THE ROLE OF PETAL CELL-SHAPE AND PIGMENTATION IN POLLINATION SUCCESS IN ANTIRRHINUM-MAJUS

The nature of the selective pressures which have resulted in the conic al-papillate shape of the cells of the adaxial epidermis of many petal s has been a matter for considerable speculation. One suggestion is th at this shape focuses light within epidermal cells resulting in an inc rease in the amount of light absorbed by the floral pigments, intensif ying the colour of the petals and possibly enhancing their attractiven ess to potential pollinators; another is that conical cells aid pollin ator orientation on the flower, either visually or when touched. The r ecent identification of a mutation at the MIXTA locus of Antirrhinum m ajus (which blocks the formation of conical petal cells) has allowed u s to test this hypothesis. We report the results of field experiments indicating that, where pollinator number limits seed-set, flowers with conical epidermal cells receive more pollinator attention than do tho se with flat cells. Through the use of double mutants we have examined whether preferences for flowers with conical cells operate through th e perception of flavonoid pigments. We have also examined the appearan ce of flowers with and without conical cells under ultraviolet light t o determine whether differences in absorption or reflectance of light at these wavelengths may influence pollinator preference.