Effects of two chelates on germination and cobalt enrichment of cobalt-stressed alfalfa seeds

In this study, the seeds of alfalfa (Medicago sativa) and hydroponics were used as the materials and culture method, respectively, to investigate the effect of two metal chelating agents, ethylene diamine tetraacetic acid (EDTA) and ethylene glycol-bis (β-aminoethyl ether)-N, N, N', N'-tetraacetic acid (EGTA), on the germination and cobalt enrichment of seeds under Cobalt (Co) stresses; to find potential solutions for plant growth in heavy metal environment and heavy metal removal from polluted soil by the plants. The seeds were first cultured at different levels of Co stresses (0.1, 0.2, 0.5, 0.7, 1.0, and 2.0 mmol·L⁻¹) to ascertain the half-maximal inhibitory concentration of Co solution against the seed germination of alfalfa (0.7 mmol·L⁻¹). The Co solution at this concentration was then applied as the Co stress to study the effect of exogenous EDTA and EGTA on the germination of Co-stressed alfalfa seeds. Through exogenous addition of EDTA (0.1, 0.2, 0.5, 0.8, 1.0, and 2.0 mmol·L⁻¹) and EGTA (0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 mmol·L⁻¹) at different concentrations to Co-treated groups, the significant recovery concentration of EDTA and EGTA for Co-stressed alfalfa seeds were determined. Meanwhile, superoxide anion generation rates (SAGRs), superoxide dismutase (SOD) activity, malondialdehyde (MDA) content, and Co content in alfalfa seedlings were determined when their germination was significantly recovered. The results revealed a significant increase (P < 0.05) in SAGRs, MDA content, and Co content (reaching 2 000 mg·kg⁻¹) in the seedlings, whereas a significant decrease in the SOD activity. Both 0.8 mmol·L⁻¹ exogenous EDTA and 1.0 mmol·L⁻¹ exogenous EGTA can significantly recover the germination rates and seedling lengths of Co-stressed alfalfa seeds when SAGRs and MDA content in the seedlings significantly decrease, and the SOD activity significantly increases. The results indicate that one of the reasons for the inhibition of alfalfa seed germination is the imbalance of ROS metabolism caused by the toxic effect of Co when it is substantially extracted by alfalfa seedlings under Co stresses. Moreover, metal chelates EDTA and EGTA can recover Co-stressed alfalfa seed germination by reducing Co enrichment in alfalfa seedlings.