Resource availability regulates reproductive meristem activity, development of reproductive structures and seed set in buckwheat ( Fagopyrum esculentum )

Grain yields in buckwheat ( Fagopyrum esculentum ) are consistently low despite a profuse and long‐lasting flowering. The mechanisms underlying this reproductive failure are still largely unknown. In this study, performed in strictly controlled conditions, the possible implication of assimilate availability in the regulation of the reproductive development was investigated in the ‘La Harpe’ cultivar, by manipulating sink/source relationships through excisions of selected leaves and/or selected inflorescences and hand pollinations of selected and precisely identified flowers. Increasing or decreasing source/sink organ ratio in buckwheat, respectively, enhanced or reduced the production of racemes by the shoot apical meristem and of cymes by the racemes, ultimately modulating the number of flowers per plant. The effect on seed production was essentially indirect and related to flower production. Seeds developing on a plant also affected reproductive morphogenesis, limiting flower production. Three stages at which flower development to mature seed may fail have been identified. First, an abnormal morphogenesis resulted in flowers with an undersized and sterile gynoecium. Secondly, after anthesis, numerous flowers with a normal sized gynoecium, were unable to develop a seed after hand pollination. Finally, a small proportion of flowers exhibited a later abortion of the seed. Flower fate is dependent on position in the raceme and time of opening: chance to develop normally and produce a ripe seed is higher for flowers at the base than at the top of the inflorescence. A mechanism, internal to the raceme, regulates flower fate, independently of assimilate availability. This mechanism may be disturbed by drastic reductions in nutrient supply.