Valérie Cawoy

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.

Comparative reproductive biology in co-occurring invasive and native impatiens species

Two alien Impatiens species are spreading in Europe and share habitats with the native Impatiens noli-tangere. We studied their reproductive biology to estimate which traits facilitate invasiveness. Flower morphology was examined, insect observations were made, and hand pollination treatments were performed. Floral biology differed among species. Impatiens glandulifera and I. noli-tangere presented large quantities of sucrose-dominant nectar, contrary to Impatiens parviflora. The latter had high autonomous selfing ability (81.4% fruit set) linked to complete self-compatibility. No inbreeding depression was detected for this species. The second alien, I. glandulifera, showed lower autonomous selfing (9.3% fruit set), with high self-compatibility and low inbreeding depression (). No evidence of autonomous selfing was found for the native, which presented the lowest self-compatibility and the highest inbreeding depression (). However, all species can be considered self-compatible. Impatiens glandulifera flowers were visited by insects up to 250 times during their life span, whereas I. noli-tangere and I. parviflora flowers received fewer than seven visits. Both exotics present traits facilitating reproductive success. Impatiens parviflora exhibits autonomous self-pollination, whereas the other species possess attractive traits with respect to insect pollination. On the other hand, the native, unable to self-pollinate, is poorly attractive to pollinators. The high fecundity of the aliens seems to contribute to their invasiveness.

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.

Floral visitors and the importance of honey bee on buckwheat (Fagopyrum esculentum Moench) in central Belgium

Summary Visitors to buckwheat (Fagopyrum esculentum Moench) flowers were studied in central Belgium during 2 months (July and September) over 2 years (2001–2002). Forty-nine different insect species, belonging to 18 families, were recorded. Over both years, species from the orders Diptera and Hymenoptera were the principal visitors. Hymenoptera were mainly represented by honey bees (Apis mellifera L.; 18.5 – 51.8% of total visitors), while Diptera were represented by syrphid flies and several other families. Variation in the visitor guild occurred at the beginning and the end of both Summers, as the proportion of honey bees was significantly higher in July than in September. No variation was detected in the relative abundance of insects between years. Honey bees appeared to be the most numerous visitors. Some syrphids and other Diptera species could, however, act as co-pollinators because of their high relative frequency and activity. In experimental cages, the effectiveness of honey bee pollination was good as these insects deposited compatible pollen on a majority (90 – 93%) of flowers without any discrimination between floral morphs.