Comparative influence of flavonoids on the toxicity of copper to Alternaria alternata

Three flavonols, morin, rutin and quercetin, and a flavonone, naringenin, were in vitro tested for their influence on the toxicity of copper to Alternaria alternata. Copper sulphate (CuSO4) displayed dose-dependent inhibition of fungal growth, resulting in a reduction in growth by one-half at 0.07mM. The addition of quercetin and morin conferred substantial protection against the inhibition of fungal growth by copper, whereas naringenin and rutin offered protection less effectively. Higher concentrations of copper could overcome the flavonoid effect, but the reduction of copper toxicity was more apparent in the presence of quercetin than morin. The metal chelating ability of quercetin was highly specific for copper among toxic metals such as Co, Ni, Zn, and Cd. The results show that the protection effect of the flavonoids against copper toxicity is highly dependent on the keto function and hydroxyl group substitution of the flavonoid skeleton.