Trichomes on vegetative and reproductive organs of Stevia rebaudiana (Asteraceae). Structure and secretory products

Trichomes from vegetativ and reproductive organs of Stevia rebaudiana were investigated by light and scanning esectron microscopy (SEM) coupled with chemical and histochemical characterisation of the secretory product. Two types of non-glandular trichomes plus sunken glandular trichomes were found on the leaves. The reproductive organs differed from the leaves mainly for the presence of stalked glandular trichomes and of non-glandular twin hairs, the latter being restricted to the achenes. The secretion product is released in consequence of the rupture of the cuticular sheath along a line of weakness in the middle of the gland head. Glandular trichomes from both leaves and inflorescences gave similar responses to the histochemical tests, the secreted material being mostly lipophilic in both cases. HRGC/MS analysis of the essential oil revealed that sesquiterpenes were the most abundant class of compounds both in the leaves and in the inflorescences, altthough their quantity was six to seven times greater in the inflorescences that in the leaves. Spathulenol, caryophyllene oxide and non-oxygenated terpenes were also detected in the secretion.