Activation of Rhizobium tibeticum With Flavonoids Enhances Nodulation, Nitrogen Fixation, and Growth of Fenugreek (Trigonella foenum-graecum L.) Grown in Cobalt-Polluted Soil

The goal of this study was to investigate the response of activation of Rhizobium tibeticum with mixture of hesperetin and apigenin to improve growth, nodulation, and nitrogen fixation of fenugreek grown under cobalt (Co) stress. The current study showed that high concentrations of Co-induced noxious effects on rhizobial growth, nod gene expression, nodulation, phenylalanine ammonia-lyase (PAL) and glutamine synthetase (GS) activities, total flavonoid content, and nitrogen fixation. Addition of a mixture of hesperetin and apigenin to growth medium supplemented with different concentrations of Co significantly increased bacterial growth. PAL activity of roots grown hydroponically at 100 mg kg−1 Co and inoculated with induced R.tibeticum was significantly increased compared with plants receiving uninduced R.tibeticum. Total flavonoid content of root exudates of plants inoculated with activated R.tibeticum was significantly increased compared with inoculated plants with unactivated R.tibeticum or uninoculated plants at variant Co dosages. Application of 50 mg kg−1 Co significantly increased nodulation, GS, nitrogenase activity, and biomass of plants inoculated with either or uninduced R.tibeticum. The total number and fresh mass of nodules, nitrogenase activity, and biomass of plants inoculated with induced cells grown in soil treated with 100 and 200 mg kg−1 Co were significantly increased compared with plants inoculated with uninduced cells. Induced R. tibeticum with flavonoids significantly alleviates the adverse effect of Co on nod gene expression and therefore enhances nitrogen fixation. Induction of R. tibeticum with compatible flavonoids could be of practical importance in augmenting growth and nitrogen fixation of fenugreek grown in a Co-contaminated agroecosystem.