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Plants are, of course, adapted to sunlight. What better defensive method could be found than causing the insects to die of what amounts to lethal sunburn? That is the conclusion that numerous plant species have come to. They produce photosensitizers, in a bewildering variety of at least one hundred different chemicals, all of which make animals liable to fatal sunburn. These photosensitizers are activated by exposure to sunlight, producing highly active free radicals, which overwhelm any protective mechanisms in the animal. They react with proteins, nucleic acids, and fats to form unworkable structures. Any organism that is exposed to sunlight after ingesting the photosensitizer is going to be severely damaged. (That is why the leaf rolling larvae that plague gardeners wrap themselves up during the day.)
Grazing animals suffer the most
All around the world, cattle and other grazing animals suffer from painful or fatal symptoms after eating the wrong sort of plant. Such plants are often declared as noxious weeds. A common example is St Johns´ Wort (Hypericum), which contains the strong photosensitizer, hypericin. Many deaths of cattle and sheep have been traced back to animals grazing on Hypericum plants.
Extracts of Hypericum are widely used (and reportedly effective) anti-depressants, and dermatologists are getting used to seeing cases of skinamage from these pills. Paradoxically, the antidepressant activity of Hypericum extracts seems to be due to a different chemical altogether, so the presence of hypericin in these pills is probably an unnecessary hazard.
Diagnosing photosensitizer injury
There are some common plants with enough photosensitizers to seriously damage our skins. If you really want to live dangerously, work around your flower garden with bare hands and arms. For most photosensitising chemicals, the patterns are pretty much all the same:
- The victim had a wet skin, either because of sweating or from dew- moistened leaves. An hour or two later the skin was exposed to bright sunlight.
- Typical symptoms are described as 'usually a slight rash or irritation, followed either at once or later by the formation of water-blisters, which, bursting, leave pink patches possessing very distinct pores. Later these patches turn dark brown, and persist for a long time (up to one or two years).' These are the classical symptoms of what physicians call phytophototoxic dermatitis. Or, to the layman, 'What a mess!'
- Some photosensitizers cause gastric irritation, nausea, nervousness and depression.
- Finally, anyone who survives the initial symptoms can spend the rest of their lives worrying if their DNA was damaged sufficiently to eventually produce skin cancer.