Solanaceae, lectins and alkaloids: the compounds that make some microgreens toxic

This continues the topic of the previous article: it is a serious mistake to assume that every edible plant can be eaten as a microgreen. Many species carry defensive chemistry that thins out or disappears in the mature edible parts but is well established in the seedling.

Here we look at three groups of compounds in more detail — glycoalkaloids in the nightshades, lectins in raw legumes, and fagopyrin in buckwheat — and, in each case, at where the evidence is solid and where it is thinner than the usual telling suggests.

Solanaceae

The Solanaceae family includes some of the most common vegetables there are: tomato, potato, aubergine and pepper. Their fruits are eaten everywhere; their stems, leaves and sprouts have never been part of the traditional human diet, and the reason is the alkaloids concentrated there during the juvenile stage.

One clarification before going further, because it is routinely got wrong. The steroidal glycoalkaloids described below are produced by tomato, potato and aubergine. Pepper (Capsicum) is also a Solanaceae, but it is not a glycoalkaloid producer — its characteristic compounds are capsaicinoids. It still belongs on the do-not-grow list for microgreens (Di Gioia, 2024), just for a different reason.

What are alkaloids and why are they dangerous?

Alkaloids are nitrogen-containing compounds that act as a chemical defence against insects, fungi and herbivores. The main ones in young nightshades are:

• α-solanine and α-chaconine (potato)
• α-tomatine (tomato)
• α-solasonine and α-solamargine (aubergine)

They disrupt cell membranes and inhibit cholinesterase, which is why the reported effects run from gastrointestinal to neurological.

Higher concentrations in young plants

Unlike a ripe fruit, a nightshade seedling has its chemical defences fully switched on, and the compounds are concentrated in the first true leaves, the early stem and the young root — precisely the parts you would be eating. A tomato or aubergine microgreen can carry more of these compounds per gram than a mature leaf of the same plant.

Symptoms of glycoalkaloid poisoning

• vomiting and diarrhoea
• abdominal pain and gastrointestinal irritation
• headache and dizziness
• in more serious exposures, confusion and other neurological effects

Severe cases are rare and are associated with large exposures rather than a garnish, but they exist in the medical literature, and horticultural and food-safety guidance is consistent: no solanaceous plant is suitable for microgreen production.

Lectins in legume microgreens

At first glance legumes look ideal for microgreens: fast, large-seeded, nutritious. The complication is lectins — proteins that bind to carbohydrates on the surface of intestinal cells, and whose function in the plant is defensive precisely because they are toxic or indigestible to herbivores.

The serious one is phytohaemagglutinin, concentrated in Phaseolus vulgaris — red and white kidney beans, and black beans. Mung bean is often lumped in with them, but it is a different genus (Vigna radiata): a good illustration of why this has to be checked species by species rather than by family.

What germination actually does to lectins

This is where the popular account overshoots, and it is worth being accurate rather than merely alarming. Thorough cooking destroys phytohaemagglutinin — that is not in doubt, and raw or undercooked kidney beans have caused well-documented outbreaks. But it is not true that germination leaves lectin levels untouched, or that a sprout is more dangerous than the seed. Measurements following white kidney beans through germination show lectin content declining substantially as the seedling develops.

So the accurate statement is narrower than “raw legume microgreens will poison you”. It is this: the decline depends on species and on how far germination has progressed, there is no threshold you can verify at home, and the compound has a documented history of serious acute poisoning. That combination — real hazard, unverifiable dose — is why Phaseolus beans stay off the microgreen list, and it is a better reason than an exaggerated one.

Buckwheat and fagopyrin

Buckwheat is popular as a microgreen for its colour and flavour, and it carries a photosensitising compound called fagopyrin. Accumulated in the body it can trigger fagopyrism: skin redness, inflammation and lesions on sun-exposed skin, and marked sensitivity to UV light.

Two corrections to the version of this story that circulates widely. First, fagopyrin is not highest in the young sprout — the published analyses put the highest concentrations in the flowers, with leaves next and the sprout lower still. Second, fagopyrism is thoroughly documented in grazing livestock eating large quantities of the plant; documented human cases are rare, and generally involve concentrated extracts or sustained heavy intake rather than a garnish.

That does not make it a non-issue — it means the sensible advice is moderation and awareness rather than alarm, and that anyone with existing photosensitivity has more reason to be careful than most.

A quick summary

Microgreens are a genuinely good addition to a diet, but the species has to be chosen deliberately. Unsuitable ones can deliver glycoalkaloids, active lectins, or photosensitising metabolites that the plant produces for its own defence during early growth.

• Solanaceae — no, including pepper, though for different chemistry
• Phaseolus beans — no
• Buckwheat — yes, in moderation, and the sprout is not the worst part of the plant
• Toxic ornamentals — never

The rule underneath all of it

Know what you are growing, choose species with a documented record as edible leaves or shoots, and do not experiment with plants whose toxicology at the juvenile stage nobody has measured. Where the evidence is thin, we have said so above rather than filling the gap with a warning that sounds authoritative — knowing which is which is the whole point.

References

• EFSA Panel on Contaminants in the Food Chain (CONTAM); Schrenk, D., Bignami, M., Bodin, L., et al. (2020). “Risk assessment of glycoalkaloids in feed and food, in particular in potatoes and potato-derived products”. EFSA Journal, 18(8), e06222. Open the study ↗
• Akiyama, R., Umemoto, N., & Mizutani, M. (2023). “Recent advances in steroidal glycoalkaloid biosynthesis in the genus Solanum”. Plant Biotechnology (Tokyo), 40(3), 185–191. Open the study ↗
• Kozukue, N., Han, J.-S., Lee, K.-R., & Friedman, M. (2004). “Dehydrotomatine and alpha-tomatine content in tomato fruits and vegetative plant tissues”. Journal of Agricultural and Food Chemistry, 52(7), 2079–2083. Paywalled; abstract free. Open the study ↗
• Vasconcelos, I.M., & Oliveira, J.T.A. (2004). “Antinutritional properties of plant lectins”. Toxicon, 44(4), 385–403. Paywalled; abstract free. Open the study ↗
• Savelkoul, F.H.M.G., Tamminga, S., Leenaars, P.P.A.M., Schering, J., & Ter Maat, D.W. (1994). “The degradation of lectins, phaseolin and trypsin inhibitors during germination of white kidney beans, Phaseolus vulgaris L.”. Plant Foods for Human Nutrition, 45(3), 213–222. Paywalled; abstract free. Open the study ↗
• Thompson, H.J., Neil, E.S., McGinley, J.N., & Lutsiv, T. (2025). “Toward Standardized Measurement of Active Phytohemagglutinin in Common Bean, Phaseolus vulgaris L.”. Foods, 14(24), 4247. Open the study ↗
• Centre for Food Safety, Food and Environmental Hygiene Department, Government of the Hong Kong SAR (2023). “Phytohaemagglutinin Poisoning”. Food Safety Focus, 208th issue, November 2023. Open the guidance ↗
• Tavčar Benković, E., & Kreft, S. (2015). “Fagopyrins and Protofagopyrins: Detection, Analysis, and Potential Phototoxicity in Buckwheat”. Journal of Agricultural and Food Chemistry, 63(24), 5715–5724. Paywalled; abstract free. Open the study ↗
• Kreft, S., Janeš, D., & Kreft, I. (2013). “The content of fagopyrin and polyphenols in common and tartary buckwheat sprouts”. Acta Pharmaceutica, 63(4), 553–560. Open the study ↗