Analysis of monoterpenoids in glandular trichomes of the catmint Nepeta racemosa

Plants within the Lamiaceae are characterized by their production of essential oils, largely composed of monoterpenoids. They also possess on their aerial surfaces different types of trichomes, including relatively large peltate glandular trichomes. Observation of leaves of the catmint Nepeta racemosa, using cryogenic scanning electron microscopy, showed that around 3000 peltate trichomes are present on a single expanded leaf. An approach has been developed for sampling directly from the subcuticular cavity of such trichomes, in order to characterize qualitatively and quantitatively their contents. Gland samples from Nepeta spp. were analysed by combined gas chromatography-mass spectrometry. These analyses have confirmed that the monoterpenoids characteristic of these species (nepetalactones) accumulate within the subcuticular cavity of peltate glandular trichomes. Qualitative analysis showed that three different nepetalactone stereoisomers were accumulated in different proportions in peltate glands from individual N. racemosa plants. Quantitative analysis showed that individual peltate glands accumulate around 30 ng of nepetalactone by maturity, and that accumulation of nepetalactone probably occurs exclusively within this gland type.