EFFECTS OF AGROCHEMICALS AND HEAVY-METALS ON FAST-GROWING RHIZOBIA AND THEIR SYMBIOSIS WITH SMALL-SEEDED LEGUMES

The effect of potentially hazardous agrochemicals including fungicides, herbicides and heavy metals on symbiotic nitrogen fixation have been investigated. The substances were tested with eight rhizobial strains from three cross-inoculation groups: Rhizobium leguminosarum b.v. trifolii, R. meliloti and R. loti in pure culture studies. Bacteria were obtained from a culture collection or from soils. Sensitivity of the bacteria to the agrochemicals and heavy metals varied. None of the bacteria were tolerant to all chemicals. No difference in tolerance between cross-inoculation groups existed. Bacteria were able to multiply at concentrations of agrochemicals equal to or higher than recommended field-application rates. Heavy metals concentrations that severely inhibited growth were far lower than the highest amounts allowed under the current Commission of the European Communities' guidelines for environmental protection. Bacterial growth in presence of the agrochemicals and heavy metals, apart from glyphosate and zinc, did not influence nodulation ability of the strains. Development of uninoculated plants was inhibited at increasing concentrations of all compounds, red clover being most sensitive. Herbicides were most harmful, with injuries occurring at levels 1/10-1/10,000 of recommended applied concentrations. Uninoculated plants were less tolerant to agrochemicals, but were more tolerant to heavy metals compared to the bacteria. Root hair deformations similar to bacterial-induced root hair deformations were induced by bentazone, chlorsulphuron and monochlorophenoxyacetic acid on uninoculated plants. Symbiotic interactions were adversely affected by several of the agrochemicals. Bacterial-induced root hair deformations necessary for nodulation decreased in the presence of benomyl, bentazone, chlorsulphuron, fenpropimorph, mancozeb and monochlorophenoxyacetic acid. Fenpropimorph and mancozeb did not cause root hair deformations at increasing concentrations, indicating that these may inhibit nodulation under field conditions. Nodule development was inhibited at increased levels of bentazone, chlorsulphuron, glyphosate and mancozeb. Dry matter production of nodulated plants was adversely affected by bentazone and chlorsulphuron, indicating disturbances in nodule function.