Floral UV-Absorption Patterns and Anthochlor Pigments in the Asteraceae

Abstract

The spatial distribution of anthochlor pigments in ray florets of ten genera (86 species) of the Asteraceae (tribes Heliantheae and Helenieae) is presented. The presence and spatial segregation of these pigments yield a floral UVabsorption pattern, visible to pollinating insects, which may play a major role in reproductive isolation and speciation in the plant groups. Anthochlor pigments (chalcones and aurones) were long thought (now disproven) to be a unique phytochemical character of the subtribe Coreopsidinae of the Heliantheae (Asteraceae). These flavonoid pigments have now been identified from 22 angiosperm families including eight genera in four tribes of the Asteraceae (Bohm 1975). No selective role had been attributed to these pigments until recently when several workers (Dement and Raven 1974; Scogin 1976; Scogin and Zakar 1976) observed a correlation between anthochlor pigment segregation pattern and UV absorption pattern in flowers. Segregation of pigments absorbing UV wavelengths permit the presentation of a floral pattern invisible to humans, but visible to some pollinating insects by virtue of their ability to see UV wavelength light. The present report details the results of a survey of ten genera of the Asteraceae containing anthochlor pigments and occurring largely in the southwestern United States and Mexico. Emphasis is placed on the spatial distribution of anthochlor pigments (hence, UV absorption pattern) in the ray petals. MATERIALS AND METHODS. The occurrence and distribution of anthochlor pigments were determined in fresh and herbarium material by placing a floral head or single ray floret in an atmosphere saturated with ammonium hydroxide. A color change from yellow to orange-red occurs within several minutes and is characteristic of anthochlor pigments. Ultra violet absorption-reflection patterns were determined experimentally in selected species by photographing fresh flowers using a Kodak-