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The lichen secondary metabolite usnic acid exists as a (−) and a (+) enantiomer, indicating a α or β projection of the methyl group at position 9b, respectively. (−)‐Usnic caused a dose‐dependent bleaching of the cotyledonary tissues associated with a decrease of both chlorophylls and carotenoids in treated plants whereas no bleaching was observed with the (+) enantiomer. (−)‐Usnic acid inhibited protophorphyrinogen oxidase activity (I50=3 μM), but did not lead to protoporphyrin IX accumulation. Bleaching appears to be caused by irreversible inhibition of the enzyme 4‐hydroxyphenylpyruvate dioxygenase by (−)‐usnic acid (apparent IC50=50 nM).