Oct
05
2007

Clover still lucky, getting poisoned is lucky.

Do you feel lucky?: Well, do you?  (photo by Martin LaBar on flickr.com)
Do you feel lucky?: Well, do you? (photo by Martin LaBar on flickr.com)
Did you know that some clover is poisonous? And, by “clover,” I mean plain old white clover, and, by “poisonous,” I mean cyanide poisonous. I didn’t know.

Before you go ahead and swear off eating clover, though, I should reemphasize the word some. It turn out that clover in colder climates is generally non-poisonous, whereas the same species, when found in warmer regions, is very often poisonous. So, Minnesota readers, go on doing whatever it is you do with clover. Mexico readers, however, should just cut out doing whatever they do with clover. That stuff is dangerous!

White clover is native to Europe and Asia, and the climate/cyanide presence relationship has long been observed there. Non-poisonous clover was introduced to the Americas about 300 years ago, and has since re-evolved. The precise mechanism for this has been something of a genetic riddle, one that scientists at Washington University in St. Louis are getting close to solving.

For a clover to be poisonous, WU scientists have found, two separate genes must be active. A gene labeled “Ac” causes cyanogenic glucoside (sugar with cyanide stuck to it, more or less) to be present in clover cells. The glucoside is harmless, until the second gene, “Li,” comes into play. “Li” codes for a particular enzyme in the cell wall, and when the clover (and therefore the cell wall) is damaged the enzyme is released, and it interacts with the cyanogenic glucoside, causing the cyanide to hydrolyze (that’s bad). Non-poisonous clover, it seems, lacks the “Li” gene.

Biologists aren’t sure of the reason for this. One thought is that in colder climates there are generally fewer herbivores, and so clovers can get by without being poisonous. If this is the case, then it would be inefficient for clovers to manufacture the Li enzyme. So they don’t.

Another hypothesis is that clovers don’t want to kill themselves. When it frosts (which obviously happens more in colder areas) cell walls in plants can rupture, which would release the cyanide-activating enzyme prematurely, and cause the clovers to die. How about that?

While we’re on the subject, let’s talk a little about cyanide. Plenty of cyanide compounds are harmless, and even useful - like gerbils. Hydrogen cyanide and the salts derived from it, however, are bad news. They are the staple of crime fiction and Nazi-officer suicides. Inhalation and ingestion are the worst ways to get the stuff in you, although absorption through the skin is possible as well.

Once in your system, “cyanide ions bind to the iron atom of the enzyme cytochrome c oxidase (also known as aa3) in the fourth complex in the mitochondrial membrane in the mitochondria of cells. This denatures the enzyme, and the final transport of electrons from cytochrome c oxidase to oxygen cannot be completed.” I think what this means is that your blood can still absorb oxygen, but your cells can’t. So, in cases of acute cyanide poisoning, your skin gets pink from the high blood oxygen concentration, and you suffer from seizures, apnea, cardiac arrest, and then death.

Plants can generally withstand much higher levels of cyanide than animals, and some, like our white clover or the giant bamboo, are known to contain cyanide compounds naturally. Leprechauns and Golden Bamboo Lemurs feed exclusively on these two plants (respectively), and absorb massive doses of cyanide with no ill effects. Just how this immunity works is not understood, although scientists are hesitant to rule out “magic,” at least in the case of one species.

Your Comments, Thoughts, Questions, Ideas

Anonymous's picture
Anonymous says:

does the warning about cyanide apply also to RED clover? would northern virginia (fredericksburg/stafford area, 22406... planting zone 7, i think) be cold enough to guarantee cyanide free clover? (for both white clover and red clover?) thanks, tom

posted on Thu, 08/18/2011 - 1:28pm
Anonymous's picture
Anonymous says:

I don't know...I'm not one to eat clover on a regular basis...I don't exactly go grazing in my backyard. But that's just me.

posted on Thu, 08/18/2011 - 5:34pm

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