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Today we have a text-and-photo essay by Athayde Tonhasca Júnior, emphasizing two of his favorite themes: history and pollination.   Athayde’s words are indented, and you can enlarge the photos by clicking on them.

Help yourself, but don’t overdo it

Throughout the history of the Roman Empire, women had hardly any employment opportunities. Locusta was an exception. This immigrant from Gaul was celebrated for her knowledge and technical skills: Agrippina, Emperor Claudius’ wife, and Agrippina’s son, Nero the unhinged, were among her clients. Nero even hired Locusta as his advisor and as a tutor for young apprentices who could absorb her expertise in an occupation in high demand in the Empire. Locusta was a professional poisoner.

From emperors to slaves, affluent merchants to muleteers, poisoning was a convenient and effective way to dispose of a difficult spouse, secure an inheritance, settle scores with an enemy or encourage an aging relative to free up some space at home; the practice was common enough that praegustator (food taster) guilds arose among slaves and freedmen (Kaufman, 1932). Women were particularly skilled at the craft, partially as self-defence in a violent and radically male-controlled society.

Locusta testing poison on a slave, by Joseph-Noël Sylvestre (1847–1926) © Bridgeman Art Library, Wikimedia Commons:

Locusta had an arsenal of poisonous plants at her disposal such as deadly nightshade (Atropa belladonna), henbane (Hyoscyamus niger) and hemlock (Conium maculatum) (Cilliers & Retief, 2000). But none of these handy tools were as reliable as aconite, aka monkshood or wolfsbane (Aconitum napellus), which the poet Ovid called ‘the mother-in-law’s poison’. Like all the 250 species in its genus, aconite is loaded with aconitine and related alkaloids that cause all sorts of neurological and cardiovascular disorders. Besides being a favourite of ne’er-do-wells and sorcerers for centuries, the plant has been on herbalists’ shelves as a local anaesthetic, tonic for the heart and for other pharmacological uses. These applications are benign but exceedingly risky, as 1 g of aconite biomass may despatch a patient to the underworld: this plant is about 100 times more lethal than strychnine.

A deceptively sweet-looking aconite. But the root of the plant’s name reveals its danger: akonitos (without dust), short for ‘without the dust of the arena’, implies biting the dust without a struggle © Llez, Wikimedia Commons:

Gardeners attracted to aconite’s violet-blue flowers may resent the plant’s mean streak, but for the plant, poisoned people are collateral damage. The alkaloids stuffing the plant to the gills are a defensive weapon against leaf munchers, root borers, seed predators and other enemies. Chemicals such as aconitine are known as secondary metabolites: they are energetically expensive to produce and have no role in plants’ day-to-day physiological processes such as photosynthesis, respiration or growth. Yet, they are vital for survival and reproductive success by repelling or killing herbivores that target the plant. But this form of chemical warfare has a drawback: secondary metabolites may leak into nectar and pollen, putting flower visitors at risk. In the case of aconite, nectar has low levels of alkaloids, but pollen is loaded with them. Understandably, aconite pollinators – mostly bumble bees – avoid the powdery stuff, so as not to end up dead. Such reluctance is not good for plant reproduction, but aconite has a cunning plan.

All Aconitum spp. flowers are hermaphrodite and dichogamous, that is, their male and female reproductive organs mature at different times. The male phase, i.e., the period when only the male bits are mature, occurs first and lasts for 5 to 6 days; the female phase, when the male organs wither and the female ones are receptive to pollen, lasts 1-2 days. This setting nudges visitors to explore male-phase flowers first and for longer, then hopefully transport pollen to a female-phase flower. To overcome bees’ unwillingness to play along, male-phase flowers produce more scents and over four times more nectar than the female-phase ones. This nutritional bribe persuades bumble bees to drop by for a sip of nectar, which is relatively harmless, and leave the toxic pollen alone. But even if not purposedly gathering pollen, a bee is unlikely to depart from a flower without some attached to its body. That’s all the aconite needs: the few pollen grains stuck to a bee are more than sufficient to assure pollination when the bee then takes nectar from a female-phase flower (Jacquemart et al., 2019).

Help yourself to a drink, but don’t take pollen away. Or else © Franz van Duns, Wikimedia Commons:

It’s worth taking a moment to appreciate aconite’s endeavours. Like most flowering plants, it needs insects to transfer its pollen and get fertilised. To attract them, it offers rewards in the form of protein (pollen) and sugars (nectar). But the plant can’t give these goodies away willy-nilly because they are metabolically expensive. Aconite prevents the excessive harvesting of pollen by spiking it with a toxic alkaloid, but promotes the involuntary taking of some pollen grains by selectively stocking male-phase flowers with better nectar.

The aconite story is not an isolated case. Many plants regulate pollen consumption with toxins, while hermaphrodite species are often gender biased regarding nectar volume and quality (Carlson & Harms, 2006), which affect the number and duration of pollinator visits, and the number of flowers visited (Parachnowitsch et al., 2019). Natural selection doesn’t manifest itself much more gloriously than through the intricate arrangements between toxic plants and their pollinators.

The pollen stuck to this bumble bee’s corbiculae (pollen baskets) will end up as food for baby bees; from the plants’ perspective, it’s a loss of resources. Judiciously dabbing it with poison could have prevented that © Tony Wills, Wikimedia Commons: