Ultraviolet patterns on rear of flowers: basis of disparity of buds and blossoms

Attractive and defensive functions of the ultraviolet pigments of a flower (Hypericum calycinum)

The flower of Hypericum calycinum, which appears uniformly yellow to humans, bears a UV pattern, presumably visible to insects. Two categories of pigments, flavonoids and dearomatized isoprenylated phloroglucinols (DIPs), are responsible for the UV demarcations of this flower. Flavonoids had been shown previously to function as floral UV pigments, but DIPs had not been demonstrated to serve in that capacity. We found the DIPs to be present in high concentration in the anthers and ovarian wall of the flower, suggesting that the compounds also serve in defense. Indeed, feeding tests done with one of the DIPs (hypercalin A) showed the compound to be deterrent and toxic to a caterpillar (Utetheisa ornatrix). The possibility that floral UV pigments fulfill both a visual and a defensive function had not previously been contemplated. DIPs may also serve for protection of female reproductive structures in other plants, for example in hops (Humulus lupulus). The DIPs of hops are put to human use as bitter flavoring agents and preservatives in beer.

Appearance of Vegetation in Ultraviolet Light: Absorbing Flowers, Reflecting Backgrounds

Flowers that uniformly absorb ultraviolet light may contrast strikingly with a bright ultraviolet-reflecting background, such as densely hairy or glaucous foliage, white soils, or the sky. Shadows will not resemble these flowers if the appearance of each in visible light is also considered. Examples are shown from Mexican heliotropiums and Michigan dune plants.