Histochemical Application for the Identification of Thymol and Carvacrol Chemotypes of Various Essential Oil Producing Plants

Histochemical methods, in combination with various microscopic imaging techniques, have been significantly utilized to study the essential oil secretion in various plants. We are interested in implementing a technique originally developed for the localization of monoterpene phenols using the reagent 4-nitrosophenol in conc. H2SO4. The plant material selected for this study consisted of the leaves obtained from three Lamiaceae species; Thymus vulgaris L., Origanum syriacum L., and Plectranthus amboinicus (Lour.) Spreng., and two Asteraceae species; Eriocephalus africanus L. and Artemisia absinthium L. Mentha piperita L. and Ocimum basilicum L. (Lamiaceae) were used as negative controls devoid of monoterpene phenols. These former species were chosen for their high content of Thymol and/ or Carvacrol which produce colored indophenols of bright red and dark green respectively. In this study, we successfully establish the method for identifying these compounds using the color reaction which was verified by the analysis of the corresponding plant leaf oils using GC and GC-MS techniques. This method could serve for a rapid screening of specific oil components that could help distinguish several plant chemotypes in the field, hence saving time spent in laboratory analyses.