Methyl Salicylate Secretory Cells in Roots of Viola arvensis and V. rafinesquii (Violaceae)

The aromatic roots of Viola arvensis and V. rafinesquii were studied in order to determine the chemical nature and anatomical localization of their volatile compounds. Gas chromatography and mass spectroscopy revealed a single detectable volatile compound, methyl salicylate. Light microscopy and differential staining with Sudan III indicates the source of this compound to be enlarged secretory cells located in the endodermis, an unusual position for such cells in roots of angiosperms. The secretory endodermal cells are sporadic, but are more frequent in primary roots than in secondary roots and the lower portion of the hypocotyl. It is hypothesized that secretory endodermal cells are restricted within Viola to subgenus Melanium where the methyl salicylate confers protection against herbivores and/or pathogens. Viola arvensis Murray and V. rafinesquii Greene, known as pansies or field pansies, are widespread in eastern North America; both are members of Viola subgenus Melanium Ging. (Melchior and Becker 1925). While V. arvensis is naturalized from Eurasia, opinions have differed over the provenance of V. rafinesquii. Some consider V. rafinesquii to be native to North America (Brainerd 1911, Radford et al 1968, Strausbaugh and Core 1970-1977), yet others treat it as a minor variant of V. kitaibellana R. & S., adventive from north Africa (Fernald 1938, 1951). Both species are winter annuals of frequent occurrence in the Richmond, Virginia area. While identifying species of violets in a plant systematics class, it was noticed that roots of both V. arvensis and V. rafinesquii possess the odor of oil of wintergreen (methyl salicylate). Radford et al (1968) note the presence of this odor in V. arvensis, but not for V. rafinesquii; curiously, this information concerning root odors in these species is reversed from that presented in Fernald (1951). Confusion in the floristic literature notwithstanding, salicylic acids have long been known in species of subgenus Melanium. For example, Wherry (1927) reported methyl salicylate in V. rafinesquii (based solely on olefaction), and Hegnauer (1962-1969) cites several European reports, all from the 19th or early 20th century. Aromatic constituents of plants are often localized in special secretory cells (Esau 1965). Such cells, however, have not been reported in accounts of the anatomy of violet roots (Freidenfelt 1904, Kaczmarek 1915, Melchior and Becker 1925, Metcalfe and Chalk 1950). The present study of V. arvensis and V. rafinesquii was undertaken, therefore, to