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 signifi-
cantly 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.