Dose-response relationships between four pesticides and phosphorus uptake by hyphae of arbuscular mycorrhizas

The effect of the fungicides carbendazim, fenpropimorph and propiconazole and of the insecticide dimethoate on plant P uptake via external hyphae of arbuscular mycorrhizal (AM) fungi was examined. Mycorrhizal plants were grown in a two-compartment system where a root-free hyphal compartment was separated from the main root compartment by nylon mesh. After 5 weeks of plant growth external hyphae of the AM fungi had spread throughout the hyphal compartment. At this time aqueous solutions of both P-32 and pesticide were added to the hyphal compartment. Resulting soil pesticide concentrations covered a wide range with the aim of describing the dose-response relationships between pesticide concentration and hyphal P uptake. Plants were harvested 6d after pesticide application. The amount of P-32 taken up into the plant was measured as was hyphal length in the hyphal compartment.

Carbendazim had the most negative effect, with concentrations above 0.006 mu g g(-1) leading to a sharp decrease in hyphal P uptake. Concentrations above 0.1 mu g g-l almost completely inhibited hyphal P uptake. This concentration is lower than the expected field concentration following carbendazim application at the recommended field rate. Hyphal P uptake was also negatively affected by propiconazole but only at concentrations above 1 mu g g(-1). This corresponds to an expected field concentration of propiconazole after application of 10 times the recommended field dosage. Fenpropimorph and dimethoate had no negative effects on hyphal P uptake even when applied at concentrations of 125 or 46 mu g g(-1) respectively. These concentrations are equivalent to expected field concentrations following pesticide applications at 100 times the recommended field rate. (C) 1998 Elsevier Science Ltd. All rights reserved.