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Pontifications on Poison

Being some ramblings on events associated with poisonous plants.

Wednesday 15th June 2011

Yesterday, the woods; today, the open road.

Anyone who has ever walked along a high speed road with no pavements knows that there is, often, no alternative than to take to the verges for safety. No matter that the grass on the verge may be wet and quite long leaving you with wet trouser legs, if vehicles are coming from both directions and look likely to pass just where you are, it would take a brave person to stay on the tarmac and hope the drivers found a solution to the problem of too much solid matter trying to occupy too little space.

Heracleum mantegazzianum, giant hogweed

But, suppose the verge is not as safe as you expect it to be. Driving down the A697, yesterday, I spotted an almost perfect example of Heracleum mantegazzianum, giant hogweed, growing on the grass verge next to the road.

It is entirely coincidental that, yesterday, I was writing about Pastinaca sativa, parsnip, and the unexpected problems it can sometimes cause and, today, it is the best known of the phytophototoxic plants that attracted my interest.

I once read that we only know 2% of what there is to know about plants and I immediately thought that was nonsense. I don’t know how you can estimate the total there is to know about plants if you don’t know it. Leaving aside anything else to do with the man, I always felt a little bit sorry for Donald Rumsfeld when he said something very profound that left him open to being pilloried by the media for being stupid. You may remember that, on Iraq, he talked about the difference between what you know, what you know you don’t know and what you don’t know you don’t know.

We may know about 2% of identified plant species which means we know we don’t know about 98% but what we don’t know is how many plant species remain undiscovered. So, putting a figure on what we don’t know about plants is just silly and takes away from the very real point that there is still a great deal to be discovered about how plants work and why they do what they do.

Heracleum mantegazzianum, giant hogweed

Especially if younger people are in the audience, I’m happy to talk about how much I don’t know and how much ‘we’, that big ‘we’ including the whole population, don’t know about plants. It’s easy to think that all the biggest discoveries have already been made, more so if you watch BBC4 documentaries, but there are still plenty of opportunities for ten year olds to grow up and become world class scientists in the full range of subjects.

I mention all this because we really don’t know why Heracleum mantegazzianum is so much more of a problem than Pastinaca sativa or Ruta graveolens, rue, another plant capable of causing burns from skin contact. We know what the substances are that produce the sensitization of the skin. They are called psoralens. Psoralen is involved in the formation of a range of compounds called furocoumarins (or, sometimes, furanocoumarins) so you will read any of these names as being the cause of the burning. I’m not going to go into the biochemistry of their relationships to each other.

I just wrote ‘cause of the burning’ which is a shorthand for what we know happens. The compounds themselves don’t cause burning. They change the structure of the skin so that it becomes more sensitive to light. It is this change of sensitivity that leads to the burns, assuming the skin is exposed to enough light. This is why the overwhelming majority of people never experience problems with growing parsnips; they don’t handle them at times when the sunlight is strong enough to make the altered skin burn. It is also why there is disagreement over how harmful rue is.

There is, of course, another factor and that is how much of these furocoumarins a particular plant species produces and why. This is where we get into the realm of what we don’t know or, at best, what we only partially know. There has been research indicating that plants produce furocoumarins to combat attack from a root fungus though the researchers themselves said more work was required to be absolutely sure of the mechanism.

Heracleum mantegazzianum, giant hogweed

So, the difference between one parsnip patch and another could be the presence or absence of the fungus. And the difference between species could be a different reaction to the fungus. That could explain why burns from Heracleum mantegazzianum are, usually, much worse than anything caused by Ruta graveolens or Pastinaca sativa.

It is said that giant hogweed was brought to the UK (from which it spread to a small degree to North America) by Victorian plant hunters who thought it would make an excellent structural plant in large gardens. There doesn’t seem to have been any appreciation of the harm it could do, unlike Rhus radicans, poison ivy, which in the 17th century was sent to the UK with clear warnings about its effects. So, and now we’re getting into pure speculation, could it be that the root fungus isn’t found where giant hogweed originates? If that is so, could it be that giant hogweed, having never before been attacked by the fungus is over-reacting and producing far more furocoumarins than other species need to combat it?

If that is the case, might the Heracleum mantegazzianum eventually learn that it doesn’t need to waste effort producing so much defence? And, could it be that a bit of plant genetic manipulation by humans would accelerate that process so that, rather than fighting the losing battle of trying to eradicate giant hogweed, we could simply make a more benign variety dominant in the wild?

We don’t know. But, at least, we know we don’t know.


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