Morphology and ultrastructure of trichomes of intact and in vitro plants of stevia rebaudiana Bertoni with reference to biosynthesis and accumulation of steviol glycosides

The ultrastructure of trichomes of intact and in vitro plants of stevia were investigated under scanning and transmission electronic microscopy. Three types of trichomes were found on both the upper and lower surfaces of the leaf: viz. rounded glands and large and small trichome. The density of trichomes on the surface of upper leaves is higher than on lower leaves, while in vitro plants are characterized with lower density of trichomes than intact ones. The positive correlation between number of trichomes on leaves and content of steviol glycosides (SGs) in them was found. Three basic stages of ontogeny of gland cells were revealed which reflect the sequence of secrete accumulation.