Floral ultraviolet patterns and flavonoid pigments for six taxa of Viguiera series Viguiera

Floral ultraviolet (UV) patterns and the distribution of flavonoid pigments were studied in relation to ligule anatomy for six taxa of Viguiera series Viguiera. This investigation used data obtained from UV photography, flavonoid chemistry, scanning electron microscopy and light microscopy.

UV photographs showed three distinct UV absorption-reflection patterns to be present on the ray florets of this series. Ligules for three taxa, V. deltoidea var. deltoidea, V., deltoidea var. Parishii, and V. dentata had UV-absorbance restricted to the proximal portion of the ligule while the distal region was UV-reflecting. The ligules of two taxa, V. laciniata and V. reticulata, were completely UV-absorbing. The ligules of V/ potosina were unique for this group, being entirely UV-reflecting, suggesting that V. potosina may not belong in this series.

Four general classes of compounds were found to be present in the ray flowers of Viguiera: flavonoid aglycones, flavone glycosides, flavonol glycosides, and anthochlor pigments. The glycosides and anthochlors are responsible for UV "nectar guide" formation. Variation for floral flavonoids divided this series into two groups. One group consisting of V. deltoidea, V. laciniata, and V. reticulata was characterized by the presence of quercetin 3-0-methyl ether compounds, the chalcone/aurone pair, coreopsin/sulfurein, and the presence of fla vonoid aglycones. The other group, composed of \/. potosina and V. dentata, lacked quercetin 3-0-methyl ether marker compounds and flavonoid aglycones. In addition, the ligules of V. dentata contained two aurone/chalcone pairs, while the ligules of V. potosina lacked anthochlors. Variation for flavonol glycosides was also detected for V. deltoidea, suggesting that the taxonomy of this species needs to be re-evaluated.

Investigation of flavonoid distribution in relation to the functional anatomy of ligules disclosed the presence of adaptive features responsible for UV-patterning in Viguiera. Flavonoid glycosides and anthochlor pigments were confined to papillate epidermal cells occurring in the solution of the vacuole. Flavonoid aglycones, when present, were localized in glandular trichomes on the abaxial ligule surface and did not contribute to UV patterning. SEM confirmed the presence of glandular trichomes and papillate cells on the ligule epidermis and disclosed other epidermal features associated with the absorption and reflection of light. Structural aspects of the ligule, in conjunction with floral flavonoid distribution, play an important role in the formation of UV "nectar guides" in Viguiera. It is discussed that UV-absorbance patterns in Viguiera may be important for reducing UV-B radiation to the reproductive organs, and may also serve as recognition cues for flower-constant pollinating insects, thus reducing cross-pollination between visibly similar species.