Comparative responses of microscopic fungi to ionizing radiation and light.

Microscopic fungi of 47 species and 24 genera isolated from radioactive soil and other radioactive substrates of a 10-km alienation zone from Chernobyl Nuclear Power Plant were investigated. Only 19 % of such fungi showed toward the ionizing irradiation source positive radiotropism. Strains with best-defended radiotropism responded to the influence of weak white light. Both of these responses (radiotropism and photostimulation) were of an adaptive nature, were found in both dark-colored and light-colored micromycetes and were probably a strain-related feature.

Atmospheric pollutants and ectomycorrhizae: more questions than answers?

Changes in research emphasis from pollution towards the effects of climate change have resulted from shifts in political and public interests. Information gained from research on the effects of pollutants as a perturbation on mycorrhizae and mycorrhizae-mediated processes may be an appropriate starting point from which to launch into climate change studies. However, the effects of pollution on mycorrhizae are not clear cut as there are many conflicting results. The methods used in experiments, levels of pollutants used and relationships between studies on seedling trees to the mature forest do not suggest unifying models of pollution effects. Some areas of work still lacking in the pollution field are highlighted. These include well replicated experiments at currently ambient pollutant levels, and areas for potential development in the global climate change arena, e.g. the interaction between C supply and nutrient uptake.

Foliar leaching of 137Cs from Eriophorum vaginatum L., Scirpus caespitosus L. and Erica tetralix L.

Evidence of extensive leaching losses of nutrients, particularly of K, suggest that loss of 137Cs by foliar leaching could be considerable and could stimulate further root uptake and redistribution of 137Cs in plants. This study investigated the foliar leaching of 137Cs from two deciduous graminoid species, Eriophorum vaginatum and Scirpus caespitosus and one evergreen shrub, Erica tetralix. Plants were labelled with 137Cs via the roots and subjected to a leaching treatment in August and November and changes in both leachate and plant 137Cs activity were determined. Leaching losses were significantly reduced in November compared with August in the deciduous species, but not in the evergreen E. tetralix. A reduction in the total activity of 137Cs of leached plants was observed not only in leaves but also in stems and roots in some instances, suggesting that 137Cs from these organs had been redistributed to replenish that lost by leaching from the leaves. The data suggest that leaching losses were greater from older and senescent leaves than from younger leaves of E. vaginatum. The extent to which this is an accurate representation of foliar leaching of field-grown plants by rainfall, and the likely fate of 137Cs lost by foliar leaching are discussed.

The distribution of metals in the forest floor of aged conifer stands at a plantation in Northern England.

The distribution of the elements Cd, Cr, Cu, Mn, Ni, Pb, Sr, V and Zn has been examined in the horizons of soils under aged Sitka spruce (Picea sitchensis (Bong.) Carr.) stands at a plantation in Northern England. The stands are under first-generation cultivation and are up to 33 years old. Cadmium, Mn, Pb and Zn concentrations were consistently higher in the organic layers than in the underlying mineral soil. This contrasted with the situation for Sr and V. Cadmium, Pb and Zn all showed an increase in concentration in the L + F horizons with stand age and a corresponding increase in the difference between L + F horizon concentrations. Soil pH declined with increasing stand age. Cadmium, Cr, Pb and Zn were all present at higher concentrations in the F horizon than in any other, while Cu and Ni were relatively constant through all the horizons studied. For all nine elements, the H horizon was the largest store of the three organic layers. Calculated rates of accumulation of Cd, Pb and Zn in the L + F horizons gave good agreement with estimated regional atmospheric deposition rates. In comparison to atmospheric deposition, biological mobilisation and deposition of Cd, Pb and Zn make a relatively minor contribution to the surface soil metal burden. Cadmium appeared to be the most readily leached of these three metals from the forest floor, although some transfer of atmospherically-derived Pb to the H+ soil horizons was indicated.

Effects of species diversity on establishment and coexistence: a phylloplane fungal community model system.

A model system was devised, evaluating the influence that species diversity (species richness) has on fungal establishment and coexistence. Seven members of the fungal phylloplane community of Vaccinium macrocarpon (American cranberry) were selected to assess how species diversity affected development and coexistence of another community member, Pestalotia vaccinii. Pestalotia was engaged in competitive interactions on 1% Malt Extract Agar (MEA) petri dishes with each of the seven individual saprotrophs (two-way interaction), in random combinations with three of the seven saprotrophs (four-way interaction), and in random combinations with five of the seven saprotrophs (six-way interaction). The saprotrophic fungi used in this study were Aspergillus sp., Alternaria alternata, Cladosporium cladosporoides, Curvularia lunata, Epicoccum purpuracens, Penicillium sp., and Pithomyces chartarum. We hypothesized that species diversity would have a significant impact on the establishment and coexistence of Pestalotia vaccinii in culture. In an effort to minimize density-dependent effects, the number of viable spores employed in the three types of interactions was kept constant. Target spore concentrations of 50 viable spores of P. vaccinii and 50 saprotroph spores were used, regardless of the number of species involved in the interaction. This proved to be a very important factor in the experiment. As our results show, species diversity had little or no effect on the establishment and coexistence of Pestalotia vaccinii; however, spore density played an extremely important role in the establishment and development of fungal propagules in our model.

Successions of sheathing mycorrhizal fungi.

Many fungi capable of forming sheathing (ecto-) mycorrhizas are associated with trees. But what are the rules governing their occurrence? Evidence from first generation woodlands/forests, where trees grow on sites that have been treeless for many years, indicates that species of fungi occurring at early stages of woodland development ('early-stage fungi') are, in due course, superseded by others ('late-stage fungi'). There is an ordered succession which seems to reflect, in large measure, the different abilities of early- and late-stage fungi to form mycorrhizas on roots growing in soils with accumulations of recalcitrant leaf litter. In second generation woodlands/forests there seems to be little evidence of early-stage fungi. This is not so surprising as it may seem, bearing in mind that their soils are already likely to have accumulations of recalcitrant litter. Instead of classifying sheathing mycorrhizal fungi by referring to the temporal stage of woodland development, it now seems more meaningful to judge them by their abilities to colonize roots in soils with or without accumulations of different types of litter.

The physiology of basidiomycete linear organs: III. Uptake and translocation of radiocaesium within differentiated mycelia of Armillaria spp. growing in microcosms and in the field.

Autoradiography and quantitative image analysis were used to measure (137) Cs distribution and translocation through mycelia of A. gallica Marxmuller & Romagn. and A. ostoyae (Romagn.) Herink growing in small microcosms in the laboratory. Rhizomorphs of A. gallica were labelled with (134) Cs in the field, and the translation of radiolabel measured after excavation and destructive harvesting. The flux of radiocaesium through rhizomorphs was estimated to be greater than through undifferentiated hyphae, and greater through rhizomorphs growing in the field than through rhizomorphs grown across homogeneous media in the laboratory'. Differentiation within mycelia resulting in melanization or rhizomorph formation cave rise to increased heterogeneity in the (137) Cs distribution through laboratory microcosms compared with that through microcosms containing undifferentiated mycelia. Radiocaesium leaked from undifferentiated hyphae into the surrounding medium, but melanized regions of the mycelium appeared to be conservative for radiocaesium. These findings provide further evidence for the importance of filamentous fungi in determining the distribution and rate of release of radiocaesium currently present in the environment as a result of weapons testing and the accident at the Chernobyl nuclear reactor.