Non-glandular trichomes in Lamiaceae and Verbenaceae species: morphological and histochemical features indicate more than physical protection

Abstract

Non-glandular trichomes are traditionally known as acting in the physical protection of plants against biotic and abiotic stresses, forming a mechanical barrier against low humidity, high light intensity and temperatures, and feeding and oviposition activities of insects. However, detailed studies involving morphology, histochemistry and ultrastructure of these epidermal appendages are lacking. We characterised the morphology and histochemistry of the non-glandular trichomes in three Lamiaceae and four Verbenaceae species and evidenced their involvement in the production, storage and/or liberation of biologically active substances. Samples of leaves were prepared according to standard methods for scanning and transmission electron microscopy. Histochemical tests were performed using fresh material. The combination of structural and ultrastructural studies and histochemical techniques allowed us to verify that the non-glandular trichomes of all studied species are comprised by living cells able to act in the synthesis, storage and/or liberation of biologically active compounds. This is the first work showing the production of compounds in non-glandular trichomes in Lamiaceae and Verbenaceae species. Our observations indicate that more than a physical protection, the non-glandular trichomes of these species participate in the chemical interaction of the plants with the environment, supplementing the work of the typically glandular trichomes.