Allelochemic function for a primary metabolite: the case of l-tyrosine hyper-production in Inga umbellifera (Fabaceae)

Abstract

Young leaves of tropical forest trees experience far higher herbivory pressure than mature leaves of the same species. Selection on young leaves has led to diverse forms of defense chemical expression. Though most allelochemicals are secondary metabolites, allelochemic function for a primary metabolite remains a possibility. We recently observed this phenomenon in the young leaves of Inga umbellifera, which accumulate the protein amino acid l-tyrosine to very high levels. We isolated l-tyrosine from young leaves of trees in Panama and characterized it using spectroscopic and chemical means. We chromatographically quantified leaf l-tyrosine levels across a range of developmental stages, showing that it was present in the youngest leaves and that its concentration increased throughout the period of expansion, reaching an average maximum of ca 10% of leaf dry mass in late-stage young leaves. This chemical phenotype was seen to be highly leaf-age specific: Free tyrosine was only present in mature leaves at very low levels. In bioassays with larvae of the noctuid moth H. virescens, l-tyrosine proved to be a potent growth inhibitor when added to artificial diet at 10% of dry mass. This suggests that a rarely observed defense strategy occurs in young I. umbellifera leaves, a hyper-produced primary metabolite functioning as an allelochemical.