A new histochemical method for localization of the site of monoterpene phenol accumulation in plant secretory structures

A new method is reported for the histochemical localization of monoterpene phenols in essential oil secretory structures. The method was adapted from a spot test originally devised for in vitro detection of phenolic compounds in organic analyses. Plant subjects were the Lamiaceae species Thymus vulgaris L., Oreganum vulgare L. and Mentha x piperha L., which accumulate essential oil in glandular trichomes. A reagent consisting of 4-nitrosophenol in cone. H2SO4 was applied to sample leaves of each species. A positive test for phenol was indicated by the production of coloured indophenols. Using this method, monoterpene phenols were identified in the trichomes of T. vulgaris (thymol) and O. vulgare (carvacrol), indicated by colour changes to red and green respectively. No phenol was detected in trichomes of M. x piperita. Results were confirmed by GC-MS analysis of leaf volatile extracts from each species, and In vitro tests with thymol and carvacrol. The method could be used in field surveys for rapid identification of potential medicinal plants and bioactive compounds. (C) 2001 Annals of Botany, Company.