Histological localization of furanocoumarins in Ruta graveolens shoots

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Abstract
Three linear furanocoumarins, psoralen, xanthotoxin, and bergapten, were localized histologically in Ruta graveolens shoots. The highest amounts were present in the epidermis, lesser amounts in the parenchyma cells, and only traces in the glands. Scrapings and filter paper rubbings from leaf and stem surfaces gave qualitative indications of their presence. As determined by liquid chromatography, large amounts (up to at least 60% of the total in the plant) were extractable into boiling water after 2-s immersion, indicating their localization on the cuticle. Xanthotoxin was usually present in the highest concentrations on the surface, but in most cases amounts of psoralen were only slightly lower. In both the scanning electron microscope and the light microscope a layer covering the cuticle was observed; its crystal-like structure varied over the surface in the regions of the glands and other parts of the epidermis. It was thicker in younger leaves, and during leaf expansion it was stretched and sometimes broken. Localization of a substantial portion of these linear furanocoumarins on the surface is a factor in the propensity of this species to induce photophytodermatitis.