Changes in Isoflavonoid and Flavonoid Content in Soybean Leaves Affected by UV-B or Copper

The production of antioxidant flavonoids is an important aspect of plant defence mechanisms under stress conditions. The hypothesis of increased antioxidant flavonol quercetin and isoflavone content in soybean leaves subjected to UV-B irradiation and copper toxicity was proposed, and their accumulation was investigated using HPLC analysis. However, the accumulation of antioxidant quercetin was not induced by either UV-B irradiation or copper treatment. UV-B irradiation strongly induced isoflavone aglycones, mainly genistein, daidzein, formononetin, and to a lesser extent, biochanin A. Isoflavone glycosides, genistin and daidzin were also induced. Copper treatment, whether applied to leaves or root media, resulted in the accumulation of genistin and its malonylated form, whereas aglycones were only slightly affected. The higher accumulation of genistein over daidzein after copper treatment could be related to the higher chelating ability of genistein and its glycosides. Soybean isoflavones, which possess antioxidant and chelating properties, rather than flavonols, play a crucial role in the plant‘s defence mechanisms against both UV-B irradiation and copper toxicity.