In the present study the effects of phosphorus nutrition and arbuscular mycorrhiza (AM) were evaluated on Cd toxicity to Trema micrantha (L.) Blum.. In a first experiment, trema seedlings were raised in a P-enriched substrate (0, 100, 200 and 400 mg dm-3) and with an AM treatment (inoculation with Glomus etunicatum). After sixty days of growth, plants were transferred to Clark nutrient solution either without Cd added or amended with this metal at concentration of 5, 15 and 45 µmol L-1. After the transplant to solution with treatments, plants were allowed to grow for additional forty days, when treatment effects were evaluated. Phosphorus effects on amelioration of Cd toxicity were also evaluated in a second experiment in which P at concentration of 0.07; 0.5; 1 and 2 mmol L-1 and Cd at 0, 5, 10 and 15 µmol L-1 were combined and simultaneously applied in the growing nutrient solution. It was found that Cd exhibited high inhibition on growth and on AM colonization even at the lowest tested concentration (5 µmol L-1). Enhanced P nutrition favored plant growth by reducing Cd concentration in plant shoots and consequently Cd phytotoxicity, whereas AM inoculation had no significant effect. An inverse relationship between P supply and Cd plant uptake and accumulation was found in the second experiment. The chemical speciation analysis of the nutrient solution showed that P application had no significant interference on Cd available forms, therefore indicating that the P-ameliorating effect on Cd toxicity results from "in plant " interactions that cause reduction in translocation of Cd absorbed by the roots.
Heavy metals; phosphate-metal interaction; mycorrhizal fungi
