Abstract <p>The effect of treatment with the flavonoid quercetin (Q) the seeds of broad bean plants (<i>Vicia faba</i> L.), inoculated with an effective strain of <i>Rhizobium leguminosarum</i>, on plant morphometric parameters, malate transport through the symbiosome membrane (SM), electrogenic activity of SM H⁺-ATPase, and respiration of root nodule bacteroids under soil contamination with cadmium (Cd) was studied. It was found that treatment (priming) of seeds with Q (50 μM/mL) contributed to increased stability of plant development, decreased variability of morphometric traits, and mitigated the effect of CdCl₂. This was manifested in the optimization of growth processes—achieving maximum plant height, biomass of the aboveground and underground parts, and nodule mass. It was also shown that in symbiosome preparations from root nodules of plants treated with Q, the rate of formation of membrane potential (Δψ), generated on SM as a result of H⁺-ATPase activity, increased compared to plants exposed to Cd. Addition of 20 μM CdCl₂ to the symbiosome incubation medium resulted in Δψ dissipation as a result of H⁺-ATPase inhibition associated with its toxic effect. The rate of malate transport through SM in the Cd²⁺ variant decreased by half compared to control plants, and Q treatment partially restored the transport of dicarboxylate. It was found that Q treatment prevented the inhibition of bacteroids respiration under the influence of Cd²⁺ by activating an alternative CN-resistant pathway in the ETC of bacteroids. The obtained results demonstrate the effectiveness of the combined use of Q, rhizobia and nitrogen-fixing legumes to reduce the effects of cadmium stress, which may have practical significance for increasing their yield and phytoremediation of soils contaminated with heavy metals.</p>