Species-specific composition of free amino acids on the leaf surface of fourSenecio species.

The specificity of the chemical composition of the leaf surface of fourSenecio species was studied in order to understand the host selection ofTyria jacobaeae, a monophagous moth that hasSenecio jacobaea as its principal host plant. Pyrrolizidine alkaloids characteristic forSenecio species were not detected in water and sulfuric acid extracts of the leaf surface ofSenecio jacobaea. Water extracts of the leaf surfaces ofS. jacobaea, S. vulgaris, S. viscosus, andS. sylvaticus contained low concentrations of free amino acids. The proportions of these amino acids were very different from those inside the leaf tissues. In a discriminant analysis, the four species could be completely separated on the basis of the relative proportions of amino acids on the leaf surface.