Adaptive significance of furanocoumarin diversity in Pastinaca sativa (Apiaceae)

Fruits of Pastinaca sativa (Apiaceae), the edible parsnip, contain six different furanocoumarins that are differentially capable of ultraviolet-mediated cross-linkage of DNA and inhibition of DNA transcription. Individually, none of the other furanocoumarins present in parsnip seeds is as toxic as the photosensitizer xanthotoxin. Nevertheless, the natural mixture of compounds is toxicologically more effective against Heliothis zea (Lepidoptera: Noctuidae), both in the presence and absence of UV light, than is an equimolar amount of xanthotoxin. The difference in toxicity diminishes with increasing light levels. Thus, a series of structurally related natural products can display toxicity lacking in individual compounds and may represent an adaptive compromise to varying environmental conditions.