Thursday, August 14, 2008

The 2-Stage Chemical Defense of Cow Parsnip

So I’ve got one more “P” to go, before I’m done going on and on about the Paunsagaunt. But here’s the thing: Tomorrow we’re headed out of town again, this time for a week by Lake Tahoe (my 3rd and last trip to California this summer.) It’ll be a great trip, and though I’m really looking forward to it, I feel I have to do at least one more Wasatch-related post this week before skipping out of town. So…. How about another Wildflower-With-A-Cool-Chemical-Defense-System post?

I mentioned a week or so ago that all the mountain wildflowers changed while I was back East. A couple times over the last 2 weeks I’ve poked around up upper Mill Creek Canyon, either on the mtn bike or with Wonder Boy and Twin B (pic right), and the fact is that there still are plenty of flowers, just different ones. These “Generation 2.0” flowers include Fireweed, Fleabane, Aster, Indian Paintbrush, and most notably, Cow Parsnip.

Cow Parsnip, Heracleum maximum, is my new candidate for Most Easily Recognizable Flower in the Wasatch. It’s way, way common at around 7,000 – 8,000 feet, and it thrives in open meadows, near streams and in Aspen glades. It stands about 3-5 feet high, with green fibrous stalks branching into numerous white disk-flower-only composite flowers. It’s easily recognizable from a distance and pollinated by dozens of different bees, flies, beetles and butterflies.

Tangent: The flowers of Cow Parsnip or double composite, meaning that the umbel, or supporting structure of each flower head, branches twice before reaching the actual disk flowers.

Heracleum, or Hogweed, is a genus of about 60 species worldwide that is part of the Carrot Family, Apiaceae, which includes such plants at Parsley, Dill, Coriander and Queen Anne’s Lace. But H. maximum is the only species of Hogweed native to North America. It’s very common, occurring across the continental US except for the Southeast.

Here’s the weird thing about Cow Parsnip: its juices can cause phytophotodermatitis in many people upon skin contact. Phytophotodermatitis, in addition to being an Elmer Fudd-sounding tongue-twister, is a condition of extreme sensitivity to ultraviolet light, such that as little as 10 minutes of subsequent exposure to the sun can cause terrible burning and blistering. The photosensitivity is caused by the presence in Cow Parsnip sap of a class of chemicals called Psoralens.

Tangent: Psoralen was actually used as a tanning accelerator in commercial sun-tanning lotions as recently as 1996. It’s also been used as a treatment for psoriasis.

Nested Tangent: I know the Psoriasis bit because Organic Chemistry Rick used it to combat psoriasis back in the late 60’s or early 70’s. And the amazing thing about Rick’s psoriasis is that it kept him out of Vietnam. Yep, Rick had a lousy lottery number, was in with the army doc for his physical. Doc’s going down the list, asking Rick if he’s ever had Rickets or Epilepsy or fantasies about George Michael (OK I made that one up) and he gets down to “Psoriasis?” and boom- he’s outta there! The ironic thing about this story is that Rick is one of the absolutely toughest, strongest, most indestructible people I know. If I had to pick one of my friends as a prototype human from which to clone an army, I’d pick Rick.

Let’s Take Apart a Molecule

Psoralen (diagram left) is a combination of 2 types of chemicals: Coumarin and Furan. Coumarin (diagram below right), which is the 2 hexagonal rings and the “tail” (C9H6O2) of the Psoralen molecule, has a sweet scent, similar to freshly-cut hay, and has been used in perfumes for over a century. It’s only mildly toxic to humans when ingested, but very toxic to rodents, and a coumarin derivative is a common ingredient in rat poison, where it acts as a super-strength anticoagulant, causing death via internal hemorrhaging.

The pentagonal ring is Furan, a colorless volatile liquid that may be a carcinogen in large quantities.

So the Psoralen-laced sap of Cow Parsnip represents a 2-stage chemical defense algorithm more nuanced than traditional floral chemical defenses such as urushiol compounds in Poison Ivy and Poison Oak. Alone, the sap is harmless. But combined with sunlight, it’s a potent weapon. The photosensitivity lasts for 6 -8 hours. The take-way from all this is:

1- Don’t bushwhack through Cow Parsnip.

2- If you have to bushwhack through Cow Parsnip, do it with long sleeves/pants and try to avoid crushing/breaking stems.

3- If you do get exposed to the sap, cover up, seek shade, keep your skin out of the sun for 6-8 hours.

Like I’ve said a gazillion times before: It turns out chemistry isn’t just some load of boring abstract classroom tedium. It’s all around us, doing freaky things in super-cool ways. Why didn’t they teach it to us this way in school??


libros said...

By sheer luck I came accross this blog and found it interesting, very much so.
Because of the way you look at things and need to know the why of them I would like to ask a question that so far no one has been able to answer: do you know of a plant sap that stains cloths according to the amount of light it receives; it starts by being transparent or white and ends up being deep brown. The catch here that it should not be toxic to humans.

Watcher said...

Hi libros- thanks for stopping by and sorry for the late reply, just saw your comment now.

I'm not aware of a plant with sap with the staining characteristics you describe. Closest I've heard of would be some of the Wisteria vines, whose sap supposedly causes stains in clothing that aren't visible until after you wash the clothing.

Based on your description, I'd suspect some species of Euphorbia, but most of those saps are toxic/irritating to people.

You might ping either Sally or Mary and see if they have an idea. Sorry I couldn't be of more help.