Milieu-dependent pro- and antioxidant activity of juglone may explain linear and nonlinear effects on seedling development.

Juglone, 5-hydroxy-1,4-naphthoquinone, is known for its wide range of biological activities. It has been suggested that juglone's excellent redox cycling properties contribute to this reputation. Many biological activities are nonlinear with low concentrations exerting stimulating effects, whereas only higher concentrations cause inhibition. Here, we corroborate studies on the nematode Caenorhabditis elegans that point out hormetic effects by showing that juglone may cause a nonlinear effect on postgerminative shoot and root growth of Sinapis alba. This effect was only significantly visible, however, when seedlings were stressed with methanol. Classic and modified versions of the deoxyribose assay were applied successfully to characterize antioxidative (purposeful generation of hydroxyl radicals) and prooxidative (no purposeful generation of hydroxyl radicals) activities. Variants of the assay with and without the addition of the iron chelator EDTA showed that the antioxidant activity is independent on chelation of iron ions by juglone; by contrast, the strength of the prooxidative activity depended on the chelation of iron ions by juglone. The hormetic effects of lower concentrations on germination of Sinapis alba, thus, may be caused by the antioxidant activities of this compound, which are especially effective when the test organism is subjected to higher oxidative challenge. The present study suggests that pronounced prooxidative activities, which are considerably accelerated by chelation of iron ions, may contribute to the toxic effects of juglone at higher concentrations.