The plant that has a nectar menu to prevent unwanted visitors from interfering with pollination

You’re familiar with plants providing nectar in their flowers for pollinators, but this isn’t the only place they can produce nectar. Some plants have nectaries for insects outside the flower, so they cannot be for pollinating insects. Back in the 19th Century, Federico Delpino classified nectaries as nuptial, helping pollination, or extra-nuptial doing something else. Now in the 21st Century, a team of Brazilian and Italian botanists are taking a closer look at nuptial and extra-nuptial nectaries, particularly when, as in the liana Amphilophium mansoanum, both types of nectary are found in the flower. In their new paper in AoB PLANTS Hannelise de Kassia Balduino and colleagues examined why a plant might have two kinds of nectary in the same flower.

Amphilophium mansoanum‘s flower has an annular (doughnut shaped) nectary around the ovary at the base of the floral tube. This attracts medium and large bees as pollinators and so is a nuptial nectary. There are also nectaries around the fringes of the calyx, the green protective coat of sepals that surround the flower. de Kassia Balduino and colleagues call these extra-nuptial nectaries, emphasising their removal from the pollination process, rather than as extra-floral, which is the description you might see elsewhere. These nectaries attracted ants, but also some cockroaches, wasps and flies.

The botanists were particularly interested to see if the two nectaries were producing two distinct types of nectar to attract two different types of visitors. They tested the nectar with high-performance liquid chromatography to look for differences in sugars, amino acids and specialised metabolites

The nuptial nectar was sucrose-dominated, while the extra-nuptial nectar has more hexoses (glucose and fructose). The nuptial nectar also had high amounts of γ-aminobutyric acid and β-aminobutyric acid which were missing from the extra-nuptial nectar. The extra-nuptial nectar had a richer and less variable amino acid chemical profile.

Another difference the botanists found was in the neuroactive compounds in the nectar. The nuptial nectar contained a theophylline-like compound. In their article, de Kassia Balduino and colleagues suggest that this dopes the pollinator for better performance.

Theophylline alkaloids were previously reported in Citrus flower tissues and nectar (Kretschmar and Baumann 1999), as well as in onion nectar (Soto et al. 2016). Caffeine also influences the long-term memory of bees (Wright et al. 2013), increases foraging frequency, dance frequency, persistence and specificity of the foraging site in honeybee workers (Couvillon et al. 2015). This occurs by blocking the receptors for the neurotransmitter adenosine, which shares structural elements with caffeine and other methylxanthines, such as theophylline (Kennedy 2014).

de Kassia Balduino et al. 2023.

In contrast, the extra-nuptial nectar contains Tyramine. It’s not yet clear what this does to an insect, but it might inhibit some senses such as smell and help protect the flower from ants. de Kassia Balduino and colleagues suggest it may be possible that these nectaries are distractions that keep ants away from the important parts of the flower. They acknowledge would be an expensive defence. They write: “However, if the nectar contains compounds with the potential to deter nectar consumption by animals that are not pollinators, this cost of nectar production might be compensated by the protection it confers.”

“Future studies on nectar trait evolution should consider the possibility that nectar trait evolution is also subjected to selective pressures exerted by interacting animals with ecological functions, other than pollination, contributing to plant fitness.”

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Balduino, H. de K., Tunes, P., Giordano, E., Guarnieri, M., Machado, S.R., Nepi, M. and Guimarães, E. (2023) “To each their own! Nectar plasticity within a flower mediates distinct ecological interactions,” AoB PLANTS, 15(2), p. lac067. Available at: https://doi.org/10.1093/aobpla/plac067.