Nectar Replaced by Volatile Secretion: A Potential New Role for Nectarless Flowers in a Bee-Pollinated Plant Species

Abstract

The presence of nectarless flowers in nectariferous plants is a widespread phenomenon in angiosperms. However, the frequency and distribution of nectarless flowers in natural populations, and the transition from nectariferous to nectarless flowers are poorly known. Variation in nectar production may affect mutualism stability, since energetic resource availability influences pollinators’ foraging behavior. Here, we described the spatial and temporal nectar production patterns of Jacaranda oxyphylla, a bee-pollinated species that naturally presents nectarless flowers. Additionally, we compared nectariferous and nectarless floral disks in order to identify histological, subcellular and chemical changes that accompanied the loss of nectar production ability. For that we used standard methods for light and transmission electron microscopy, and gas chromatography coupled to mass spectrometry for chemical analyses. We verified that 47% of flowers did not produce nectar during the whole flower lifespan (nectarless flowers). We also observed remarkable inter-plant variation, with individuals having only nectarless flowers, others only nectariferous ones and most of them showing different proportions of both flower types, with variable nectar volumes (3–21 μl). Additionally, among nectariferous flowers, we registered two distinct rhythms of nectar production. ‘Early’ flowers produced nectar from 0 to 24 h, and ‘late’ flowers produced nectar from 24 to 48 h of anthesis. Although disks from nectariferous and nectarless flowers displayed similar histological organization, they differed strongly at subcellular level. Nectariferous (‘early’ and ‘late’) flowers exhibited a cellular apparatus typical of nectar secretion, while nectarless flowers exhibited osmophoric features. We found three aliphatic and one aromatic compound(s) that were detected in both the headspace of flowers and the disks of nectarless flowers, but not the disks of nectariferous flowers Although the remarkable variation in nectar availability may discourage pollinator visits, nectarless flowers might compensate it by producing volatile compounds that can be part of floral scent, acting as chemical attractants. Thus, nectarless flowers may be helping to maintain pollination in this scenario of trophic resource supply scarcity. We suggest that J. oxyphylla can be transitioning from a nectar-based pollination system to another resource-based or even to a deceit mechanism of pollination.

A sensory bias overrides learned preferences of bumblebees for honest signals in Mimulus guttatus

“…However, upon reaching E. helleborine flowers, these wasps are able to obtain nectar as a reward, which may motivate them to visit additional E. helleborine flowers and ensure pollination [35]. The bee-pollinated Jacaranda oxyphylla presents both nectarless and nectar-producing flowers, yet only nectarless flowers emitted volatile compounds produced by flowers of other bee-pollinated species [40].…”

Nectar secretion in a dry habitat: structure of the nectary in two endangered Mexican species of Barkeria (Orchidaceae)

“…Furthermore, differences in the distribution of nectariferous and nectarless flowers at the population or individual plant level have also been reported for several natural populations of a range of different angiosperm species. Thus, differences in the nectariferous status of these plants may be related to the conservation of resources, whilst still ensuring the attraction of pollinators ( Guimarães et al, 2018 and references therein; Pacini & Nepi, 2007 ; Thakar et al, 2003 ).…”

“…The nectar of both investigated species of Barkeria had a sweet taste, but also contained lipid-like compounds. Nectar is predominantly an aqueous solution of various sugars, but nearly all nectars investigated to date also contain a variety of additional components, including amino acids, organic acids, terpenes, alkaloids, flavonoids, phenolics, metal ions, oils, and proteins ( Guimarães et al, 2018 ; Nepi, 2014 ; Nicolson & Thornburg, 2007 and references therein; Palmer-Young et al, 2019 ; Roy et al, 2017 ; Wiese et al, 2018 ). Secondary compounds in nectar may play a part in plant-animal interactions by repelling herbivores, regulating nectar ingestion, as well as having a significant role in defence against infection by pathogens.…”

“…Such cells are typical of nectary cells in terms of their small size, their possession of a relatively large nucleus and dense cytoplasm, with numerous secretory vesicles that gather close to the cell wall. Small plastids with a dense stroma and electron-dense plastoglobuli closely resemble those present in nectary cells that secrete heterogeneous material, such as those of Epidendrum and Maxillaria ( Davies & Stpiczyńska, 2019 ; Stpiczyńska, Davies & Gregg, 2004 ; Stpiczyńska et al, 2018 ), as well as those of certain non-orchidaceous genera e.g., Jacaranda (Bignoniaceae) ( Guimarães et al, 2018 ). An atypical feature of the nectariferous cells of Barkeria was the absence of starch in plastids.…”

I’ve been robbed! – Can changes in floral traits discourage bee pollination?

“…caroba flowers, may benefit from not being completely honest, because pollinator visits remain constant. In addition, as another Jacaranda species [48], J . caroba may replenish the nectar throughout the anthesis, which can make flowers worth the risk for pollinators.…”