The Role of Trichomes in Plant Defense

Trichomes occur in a multitude of forms and sizes. Although they have been used widely for taxonomic purposes, their adaptive significance has been all but ignored by the evolutionist and ecologist. It is clear that trichomes play a role in plant defense, especially with regard to phytophagous insects. In numerous species there is a negative correlation between trichome density and insect feeding and oviposition responses, and the nutrition of larvae. Specialized hooked trichomes may impale adults or larvae as well. Trichome may also complement the chemical defense of a plant by possessing glands which exude terpenes, phenolics, alkaloids or other substances which are olfactory or gustatory repellents. In essence, glandular trichomes afford an outer line of chemical defense by advertising the presence of "noxious" compounds. In some groups of plants, protection against large mammals is achieved by the presence of stinging trichomes. Intraspecific variation for trichome type and density is known in many species, and often is clinal in accordance with ecographic parameters. The presence of such correlations does not imply that differences in predator pressure are the causal factors, although this may indeed be the case.