Ferric reducibility assay
The fresh peanut roots were washed twice with distilled water, weighed, and then immersed in a reagent consisting of 0.4 mM 2,2ʹ-dipyridyl (CAS No. 366-18-7) and 0.1 mM EDTA-Fe for 2 h without light (100 ml of solution per root). Then, the absorbance of the reaction solution was measured at 520 nm using an ultraviolet spectrophotometer (T2600; Yoke Instrument Co., Ltd., China). Ferric reductase activity was calculated with the formula below:
Ferric reductase (μmol·g−1·t−1) = A520 / (FW * 8650) * 106,
where FW is the fresh weight of the root, and 8650 is the molar absorbance of Fe(Ⅱ)–dipyridyl (L·mol−1·cm−1).