Ecto‐ and endomycorrhizal symbiosis can play a crucial role in protecting plant roots from heavy metals (HMs). The efficiency of protection, however, differs between distinct isolates of mycorrhizal fungi and different HMs. Fungal ecotypes from HM‐contaminated sites seem to be more tolerant to HMs than reference strains from non‐contaminated sites. The abundance of the extramatrical mycelium was shown to he important for HM binding by the fungus. Most of the HMs were demonstrated to be bound to cell wall components like chitin, cellulose. cellulose derivatives and mela‐nins. The chemical nature of HM‐binding substances in the fungal cells is not clear. Polyphosphate granules, which were proposed to have this function, seem to be artifacts of specimen preparation. The high N and S concentrations associated with the polyphosphate granules rather indicate the occurrence of HM‐thiolate hinding by metallothionein‐like peptides.

SUMMARYThe objective of this study was to examine the ability of arbuscular mycorrhizal (AM) fungi to take up nitrogen from soil and transport it to the host plant. Maize {Zea mays L.) associated with Glomus intraradices Schenck and Smith or left uninoculated was grown in containers which were divided by a nylon net into a root compartment and a hyphal compartment. A 40 /im pore size nylon net was used to exclude plant roots while allowing fungal hyphae to grow into soil confined by the net. ^^N tracer was supplied either as inorganic N or as organic N to the hyphal compartment at a distance of 5 cm from the net.Inoculation with the AM fungus increased the '^N content of maize compared to the non-mycorrhizal controls when N was applied as (^^NHJ^SO^. However, there was no conclusive evidence that AM hyphae could derive N from organic "N sources. Most of the increased N uptake of mycorrhizal plants occurred by hyphal translocation from the hyphal compartment to the root compartment. Higher N uptake by mycorrhizal plants with access to the hyphal compartment was indicated by depletion of total ^'N in the soil of that compartment. Cutting the extraradical hyphae in the hyphal compartment in order to sever their connection with the host roots decreased the ^*N uptake of the maize plants. A time-course study with inorganic ^^N over 26 d showed that G. intraradices transported most of the '°N between 10 and 15 d after ^'N application, whereas the non-mycorrhizal control plants had a consistently low concentration of ^'N throughout the period of sampling.Nitrogen transport by external hyphae of three AM fungi, G. intraradices, Acaulospora laevis Gerdemann and Trappe and Gigaspora margarita Becker and Hall associated with maize, was further investigated. The results indicated that different isolates of AM fungi differ in the efficiency of hyphal N transport as a consequence of the different patterns of hyphal spread in the soil or of the different capacity for uptake by unit length of hyphae.

SUMMARYIn order to examine wbetber bypbae of VA mycorrbizal fungi may aid tbe transfer of symbiotically fixed nitrogen from a legume to a non-legume, tbe roots of berseem plants inoculated with the VAM fungus Glomus intraradices and Rhizobium leguminosarum biovar. trifolii (strain RCR 5) were separated by a 2 cm root-free zone from tbe roots of maize plants. A 40 (xm pore size nylon net allowed VAM bypbae to pass tbrougb tbe root-free zone but prevented root penetration. Replacement of tbis net by a 0-45 ixm pore size membrane prevented penetration of VAM bypbae to tbe root-free compartment. Wben tbe berseem plants were 63 d old, tbe plant chambers were sealed in plastic bags to allow tbe roots of tbe plants to be exposed to a ^"N2-enriched atmospbere for 5 d.Berseem plants tbat were exposed to a ^''Ng-enricbed atmospbere fixed more ^^N tban did berseem plants maintained under air at natural ^"N abundance. Tbe total '^N excess content of berseem was similar wbetber or not tbe mycorrhiza bad access to tbe maize roots. Plant mass (dry weight) of maize was not affected by tbe mycorrbizal fungus, but total nitrogen content tended to be bigber in VAM-colonized maize tban in non-colonized maize. Furtbermore, botb atom % ^^N excess and ^^N excess content of mycorrbizal maize were significantly increased as compared witb non-mycorrbizal maize or witb maize controls witbout berseem. Tbe amounts of ^^N transferred were small, accounting for less tban 4% of tbe ^^N in tbe Ng-fixing plant.In spite of tbe 2 cm root-free separating zone, tbe presence of tbe mycorrbizal fungus in tbe non-legume increased tbe transfer of symbiotically fixed nitrogen from berseem to maize plants tbat were physiologically younger. Tbe implications of tbese results for nitrogen transfer from a legume to an accompanying non-legume via VAM bypbae are discussed.

To study the response of non-mycorrhizal and mycorrhizal maize plants to drought, the changes in the pools of non-structural carbohydrates and amino acids were analysed in leaves and roots of two maize cvs. Plants well colonized by the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. & Gerd.) (60 % of root length infected) and comparable non-mycorrhizal plants were subjected to moderate drought stress by reducing the water supply. This stress induced a conspicuous increase in the trehalose pool in the mycorrhizal roots, probably because it was accumulated by the fungal symbiont. Furthermore, glucose and fructose were accumulated in leaves and roots of non-mycorrhizal plants but not in the mycorrhizal ones. Starch disappeared completely from the leaves of both mycorrhizal and non-mycorrhizal plants in response to drought. Activities of soluble acid invertase and trehalase were also measured. Acid invertase activity increased during drought in the leaves of both non-mycorrhizal and mycorrhizal plants whilst in the roots it was unaffected in non-mycorrhizal plants and decreased in the mycorrhizal ones. Without drought stress, trehalase activity was considerably higher in the leaves and roots of mycorrhizal plants than in those of non-mycorrhizal plants. It increased conspicuously during drought, primarily in the leaves of non-mycorrhizal plants. A drought-induced accumulation of amino acids as well as imino acids was found in roots and leaves of both mycorrhizal and non-mycorrhizal plants ; leaves of mycorrhizal plants accumulated more imino acids than those of non-mycorrhizal ones. Our results show that drought stress and the presence of a mycorrhizal fungus have a considerable effect on carbon partitioning, imino acid and amino acid accumulation in maize plants.

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