Arbuscular mycorrhizal colonization and growth of soybean (Glycine max) and lettuce (Lactuc sativa) and phytotoxic effects of olive mill residues

We studied the influence of olive mill dry residue (DOR) on growth of soybean (Glycine max) and lettuce (Lactuca sativa) colonized with the arbuscular fungi Glomus mosseae or G. deserticola. The DOR has 6 mg g−1 of soluble phenolic compounds and the application of 2.5 g kg−1 of DOR (15 mg kg−1 of phenolic content) to soil decreased the growth of plants colonized with the AM fungi. The application to the soil of 10 g kg−1 of DOR (60 mg kg−1 of phenolic content) decreased the dry weight of nonarbuscular mycorrhizal (AM) plants. The application of DOR decreased the percentage of AM colonization of plants except those were inoculated with G. mosseae or G. deserticola 4 weeks before the application of DOR. No effect of the residue on the colonization of these previously AM inoculated plants was observed. DOR had no effect neither on the fungal succinate dehydrogenase activity nor on the most probable number of AM propagules in all treated soils. The obtained results showed than AM fungi increased the phytotoxicity of DOR in lettuce and soybean.