Ectomycorrhizas and cadmium toxicity in Norway spruce seedlings

We studied the effects of ectomycorrhizal colonization by Laccaria bicolor (Maire) Orton S238 and Paxillus involutus (Batsch) Fr. 533 on cadmium (Cd) toxicity in Norway spruce seedlings (Picea abies (L.) Karst.). Both mycorrhizal and nonmycorrhizal seedlings were exposed to 0 (control), 0.5 or 5 &mgr;M CdSO(4) for 9 weeks in a sand culture system with frequent addition of nutrient solutions. In pure culture, P. involutus and L. bicolor showed similar Cd tolerance. However, in symbiosis, the Cd treatments decreased colonization by L. bicolor but not by P. involutus. Paxillus involutus ameliorated the negative effects of 0.5 &mgr;M Cd on shoot and root growth and chlorophyll content of old needles, whereas L. bicolor did not. Mycorrhizal colonization did not affect Cd concentrations of old needles and roots of seedlings. Despite differences between the ectomycorrhizal fungi in colonization and ability to alleviate Cd toxicity of seedlings, both species reduced Cd concentrations of young needles to a similar degree compared with nonmycorrhizal seedlings. However, in the 0.5 &mgr;M Cd treatment, the Cd content of needles of seedlings colonized by P. involutus was increased, whereas the Cd content of needles of seedlings colonized by L. bicolor was similar to that of needles of nonmycorrhizal seedings. When the amount of Cd translocated to needles was expressed on a root length basis to account for differences in the size of the root systems, the amount of Cd translocated to the needles was similar in seedlings mycorrhizal with P. involutus and in nonmycorrhizal seedlings. All mycorrhizal seedlings were similarly affected by 5 &mgr;M Cd, indicating that the amelioration efficiency of ectomycorrhizal fungi is dependent on the metal concentration to which the roots are exposed. Concentrations of P, K, Ca, Mg and Mn were decreased by 5 &mgr;M Cd to a similar extent in both nonmycorrhizal and mycorrhizal seedlings. In contrast to L. bicolor, P. involutus increased P uptake and altered patterns of root branching. We conclude that mycorrhizas alleviate Cd-induced reductions in growth of Picea abies seedlings. Although the two mycorrhizal fungi examined differed in their ability to alleviate Cd toxicity, these differences were not related to differences in Cd uptake or translocation to the shoot of the mycorrhizal seedlings. We suggest that amelioration of Cd toxicity by P. involutus may be a result of improved P nutrition or changes in root morphology, or both.