The morphology and activity of the extrafloral nectaries in Reynoutria × bohemica (Polygonaceae)

Reynoutria × bohemica is an invasive species causing significant damage to native ecosystems in North America and Europe. In this work, we performed an in-depth micromorphological characterisation of the extrafloral nectaries (EFN), during their secretory and post-secretory phases, in combination with field monitoring of nectary activity over time and the qualitative pool of insect visitors. EFN consist of secretory trichomes and vascularised parenchyma. Polysaccharides, lipids and proteins were histochemically detected in all trichome cells; phenolic substances were detected in parenchyma cells. Our data indicate that all nectary regions are involved in nectar production and release, constituting a functional unit. Moreover, the main compound classes of nectar and their transfer change over time: first, granulocrine secretion for sugars prevails, then eccrine secretion of the lipophilic fraction takes place. Active nectaries are mainly located in the apical portion of the stem during the growth phase (April–May), when we detected the highest number of individuals visited by ants; from mid-August onwards, during flowering, the number of active nectaries declined then ceased production (September), with a concomitant decrease in visits by the ants. The spectrum of nectar-foraging ants mainly included representatives of the genera Formica, Lasius and Camponotus. Reynoutria × bohemica produces an attractive secretion able to recruit local ants that may potentially act as ‘bodyguards’ for protecting young shoots, reducing secretions during the blooming stage. This defence mechanism against herbivores is the same as that displayed by the parental species in its native areas. © 2019 German Society for Plant Sciences, Royal Botanical Society of the Netherlands