The objectives of the present study were to evaluate the effects of arbuscular mycorrhiza, the plant P nutritional status and increasing levels of P in nutrient solution on Zn toxicity to Trema micrantha (L.) Blum. In a first experiment trema seedlings were grown in a substrate with increasing P rates [0, 100, 200 and 400 mg dm-3 as Ca(H2PO4)2], besides a mycorrhizal treatment inoculated with Glomus etunicatum (Ge). After 60 days of growth, the plants were transferred to pots with Clark's nutrient solution containing 2, 75, 150 and 225 µmol L-1 of Zn, grown for 40 days, harvested and evaluated. In a second experiment the ameliorating effects of P on Zn phytotoxicity were also evaluated in nutrient solution containing increasing P rates (0.07; 0.5; 1 and 2 mmol L-1 supplied by different sources) and Zn (2, 75, 150 and 225 µmol L-1 as ZnSO4.7H2O) combined and simultaneously applied to the solution where trema seedlings were left to grow for 40 days. It was found that high Zn concentrations were highly inhibitory to plant growth and mycorrhizal colonization. Enhanced P nutrition reduced translocation of Zn from plant roots to shoots, but such effects as well as mycorrhizal colonization had no effect on trema growth under Zn excess in solution. In the second experiment, higher P concentration in solution improved the plant nutritional status and protected them against the excess of Zn. Considering that chemical speciation of the solution showed no significant effect on Zn forms, the results suggested that the ameliorating effects of P occur within the plant (in planta), possibly by reducing Zn translocation from roots to shoots.
woody species; phytotoxicity; phosphate; mycorrhizal fungi; heavy metals; tolerance
