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

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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 32P 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 6 d after pesticide application. The amount of 32P 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 μg g −1 leading to a sharp decrease in hyphal P uptake. Concentrations above 0.1 μg g −1 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 μg 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 μg g −1, respectively. These concentrations are equivalent to expected field concentrations following pesticide applications at 100 times the recommended field rate.