Trichomes as chemical defenders: Ecological implications of evolutionary adaptations in angiosperms

Abstract The hair‐like structures on the surface of plants are known as trichomes, and they play a number of physiological and ecological roles including protection against herbivores, minimizing water loss, and modifying gas exchange. This review provides an extensive overview of the phytotomy diversity of plant trichomes of 82 species, focusing on angiosperm species. The data compiled reveal a rich diversity of trichomes within the families Asteraceae, Lamiaceae, and Solanaceae, among others. The review demonstrated that the greatest trichome occurrence came from Lamiaceae (17.4%), Solanaceae (16.3%), and Asteraceae (14%) while the least occurred in families at 10%. Regarding trichome morphologies glandular types prevailed (45.1%), with capitate (15.7%) and simple (15.2%) being the most occurring subtypes. The trichomes on these species include all glandular and non‐glandular trichomes. Glandular trichomes may be unicellular or multicellular, and often include capitate and peltate types; these are very common and have a substantial role in the secretion of secondary metabolites such as terpenoids, flavonoids, alkaloids and phenolics. Non‐glandular trichomes, may be unicellular or multicellular, and with further subtypes as simple, stellate, peltate, scales or dendritic. They, contribute to mechanical defense and reduce water loss. The localization and structure of the trichomes and their secondary metabolites vary between species and organs, although leaves, stems, flowers and fruits are common sites, and they are more frequent in the leaves. The conducted literature review underscores the functional and ecological importance of trichomes on angiosperms as revealed by the selection of the reviewed literature, which is key to the survival of a plant species and its interactions with the environment and endurance under unfavorable climatic conditions.