Morph Differences and Honeybee Morph Preference in the Distylous Species Fagopyrum esculentum Moench

The relatively low reproductive success of buckwheat (Fagopyrum esculentum Moench, Polygonaceae) is poorly understood. The question arises as to whether this distylous species is pollen or resource limited. We investigated the reproductive biology of buckwheat under controlled conditions in growth rooms and in the field in central Belgium in order to determine whether floral morph and pollination events may affect its reproductive success. In controlled conditions, flowering phenology and flower morphology of the two floral morphs did not differ. However, thrum flowers produced larger and fewer pollen grains and secreted more nectar, with a higher proportion of sucrose, than pin flowers. In the field, thrum flowers were preferentially visited by honeybees, but fewer pollen grains were deposited on their stigmas. However, numbers of pollen tubes growing in styles, seed set, and seed weight did not differ between morphs. Seed set was low under field conditions and did not increase after hand cross-pollinations, suggesting that there was no pollen limitation. These results indicate that factors other than floral morph or pollination events were governing female fertility in buckwheat.