Flowering, nectar secretion, and structure of the nectary in the flowers of Acer pseudoplatanus L.

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Abstract
Flowering and nectar release in Acer pseudoplatanus were investigated between 2011 and 2013. The micromorphology of the floral and nectary elements was observed using a scanning electron microscope and the anatomy of nectaries was examined by light microscopy. The inflorescence of the sycamore was found to contain flowers, which were functionally male or functionally female. The life-span of the former was on average 5 days, whereas the latter lived on average 4 days. Both types of A. pseudoplatanus flowers had yellow-green nectaries with a similar structure. The nectary tissue formed an elliptical or circular layer located on the receptacle surface between the petals and the pistil. The filament bases were located within the nectary recesses but were not fused with this organ. The nectary margins were slightly undulated. The outer diameter of the nectary was in 3.0–3.25 mm and the thickness of the nectary tissue was in the range of 532–1,023 μm. The nectary of the sycamore flower comprised a single layer of epidermal cells and several layers of secretory parenchyma cells. The average depth of epidermal cells was 16.8 μm. The stomata, which were involved in the secretion process, were arranged unevenly on the nectary surface and were surrounded by six–eight epidermal cells. Visible droplets of nectar accumulated on the nectary surface and thus they were easily accessible to insects. The average weight of nectar from 10 flowers was 16.54 mg (range: 11.0–23.75 mg) and the content of sugars in the nectar was found to be in the range of 23.5–50%, with an average of 37.3%. The calculated weight of sugars in the nectar from 10 flowers was on average 6.11 mg and so the average sugar yield from one sycamore tree was estimated to be 0.65 kg.

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