Twenty-one isolates of the ectomycorrhizal fungus Suillus luteus were screened for their tolerance to the heavy metals Zn, Cd, Cu and Ni, measured as inhibition of radial growth and biomass production. Two populations from even-aged pine stands were investigated: 10 isolates were obtained from an area polluted with high levels of Zn, Cd and Cu, and 11 isolates were obtained from a control population located in a nearby unpolluted area. RFLP patterns of the internal transcribed spacer region of the isolates confirmed the morphological identification of the carpophores. All isolates were maintained on basic medium without elevated metals to avoid phenotypically acquired metal tolerance. The in vitro Zn and Cd tolerance of the S. luteus isolates from the polluted habitat were significantly higher than the tolerances measured in the isolates from the nonpolluted site. This observation suggests that the elevated soil metal concentrations might be responsible for the evolution of adaptive Zn and Cd tolerance. Tolerance was maintained in an isolate not exposed to elevated metals for 3 yr. The two S. luteus populations did not differ in tolerance to Cu and Ni. The mechanisms for the adaptive Zn and Cd tolerance are not identical as there was no correlation between response to the two metals; the most Zn-tolerant isolate was the most sensitive for Cd in the metal-tolerant population. Zinc did not accumulate in basidiocarp tissue, whereas Cd levels in basidiocarps were significantly higher in the population on the polluted site. Inter-simple sequence-repeat fingerprints showed that 90% of the isolates were from different individuals. The genetic variation in the population from the unpolluted site was considerably larger than that observed at the polluted site.

The activities of nitrate reductase and glutamine synthetase were evaluated in young plants of Faidherbia albida, a tropical woody legume, fed with different N sources under hydroponic conditions. Results showed that assimilation of both NO3− and NH4+ preferentially took place in shoots. A basal amount of nitrate reductase activity was detected in shoots of plants grown with an NO3−-free solution or placed under N2-fixing conditions, and also in nodules of N2-fixing plants. This strongly suggests that constitutive nitrate reductase activity is present in these organs. Analyses of the soluble nitrogenous content showed that the major form of N in the different organs was α-amino acids (particularly amides), irrespective of the N status of the culture conditions. The same result was obtained for nodulated plants grown in local sandy soil. In this case, amide-N generally accounted for more than 40% of the total soluble N. This was especially true in nodules. Ureide-N never exceeded 9% of the total soluble N and did not appear to increase with increasing nodule nitrogenase activity. Amides were also predominant in three N2-fixing Sahelian acacias (Acacia seyal, A. nilotica and A. tortilis), showing that F. albida does not differ from Sahelian Acacia in terms of the metabolism of fixed N. However, like another Sahelian acacia growing preferentially near water (A. nilotica), F. albida can be distinguished from acacias growing strictly in arid zones (A. seyal and A. tortilis) in terms of initial growth, water and nitrate management.

There has been conflicting debate over many years regarding the trophic status of Matsutake. Here we examined the morphology, structure and ultrastructure of Pinus densiflora roots collected from a Tricholoma matsutake Shiro within a pure Japanese red pine stand. Molecular investigations (PCR–RFLP analyses) indicated that T. matsutake was the highly dominant fungus within both the Shiro and the colonized root tips, suggesting that reported root morphology modifications can be attributed to T. matsutake infection. The external morphology of Matsutake-colonized roots is consistent with previous descriptions. The presence of extraradical mycelium, mantle, and intracortical Hartig net hyphae indicates clearly that T. matsutake forms an ectomycorrhizal association with P. densiflora in naturally occurring Shiros. The elucidation, for the first time, of the Hartig net ultrastructure at the host–fungus interface provides further and convincing evidence of a conventional ectomycorrhizal association. The progressive blackening, observed from base to tip in dominant mycorrhizal types, due to increased deposition of polyphenol and subsequent necrosis, appears to be a result of infection. However, the presence of highly nucleated vascular tissue indicates the viability of the vascular cylinder in these roots bearing necrotic cortices. Such a preponderance of black necrotic cortical tissues among colonized roots may reflect some atypical behaviour of T. matsutake.

Arbuscular mycorrhizal (AM) symbioses are a potentially important link in the chain of response of ecosystems to elevated atmospheric [CO2]. By promoting plant phosphorus uptake and acting as a sink for plant carbon, they can alleviate photosynthetic down-regulation. Because hyphal turnover is likely to be fast, especially in warmer soils, they can also act as a rapid pathway for the return of carbon to the atmosphere. However, most experiments on AM responses to [CO2] have failed to take into account the difference in growth of mycorrhizal and non- mycorrhizal plants; those that have done so suggest that AM colonization of roots is little altered by [CO2], although this issue remains to be resolved. Very little is known about the effects of other factors of global environmental change on mycorrhizas. These issues need urgent attention. It is also necessary to understand the potential for the various AM fungal taxa to respond differentially to environmental changes, including carbon supply and soil temperature and moisture, especially because of the differential abilities of plant and fungal species to migrate in response to changing environments. Indeed, there is a need for a new approach to the study of mycorrhizal associations, which has been too plant-centred. It is essential to regard the fungus as an organism itself, and to understand its biology both as an entity and as part of a symbiosis.

The photobiont ultrastructure of the epiphytic lichens Bryoria fuscescens and Bryoria fremontii was studied along the pollution gradient from two Cu-Ni smelters in Nikel and Monchegorsk in northern Finland and north-western Russia. The relationship between ultrastructural characteristics of B. fuscescens and environmental factors (i.e. climate, atmospheric SO2 and bark element concentrations) was studied by using a principal component analysis (PCA) aiming to assess the air quality in a northern environment. Based on PCA, increased plasmolysis and mitochondrial changes in the Trebouxia photobiont were significantly correlated with elevated pollutant concentrations. Degenerated cells, showing altered chloroplasts and electron-translucent pyrenoglobuli, occurred in lichens growing 35–50 km from the Monchegorsk smelter. Cell wall and cytoplasmic lipid volumes, and size of pyrenoglobuli, positively correlated with the distance from the Monchegorsk smelter. Vacuoles and electron-opaque vacuolar deposits were significantly increased at the Finnish site in the vicinity of a pulp mill. Swelling of mitochondrial cristae and thylakoids showed little correlation with environmental factors, but indicated of initial stage of injuries and were observed at several slightly polluted sites in northern Finland and north-western Russia. The results suggest that the severe photobiont injuries of lichens are strongly associated with poor air quality.

Accumulation of lead in the crustose lichen Acarospora smaragdula sensu lato is reported in the vicinity of an ore- processing plant where it is subjected to acidification and metal particulate fallout. A combination of light microscopy, X-ray element mapping, field emission scanning electron microscopy (FESEM) and other analytical techniques identifies Pb accumulation within specific fungal tissues derived from smelter particles (PM10s). No Pb was detected within the photobiont layer. Our studies suggest that Pb is highly mobile under the prevailing acidic conditions, and is fixed within the lichen cortex and melanized apothecia. Lead is also accumulated within the medulla and at the rock–lichen interface where it may precipitate as amorphous botryoidal encrustations on medullary hyphae and iron-rich particles. Modern FESEMs and microprobes enable analysis of minute quantities of material, and are important tools in understanding the fate of metals within lichens necessary to develop their use as predictive and sensitive bioindicators of aerial particulate contaminants. We suggest that crustose lichens, hitherto largely ignored in metal pollution studies, may be useful bioindicators of aerial particulate contaminants in polluted areas where macrolichens are absent.

The morphology and anatomy of ectomycorrhizas of Rhizopogon parksii, Rhizopogon vinicolor and Rhizopogon subcaerulescens, and a hypogeous relative, Truncocolumella citrina, synthesized on Douglas fir in glasshouse conditions using spore slurries as inoculum, are described and compared. Mycorrhizas formed with R. parksii and R. vinicolor did not exhibit their characteristic subtuberculate morphology in these tests, but rather had a pinnate form. All species had diagnostic features of ectomycorrhizas: a well-developed Hartig net and a fungal mantle. In addition, several species exhibited crystal inclusions in the outer mantle, usually at the interface between the mantle and soil. Truncocolumella citrina had crystal-like inclusions within the mantle but external to fungal hyphae, a feature rarely described in ectomycorrhizas.

In this review, we discuss the potential for mycorrhizal fungi to act as a source or sink for carbon (C) under elevated CO2 and nitrogen deposition. Mycorrhizal tissue has been estimated to comprise a significant fraction of soil organic matter and below-ground biomass in a range of systems. The current body of literature indicates that in many systems exposed to elevated CO2, mycorrhizal fungi might sequester increased amounts of C in living, dead and residual hyphal biomass in the soil. Through this process, the fungi might serve as a negative feedback on the rise in atmospheric CO2 levels caused by fossil fuel burning and deforestation. By contrast, a few preliminary studies suggest that N deposition might increase turnover rates of fungal tissue and negate CO2 effects on hyphal biomass. If these latter responses are consistent among ecosystems, C storage in hyphae might decline in habitats surrounding agricultural and urban areas. When N additions occur without CO2 enrichment, effects on mycorrhizal growth are inconsistent. We note that analyses of hyphal decomposition under elevated CO2 and N additions are extremely sparse but are critical in our understanding of the impact of global change on the cycling of mycorrhizal C. Finally, shifts in the community composition of arbuscular and ectomycorrhizal fungi with increasing CO2 or N availability are frequently documented. Since mycorrhizal groups vary in growth rate and tissue quality, these changes in species assemblages could produce unforeseeable impacts on the productivity, survivorship, or decomposition of mycorrhizal biomass.

There is considerable uncertainty about how rates of soil carbon (C) and nitrogen (N) cycling will change as CO2 accumulates in the Earth's atmosphere. We summarized data from 47 published reports on soil C and N cycling under elevated CO2 in an attempt to generalize whether rates will increase, decrease, or not change. Our synthesis centres on changes in soil respiration, microbial respiration, microbial biomass, gross N mineralization, microbial immobilization and net N mineralization, because these pools and processes represent important control points for the below-ground flow of C and N. To determine whether differences in C allocation between plant life forms influence soil C and N cycling in a predictable manner, we summarized responses beneath graminoid, herbaceous and woody plants grown under ambient and elevated atmospheric CO2. The below-ground pools and processes that we summarized are characterized by a high degree of variability (coefficient of variation 80–800%), making generalizations within and between plant life forms difficult. With few exceptions, rates of soil and microbial respiration were more rapid under elevated CO2, indicating that (1) greater plant growth under elevated CO2 enhanced the amount of C entering the soil, and (2) additional substrate was being metabolized by soil microorganisms. However, microbial biomass, gross N mineralization, microbial immobilization and net N mineralization are characterized by large increases and declines under elevated CO2, contributing to a high degree of variability within and between plant life forms. From this analysis we conclude that there are insufficient data to predict how microbial activity and rates of soil C and N cycling will change as the atmospheric CO2 concentration continues to rise. We argue that current gaps in our understanding of fine-root biology limit our ability to predict the response of soil microorganisms to rising atmospheric CO2, and that understanding differences in fine-root longevity and biochemistry between plant species are necessary for developing a predictive model of soil C and N cycling under elevated CO2.

Plant species that persist during succession, from the colonization to the stabilization stages, face major environmental changes. Such changes are believed to have significant effects on species performance. In subarctic coastal dune systems, Leymus mollis colonizes the embryo dunes, on the upper limit of the beach. It reaches its maximum density on the foredune, but also grows on older, stabilized ridges. This paper reports on the phenotypic variations of some ecophysiological traits associated with the persistence of L. mollis on a dune system on the east coast of Hudson Bay (northern Quebec). Leymus mollis ramets tend to have a lower net carbon assimilation rate and water use efficiency, and a higher substomatal CO2 concentration on the stabilized dune than on the foredune. However, these physiological differences cannot be explained by differences in leaf morphology or nitrogen content. Under controlled conditions, ecophysiological differences observed in the field disappear, suggesting that these are not genetic but determined by environmental changes along the foredune-stabilized dune gradient. We propose that higher net carbon assimilation rate on the foredune might be related to higher sink strength in relation to the growth-stimulating effect of sand burial.

Three ectomycorrhizal legume trees, Microberlinia bisulcata, Tetraberlinia bifoliolata and T. moreliana, form discrete groves in the southern part of Korup National Park, in southwest Cameroon and contribute c. 45–70% of stand basal area locally in a matrix of otherwise species-rich arbuscular mycorrhizal forest. A transplant experiment was performed to assess the importance of ectomycorrhizal infection associated with proximity to parents in seedling establishment of the grove-forming species. Nonectomycorrhizal seedlings of the three species were transplanted into plots of two forest types, one of high (HEM, within-grove) and one of very low (LEM, outside the grove) abundance of all three species as adult trees. For two species (T. moreliana and M. bisulcata) there was no difference in survival over 16 months, but for the third (T. bifoliolata) survival was best in HEM forest, and correlated with the basal area of adult trees of ectomycorrhizal species. Only one species (T. moreliana) increased in biomass over the experimental period; the others declined. There was no effect of forest type on overall growth of any species, but the survivors of two (T. moreliana and M. bisulcata) had heavier stems in the HEM forest. Differences in survival and growth of transplants between the three species were in accord with the ecology of the species as inferred from the frequency distributions of adult tree size in the forest. Seedlings became infected with ectomycorrhizas in both forest types; where there was a difference in extent of infection (T. moreliana) this was not related to survival or growth; and where there was a difference in survival (T. bifoliolata) this was not related to extent of infection. These results confirm that mycorrhizal inoculum associated with conspecific adults is neither a prerequisite nor a guarantee of seedling establishment, but indicates that in some circumstances there might be benefits of being close to parents. Further research is required to unravel the complexities of ectomycorrhizal community structure in this spatially and temporally heterogeneous forest, and to clarify the extent to which the various hosts share ectomycorrhizal partners.

The representation of root activity in models is here confined to considerations of applications assessing the impacts of changes in climate or atmospheric [CO2]. Approaches to modelling roots can be classified into four major types: models in which roots are not considered, models in which there is an interplay between only selected above-ground and below-ground processes, models in which growth allocation to all parts of the plants depends on the availability and matching of the capture of external resources, and models with explicit treatments of root growth, architecture and resource capture. All models seem effective in describing the major root activities of water and nutrient uptake, because these processes are highly correlated, particularly at large scales and with slow or equilibrium dynamics. Allocation models can be effective in providing a deeper, perhaps contrary, understanding of the dynamic underpinning to observations made only above ground. The complex and explicit treatment of roots can be achieved only in small-scale highly studied systems because of the requirements for many initialized variables to run the models.

Very little is known about the physiological interactions between plant hosts and symptomless endophytic fungi despite their widespread occurrence. We investigated the impact of two such fungi, Colletotrichum musae and Fusarium moniliforme, upon the photosynthetic capacity of two crop plants, banana and maize, respectively. Endophyte-free plants were obtained first and then infected with the fungi. Measurements of total chlorophyll content revealed very little difference between endophyte-free and infected plants of banana, whereas in maize they showed 50% reductions in the endophyte-infected plants. The maximum photochemical capacity (Fv/Fm ) was measured in order to determine if the plants had any photoinhibitory effect caused by biotic or abiotic factors. After 45 d of growth, endophyte-free banana plants had similar values of Fv/Fm to plants typical of nonstressed conditions, whereas the endophyte-infected plants showed a reduction of approx. 15%. Unlike banana, infected maize plants displayed values of Fv/Fm similar to those of control and endophyte-free plants, indicating that the maximum photochemical capacity was not affected by infection. The light response curves of both species showed that the photosynthethic capacity was severely reduced in endophyte-infected plants, reaching saturation at c. 400 μmol m−2s−1 whereas the control and endophyte-free plants were saturated at much higher photon flux densities. In banana the effect seemed to be due to an impairment of electron transport in the thylakoid membranes. By contrast, reduction of the photosynthetic capacity in maize was due to a reduction in chlorophyll content, leading to a decrease in the electron transport components and a consequent reduction in carbohydrate synthesis. It is possible that the reduction in the maximum yield of photosynthesis in both crops was caused by toxins produced by the fungi. Nevertheless there were no major macroscopic effects on the plants to indicate disease symptoms.

New Phytologist147 (2000), 73–85

In the July 2000 issue of New Phytologist, we published the research paper entitled ‘Dynamics of root systems in native grasslands: effects of elevated atmospheric CO2’ by J. A. Arnone III, J. G. Zaller, E. M. Spehn, P. A. Niklaus, C. E. Wells and C. Körner. Since its publication, it has come to our attention that there are two important errors. In Table 1, for alpine grassland (Switzerland, 2480 m) in year 2 (cool), the observed soil depth should be given as 0–10 cm (as originally printed there is no soil depth given). In Table 2, the period covered should be 28 April 1994 to 6 April 1995, not 28 April 1994 to 6 April 1996, as erroneously stated.

We apologize to the authors and to our readers for these mistakes.

A survey of the endophytic fungi in fronds of Livistona chinensis was carried out in Hong Kong. The endophyte assemblages identified using morphological characters consisted of 16 named species and 19 ‘morphospecies’, the latter grouped based on cultural morphology and growth rates. Arrangement of taxa into morphospecies does not reflect species phylogeny, and therefore selected morphospecies were further identified based on ribosomal DNA (rDNA) sequence analysis. The 5.8S gene and flanking internal transcribed spacers (ITS1 and ITS2) regions of rDNA from 19 representative morphospecies were amplified by the polymerase chain reaction and sequenced. Phylogenetic analysis based on 5.8S gene sequences showed that these morphospecies were filamentous Ascomycota, belonging in the Loculoascomycetes and Pyrenomycetes. Further identification was conducted by means of sequence comparison and phylogenetic analysis of both the ITS and 5.8S regions. Results showed that MS704 belonged to the genus Diaporthe and its anamorph Phomopsis of the Valsaceae. MS594 was inferred to be Mycosphaerella and its anamorph Cladosporium of the Mycosphaerellaceae. MS339, MS366, MS370, MS395, MS1033, MS1083 and MS1092 were placed in the genus Xylaria of the Xylariaceae. MS194, MS375 and MS1028 were close to the Clypeosphaeriaceae. MS191 and MS316 were closely related to the Pleosporaceae within the Dothideales. The other 5 morphospecies, MS786, MS1043, MS1065, MS1076 and MS1095, probably belong in the Xylariales. The value of using DNA sequence analysis in the identification of endophytes is discussed.

Measurements of the electric potential difference across the hyphal wall and the cell membrane were made on external hyphae of three species of arbuscular mycorrhizal fungus Gigaspora margarita, Scutellospora calospora and Glomus coronatum and on germ tubes of Gi. margarita. The values of transmembrane electric potential difference recorded (∼ –40 mV) are less negative than those previously reported from hyphae of arbuscular mycorrhizal fungi closely associated with roots and from filamentous fungi. The external hyphae of arbuscular mycorrhizal fungi grown in soil had similar values of electric potential difference to those grown in soil-less culture, and to germ tubes. Thermodynamic calculations showed that despite these low values of electric potential difference, efficient high-affinity uptake of phosphate is possible. The transmembrane electric potential difference of germ tubes of Gi. margarita became more negative when plant root extract was added to the medium, showing for the first time that the early stages of interaction between plant and fungus occur via direct effects on the plasma membrane rather than via effects on gene expression. Addition of K+ reversibly depolarized the transmembrane electric potential difference of germ tubes of Gi. margarita, indicating that despite the low electric potential difference the fungus has control over the permeability of the plasmamembrane to K+.