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

Abstract

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.

Colleters, Extrafloral Nectaries, and Resin Glands Protect Buds and Young Leaves of Ouratea castaneifolia (DC.) Engl. (Ochnaceae)

“…In the studied EFNs of O. castaneifolia, the highest concentration of calcium oxalate crystals coincides with the vascularized portion of these structures. The presence of these crystals is associated with the control of cytosolic calcium levels [53], which seems to be an essential factor for nectar secretion [54]. It is not by chance that the presence of these crystals is frequently reported in the nectaries of different plant taxa [23,[55][56][57][58][59][60][61].…”

First Report of Colleters in Araceae: A Case Study in Anthurium andraeanum Reveals Diverse Mucilage Glands Associated with the Developing Shoot

“…As a result of this great diversity and adaptive capacity, Araceae species would be expected to show a diverse range of secretory structures involved in several distinct plant–environment interactions. Evidence for secretion-mediated interactions in aroids includes the observation of various exudates, such as nectar, stigmatic secretions, and volatile fragrant substances (see [ 3 ] and references therein). However, the source and chemical nature of these exudates are often only indirectly inferred, and the anatomy and secretory activity of the secreting structures still need to be explored.…”

“…However, the source and chemical nature of these exudates are often only indirectly inferred, and the anatomy and secretory activity of the secreting structures still need to be explored. This practice renders the knowledge of secretory systems in Araceae deficient and imprecise, making it difficult to understand interactions mediated by aroid glands or the evolutionary relationship between these structures [ 3 ]. Despite the diversity of glands within the family, records of their occurrence are restricted to a few species and often lack detailed studies.…”

Transcriptomic analysis of deceptively pollinated Arum maculatum (Araceae) reveals association between terpene synthase expression in floral trap chamber and species-specific pollinator attraction

“…No rewards are provided to pollinators within the chamber. A small amount of stigmatic secretions have been observed, but the low (9–12.5%) sucrose content suggests that this fluid is not nectar ( Lack and Diaz 1991 ), and more likely is produced to collect pollen grains ( Paiva et al 2021 ). The following morning, after the female florets are no longer receptive, pollen is released onto the trapped pollinators.…”