Morphology of nectaries and biology of nectar production in the distylous species Fagopyrum esculentum.

BACKGROUND AND AIMS: The mechanisms of floral nectar production in buckwheat (Fagopyrum esculentum, Polygonaceae), a distylous pseudo-cereal, have received relatively little attention, prompting an investigation of the factors that regulate this process. The aim was to perform a refined study of the structures that secrete nectar and of the internal and external parameters influencing nectar volumes and sugar concentrations. METHODS: In order to control environmental parameters, plants were cultivated in growth rooms under controlled conditions. The structure of nectaries was studied based on histological sections from flowers and flower buds. Nectar was extracted using glass micropipettes and the sugar concentration was measured with a hand refractometer. Sugar concentration in the phloem sap was measured using the anthrone method. To test the influence of photosynthesis on nectar production, different light and defoliation treatments were applied. KEY RESULTS: Unicellular trichomes were located in the epidermis at the ventral part of eight nectary glands situated on the flower receptacle alternately with stamens. Vascular bundles consisting of both phloem and xylem were identified at the boundary between a multilayered nectary parenchyma and a sub-nectary parenchyma with chloroplasts. A higher volume of nectar in thrum morphs was observed. No other difference was found in morphology or in sugar supply to inflorescences between morphs. Nectar secretion was strongly influenced by plant age and inflorescence position. Nectar volumes were higher in the upper inflorescences and during the flowering peak. Light had a dual role, (1) acting directly on reproductive structures to trigger flower opening, which conditions nectar secretion, and (2) stimulating photosynthetic activity, which regulates nectar accumulation in open flowers. CONCLUSIONS: In buckwheat, nectar is secreted by trichomes and probably proceeds, at least in part, from phloem sap. Nectar secretion is strongly influenced by floral morph type, plant age, inflorescence position and light.

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.

Inflorescence structure and control of flowering time and duration by light in buckwheat (Fagopyrum esculentum Moench).

Morphogenesis of the reproductive structure of buckwheat and the impact of light conditions on flowering time and duration have been investigated using the variety 'La Harpe'. Inflorescences were initiated acropetally, in leaf axils, by the shoot apical meristem until its arrest of functioning which was accompanied by the abortion of the last inflorescence produced. The buckwheat inflorescence is a compound raceme that produces laterally flowered cymose clusters, the number of which was affected by the position of the inflorescence along the main stem. Similarly, the number of flowers in a lateral cluster was dependent on the inflorescence's position on the stem. The development of each inflorescence was stopped as its meristem stopped functioning and, in a situation reminiscent of the shoot apical meristem, the latest initiated cyme aborted. The development of each cyme was also terminated with the abortion of a few young flowers. The variety 'La Harpe' is a facultative short-day plant: the number of nodes generated before the initiation of the first inflorescence and the number of days from sowing to macroscopic appearance of this inflorescence were reduced in 8 h days as compared with 16 h days. The number of inflorescences, and thus flowering duration, was also strongly reduced by short days. It was unaffected by light irradiance in 8 h days while, in 16 h days, it was prolonged when light intensity was increased, suggesting the interaction of two different mechanisms for its regulation. Buckwheat is a distylous species, but inflorescence structure and flowering behaviour were not affected by floral morph.