Plectranthus madagascariensis:: Morphology of the glandular trichomes, essential oil composition, and its biological activity

Glandular and nonglandular trichomes are spread over the vegetative and reproductive organs of Plectranthus madagascariensis. Two morphologically distinct types of glandular trichomes (capitate and peltate) are described. Capitate trichomes have an ovoid unicellular head and a short stalk cell or an elongated two-to three-celled stalk slightly enlarged at the base. Peltate trichomes, which show in vivo a characteristic orange-to-reddish color, are large, flattened-to-depressed in the center, and often appear as a ring of minute gems. They are composed of a large head with eight glandular cells arranged in a single Layer. An unusual kind of capitate trichomes, reported for the first time in Lamiaceae, occurs typically restricted to the calyx. These trichomes possess a two-or three-celled stalk and a long, unicellular conical, glandular head. The several types of trichomes differ in the secretion process. The essential oils of P. madagascariensis, isolated by hydrodistillation and by distillation-extraction separately from flowers and from leaves collected during the flowering and the vegetative phases of the plant were analyzed by gas chromatography and gas chromatography-mass spectrometry (GC and CC-MS). The main component of the oils was a diterpene, 6,7-dehydroroyleanone, isolated as orange-to-reddish crystals, which represented 28%. 87%, and 41% of the oils from the flowers and from the leaves collected during the flowering and vegetative phases, respectively. The essential oils of P. madagascariensis showed bactericidal activity against Bacillus subtilis, Micrococcus sp., Staphylococcus aureus, and Yersinia enterocolitica, and revealed a modest antioxidant activity.