Morphology and mass spectrometry-based chemical profiling of peltate glandular trichomes on Mentha haplocalyx Briq leaves

Mentha haplocalyx Briq (M. haplocalyx) is a herbaceous plant that has long been used as a food, medicinal spice, and flavoring agent in traditional Chinese medicine. Its secondary metabolites, having high commercial values, are mainly produced in tiny specialized structures called glandular trichomes (GTs). The primary purpose of this study was to examine the morphology and metabolites of peltate GTs in M. haplocalyx.Peltate GTs possessed globular dome shapes and intense auto-fluorescence on the surfaces of M. haplocalyx leaves. Structure subsidence and cuticle rupture were found throughout the aging stage of peltate GTs. According to histochemical staining results, the secretion of peltate GTs contained anthraquinone, flavonoids, phenolic acid and terpenoids. In M. haplocalyx peltate GTs and leaf tissues without peltate glandular trichomes, ten and two volatile compounds were identified respectively. Peltate GTs contained 42 non-volatile chemicals with a variety of structural types, including 20 flavonoids, 17 phenolic acids,1 diterpene, 3 anthraquinone and 1 alkane. Meanwhile, 15 non-volatile compounds were discovered in leaf tissues without peltate glandular trichomes, and they were all included in the list of peltate GTs' 41 components. Therefore, Peltate GTs were shown to be the primary site of not just volatile compounds but also non-volatile chemicals in M. haplocalyx. This study provides an important theoretical basis and technical approach for clarifying the bio-active metabolite biosynthesis in M. haplocalyx.