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.

The role of glutathione (GSH) in protecting plants from chilling injury was analyzed in seedlings of a chilling-tolerant maize (Zea mays L.) genotype using buthionine sulfoximine (BSO), a specific inhibitor of gamma-glutamylcysteine (gammaEC) synthetase, the first enzyme of GSH synthesis. At 25 degrees C, 1 mM BSO significantly increased cysteine and reduced GSH content and GSH reductase (GR: EC 1.6.4.2) activity, but interestingly affected neither fresh weight nor dry weight nor relative injury. Application of BSO up to 1 mM during chilling at 5 degrees C reduced the fresh and dry weights of shoots and roots and increased relative injury from 10 to almost 40%. Buthionine sulfoximine also induced a decrease in GR activity of 90 and 40% in roots and shoots, respectively. Addition of GSH or gammaEC together with BSO to the nutrient solution protected the seedlings from the BSO effect by increasing the levels of GSH and GR activity in roots and shoots. During chilling, the level of abscisic acid increased both in controls and BSO-treated seedlings and decreased after chilling in roots and shoots of the controls and in the roots of BSO-treated seedlings, but increased in their shoots. Taken together, our results show that BSO did not reduce chilling tolerance of the maize genotype analyzed by inhibiting abscisic acid accumulation but by establishing a low level of GSH, which also induced a decrease in GR activity.

Abstract. Maize plants (Zea mays L. cv. Honeycomb F-l) were grown on quartz sand containing amounts of Cd or Cu which resulted in comparable internal contents in the roots. Fresh and dry weights and the content of Cd or Cu were measured in roots and shoots after eight weeks. In addition, cysteine, y-glutamylcysteine (yEC), glutathione (GSH) and the thiols in heavy-metal-binding peptides (HMBPs) were determined in the roots.

SUMMARY The effect of cadmium on assimilatory sulphate reduction and thiol content was studied in non‐mycorrhizal and mycorrhizal Norway spruce seedlings (Picea abies) and its ectomycorrhtzal fungus Laccaria laccata. The distribution of cadmium was also investigated. Isotope dilution experiments indicated that the fungus reduced sulphate via adenosine 3′‐phosphate 5′‐phosphosulphate sulphotransferase, whereas Norway spruce seedlings assimilated sulphate via adenosine 5′‐phosphosulphate sulphotransferase in both roots and needles. In mycorrhizal roots only the plant sulphotransferase activity could be measured. Mycorrhizal and non‐mycorrhizal roots and the mycelium of Laccaria laccata contained increased activities of sulphotransferase and more acid‐soluble thiols when cultivated with cadmium. The increase in acid‐soluble thiols was due to phytochelatins in roots and to glutathione in Laccaria laccata, where neither phytochelatins nor metallothioneins could be detected. Even though the cadmium content of mycorrhizal roots was slightly higher than that of non‐mycorrhizal roots, concentrations of phytochelatin were only half as high as in non‐mycorrhizal roots. Cadmium content of needles of mycorrhizal plants was significantly lower than that of non‐mycorrhizal plants. Most of the cadmium in Laccaria laccata was associated with the cell walls and could be exchanged with Ni2+.

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