Sunflowers and Honeybees: A Study of the Mutualistic Relationship from a Biochemical and Morpho-Anatomical Perspective

The mutualistic relationship between Helianthus annuus (Asteraceae) and Apis mellifera is reflected in their co-evolutionary adaptations. The corolla morphology and pigmentation of sunflowers help form a target pattern under UV, recognizable by bees. While collecting rewards, bees cross-pollinate the disk florets. Morpho-anatomical co-adaptations of the sunflower and honeybee were studied with LM, SEM, and CLSM. This study reports for the first time the presence of one to three rows of transitional papillae on stigma, which may function in protection of the receptive stigma from self-pollination. A model of the cross-pollination of sunflower inflorescence by honeybees is presented. The chemical characterization of flavonoid pigments in disk florets, known to contribute to the target pattern of the inflorescence, accomplished with chromatographic and MS techniques, revealed the presence of luteolin and pelargonidin pigments. This is the first report on the presence of luteolin and pelargonidin in sunflower disk florets. Results of this study will contribute to the metabolomics of the phenylpropanoid pathway in H. annuus in addition to enhancing understanding of mutualism and biosemiotic relationships between flowering plants and pollinators.