroxy)-2″,2″-dimethylpyrano-(5″,6″:7,8)-flavone (2) by means of spectroscopic methods. Also present was ephedroidin (4′,5,7-trihydroxy-8-(2-hydroxy-3-butenyl)-flavone).