Trichome diversity in the Vernonieae (Asteraceae) of Mexico I: Vernonanthura and Vernonia (Vernoniinae)1

Abstract

Redonda-Martínez, R., J. L. Villaseñor, and T. Terrazas (Instituto de Biología, Universidad Nacional Autónoma de México, Departamento de Botánica, Apartado postal 70-367, 04510 México, D.F.). Trichome diversity in the Vernonieae (Asteraceae) of Mexico I: Vernonanthura and Vernonia (Vernoniinae). J. Torrey Bot. Soc. 139: 235–247. 2012.—Trichomes on the leaves and florets of 17 Mexican species of Vernonanthura and Vernonia (Tribe Vernonieae, subtribe Vernoniinae) were studied using light and scanning electron microscopy (SEM), to assess their taxonomic value in distinguishing between these two genera. Earlier studies of North American Vernonieae by Jones and colleagues (Faust and Jones 1973, King and Jones 1975) showed that trichome morphology and their presence on leaves and reproductive structures could be taxonomically informative, but this work included few Mexican taxa. Glandular and nonglandular trichomes, each with several variants, were found in the two genera. Glandular trichomes were particularly abundant on the florets and the achenes while the nonglandular were more common on the leaves. A cluster analysis shows two main species groups based on trichome traits. The first group linked species lacking glandular trichomes in vegetative or reproductive organs, whereas in the second the species were grouped by the occurrence of uniseriate and glandular trichomes. Within this second group, two species possess unique features: Vernonanthura cordata has long uniseriate subtype b2 trichomes while in Vernonia greggii there are T-shaped trichomes on the leaf abaxial surface. Our results also showed that trichomes were useful in differentiating species with similar ecological and/or geographical features (e. g., Vernonia larsenii and V. lindheimeri), but were of limited taxonomic value at the generic level in distinguishing Vernonanthura from Vernonia. These two genera share various trichome types, but L-shaped trichomes were only found in Vernonanthura. An ongoing study of the Mexican Vernonieae may reveal informative trichome characters at different taxonomic levels, such as among subtribes, or between species of other more distantly related genera.