Plant traits may play multiple functional roles simultaneously. Leaf trichomes have been related to resistance against herbivores as well as to enhanced water economy in the plant. In a greenhouse study, we evaluated the interactive effect of damage (control vs. mechanical damage) and water availability (control vs. low watering) on the expression of glandular and non-glandular leaf trichomes in the annual Chilean tarweed Madia sativa (Asteraceae). We found that the overall trichome density increased both after damage and when plants were grown under water shortage. Interestingly, the type of trichome induced after damage varied with each water environment. While damage induced glandular trichomes only under control watering, non-glandular trichomes were induced by damage only under experimental drought. Results indicate that in M. sativa glandular trichomes are equally induced by drought or damage but there is no additive effect of these factors. In both cases glandular trichome density apparently reached a limit, which may be interpreted in terms of constraint or efficacy. On the other hand, the synergistic effect of damage and drought on non-glandular trichomes might suggest that, compared to glandular ones, these trichomes are less responsive to each stress factor separately. Thus, for plants to induce non-glandular trichomes they must be subjected to a degree of stress above a threshold that was not reached for each individual factor in our experimental setting. We did not detect a significant correlation between trichome types. Thus, the contrasting patterns observed likely reflect independent responses of trichomes to the evaluated factors. © 2007 Elsevier Masson SAS. All rights reserved.