Ecology of mycorrhizae: a conceptual framework for complex interactions among plants and fungi.

Mycorrhizae regulate elemental and energy flows in terrestrial ecosystems. We understand much of how mycorrhizae work, but integrating all possible mechanisms into a whole has proven elusive. Multiple evolutionary events and the long evolutionary history mean that different plants and fungi bring independent characteristics to the symbiosis. This variety results in extensive physiological variation. How do we integrate functional responses with diversity to understand the role of mycorrhizae in ecosystems? We review ecophysiological mechanisms of mycorrhizae and organize these into functional groups. Species-area relationships are not curvilinear, but resemble the "broken stick" model. We coupled functional groups with a metacommunity analysis to show how complex behavior can be generated using a simple matrix model of resource exchange. This approach provides insights into how we might integrate diversity and function across landscapes.

Reconstruction of the historical changes in mycorrhizal fungal communities under anthropogenic nitrogen deposition.

Archived soil samples (1937-1999) and historic air quality data from the Los Angeles Basin were used for reconstructing the record of change between atmospheric NO(x) loads, soil delta(15)N values and the diversity of arbuscular mycorrhizae (AM), which are ubiquitous plant-fungus mutualists that control plant community productivity. A tripling of atmospheric NO(x) loads between 1937 and the 1970s was paralleled by soil nitrogen enrichment (delta(15)N = 3.18). From 1975 onwards, atmospheric NO(x) declined, but soils became nitrogen saturated (delta(15) N = -4 and NO(3)-nitrogen = 171mgkg(-1)). The shifts in the AM community followed 28 years of atmospheric nitrogen enrichment and coincided with the onset of soil nitrogen saturation. Such changes were manifest in the loss of AM productivity, species richness (one species per year), three genera (Acaulospora, Scutellospora and Gigaspora) in the spore community and Gigaspora within the roots. Nitrogen enrichment also enhanced the proliferation of potentially less mutualistic species of Glomus. Autoregressive models implied that such patterns will persist and be driven by soil nitrogen cycling patterns. Chronic nitrogen enrichment from air pollution thus alters the diversity and mutualistic functioning of AM communities, which, in turn, may influence the plant community.

Isolation and structural identification of a germination inhibitor in fire-recruiters from the California chaparral.

A role for inhibitors as regulators of seed dormancy in fire-dependent annuals and their nondormant (fire-independent) congeners was examined in the family Hydrophyllaceae. From seeds of the fire-dependent species, Emmenanthe penduliflora, Phacelia minor, P. brachyloba, and P. grandiflora, extracts were obtained that were found to be largely self-inhibitory and potent inhibitors of seed germination in nondormant congeners. Lower activity was detected in the fire-independent species, P. tanacetifolia and P. campanularia. The inhibitory activity was associated with the new sucrose ester, 6-O-linoleyl-alpha-D-glucopyranosyl-beta-D-fructofuranoside.

Levels of mineral nutrients in fresh- and frozen bulk hydrated biological specimens: a comparison of EDS data collected in the environmental SEM and a conventional cryo-SEM.

Energy-dispersive X-ray microanalysis (EDS) was compared in fresh- and frozen bulk hydrated tissues using the Environmental SEM (ESEM) and conventional cryo-SEM, respectively. Analysis of globoid inclusions of Eucalyptus calophylla seed from two soil types demonstrated that higher levels of cations (K, Ca, Mg, Al, Fe, Mn, Cu, Zn) occurred in seeds from soils containing higher levels of Al, while EDS-detectable levels of S and P were dependent upon the techniques utilised. Cumulative changes in ESEM-EDS-detectable levels of S and P were characterized by collecting cumulative spectra from nutrient standards and compared with those for K. Progressive increases in K occurred and were consistent with an enriching effect. Levels of S and P increased during early analysis (40-60 sec live time) and decreased thereafter. The semi-conductive nature of biological samples, the loss of anions and gain of cations from the net negatively-charged electron interaction volume contributed to an electrochemical bias. These local modifications in fluid chemistry were reversible. Dehydration effects also occurred in stable, 'wet' samples. These differences indicated that EDS in ESEM may be limited to cations rather than anions, and that changes in fluid electrochemistry and dehydration may affect the level and distribution of elements.

Microanalytical studies of metal localization in biological tissues by environmental SEM.

The presence and distribution of Al and Mn in floral and seed tissues of eucalypts from Al-contaminated soils was analyzed using energy-dispersive X-ray microanalysis (EDS) in an environmental scanning electron microscope (ESEM). EDS by ESEM determined the distribution of elements between tissue types was suitable for intact samples or those with lower available moisture or intact specimens. The analytical technique was not appropriate for highly vascular samples. Other factors influencing the detection of elements by ESEM-EDS were electron scattering and the relative concentration and localization of elements within the tissues. EDS-detectable levels were significantly correlated with tissue concentrations determined by atomic absorption spectrophotometry for Mn but not for Al.