Symbiosis between Rhizobium leguminosarum bv. trifolii strain TA1 and a white clover cultivar benefits clover tolerance to cadmium toxicity

New Zealand pastoral productivity relies heavily on biological nitrogen (N) fixation from rhizobia bacteria. Cadmium (Cd), present in pasture soils as an impurity in phosphate fertilisers, may have toxic effects on both plant growth and rhizobia activity. Effects of Cd on growth of seedlings of a white clover cultivar 'Grasslands Tribute', in the absence and presence of the N 2 -fixing Rhizobium leguminosarum bv. trifolii strain TA1, were assessed in vermiculite microcosm assays. The lowest observable effect concentrations resulting in significantly lower white clover shoot weights than those grown without Cd were 0.07 mg Cd kg −1 (measured as Ca(NO 3 ) 2 -extractable Cd, Cd extractable ) in the absence of Rhizobium, and 2.33 mg Cd extractable kg −1 for Rhizobium-inoculated seedlings. Shoot N content was unaffected in white clover exposed to