True nectar or stigmatic secretion? Structural evidence elucidates an old controversy regarding nectaries in Anthurium.

Premise: Floral rewards are essential in understanding floral function and evolution of the relationships between flowers and pollinators. Whether sugars are present in stigmatic exudates in Anthurium and whether it has floral nectaries have remained controversial because of the scarcity of structural studies. To solve these questions, we investigated the floral anatomy of A. andraeanum to elucidate whether (1) tepals are secretory organs, (2) tepals possess a structurally recognizable nectary, and (3) tepalar secretion differs from stigmatic secretion.
Methods: Floral structure was assessed through light and electron microscopy of samples of immature, pistillate, and staminate flowers. The dynamics of the starch reserve was investigated using histochemical tests, and the sugar content in the floral exudates was assessed using thin-layer chromatography.
Results: Sugar analysis did not detect sucrose, glucose, or fructose in stigmatic secretions, but confirmed their presence in tepalar secretions. Stigmatic secretion was produced by secretory stigmatic papillae; tepalar exudates were produced by nonvascularized nectaries in the apex of tepals. These nectaries were characterized by modified stomata and cells with cytoplasm rich in organelles, and a high content of calcium oxalate crystals.
Conclusions: Our results showed for the first time nectaries on tepals and true nectar secretion for A. andraeanum. Stigmatic secretion appears to be a distinct substance, and its often-reported sugar content seems to be a result of sample contamination. Nectar and stigmatic secretions have been often mistaken in other Anthurium species and deserve a revision for this genus.
(© 2021 Botanical Society of America.)