Possible ecological significance of within-fruit and seed furocoumarin distribution in twoPsoralea species

Furocoumarin distribution patterns in the fruits and seeds ofPsoralea macrostachya andP. onobrychis were investigated. Both species contain the linear furocoumarin psoralen and its angular isomer, angelicin. In the monospermous indehiscent fruit ofP. macrostachya, furocoumarins occur in the pericarp and all seed parts. InP. onobrychis, the pericarp of which is easily detached at ripeness, no furocoumarins were found in the pericarp tissues and only traces occur in the embryo axis; cotyledons are the preferential accumulation site. The within-fruit and -seed furocoumarin variations associated with the developmental stages of fruit were followed inP. onobrychis, in view of changes in the defensive value of the pericarp before and after ripening. Rapid furocoumarin biosynthesis after fertilization was observed in both pericarp and seed; ripening is associated with furocoumarin decrease in the seed and complete disappearance in the pericarp tissues. Such findings are consistent with the chemical defense role of these substances. The cooccurrence of linear and angular isomers seems to be a chemical marker of the genusPsoralea: the biosynthetic pathway leading to the angular isomer as an evolutionary response to selective pressure from herbivore insects is suggested.