Poison Ivy and Berries

Five weeks ago I was out in my black raspberry patch picking the berries that every year Deborah makes into jam. Usually the raspberries are ready in late June, but this “Summer in March” year had sped up the flowering of the plants and the ripening of the berries by a couple of weeks. Unfortunately, though, the very dry weather had greatly reduced the crop. The limited numbers of berries on the canes were small and nearly as dry as raisins. We ended up with about half the usual crop and half the number of jars of jam. We will have to be careful to make this supply last next June!
I was wading through the berry patch in the early morning. Temperatures were pushing 80 degrees already, but I was wrapped in long sleeves, long pants and boots to avoid excessive lacerations by the raspberry thorns. I was also being careful not to have too much contact with another major floral component of my berry patch: poison ivy.

Poison ivy

Poison ivy grows abundantly in almost any habitat affected by humans. It thrives on disturbance and favors the edges and overlaps of fragmented ecosystems. It grows abundantly in my berry patch and in the ecotones between my mowed field and the surrounding woods. 
As I have previously noted about poison ivy  many animals greatly benefit from poison ivy’s presence. Songbirds eat its white berries, and deer browse on its tender leaves. If fact, there is only one animal that has a problem with this plant: humans. In a sensitized human the delayed hypersensitivity reaction following exposure to the urushiol oil of poison ivy can range from an annoying itch to a life threatening anaphylactic shock.
A very logical question to ask about this impact of urishiol on humans is “why?” 
There are scores of websites that talk about this property of poison ivy’s urishiol and relate it to a protective function for the plant. If it is a known cause of such toxic unpleasantness, then people will leave it alone. And, if people leave the plant alone, it will grow and thrive in its ecosystems. 
That is very logical, but there is no long-term, evolutionary relationship between humans and poison ivy that could have led to the selection of this “protective” feature. Further, no other animals generate the allergic reaction to urishiol that humans do. Birds avidly eat its berries (which contain urishiol), and deer and other browsers readily consume its urishiol-rich leaves. How could urishiol be protective when it has no impact on these potential consumers?
I came back in with my small bowl of raspberries and carefully put my socks and pants that had borne the brunt of the poison ivy contact directly into the washer. The urishiol oils can travel on clothing (or gloves or boots or even pets!) and trigger the allergic reaction in any sensitized person with whom they come in contact. I then went to my computer and started reading up on urishiol to try to figure out what it is and how it might benefit the plants that go to all of the trouble and cost (metabolic energy!) to make it. One book that I found through the Penn State library was Edward Frankel’s “Poison Ivy, Poison Oak, and Poison Sumac and Their Relatives.”
Frankel splits poison ivy into two species within the genus Toxicodendron. There is the very abundant climbing poison ivy (Toxicodendron radicans) that is found throughout southern Canada, the eastern United States, Mexico, and Central America, and also across the Pacific Ocean into China, Japan, and Taiwan. There is also the non-climbing form (T. rydbergii) whose distribution overlaps the climbing species and extends even further northward and westward. Also within the genus Toxicodendron are the poison oaks (an eastern and western species) and poison sumac. All of these species contain urishiol oils and all are capable of triggering reactions in sensitized individuals. Poison oak has a somewhat contentious naming history. Many taxonomists felt that the “oaks” were really varieties of the “ivies” and, therefore, should be considered members of the same species. This is the position I describe in my species page about poison ivy. Frankel, though, strongly argues that not only is poison oak different from poison ivy but that there are two distinct species (a western and an eastern poison oak).
But our question here is not concerned with the taxonomy and systematic of these plants. The question concerns what is the function of the urishiol oil in these species?
An Asian relative of these species may point us toward a possible answer.  The Oriental Lacquer tree is a Toxicodendron species that grows in China and Japan.  The sap of this tree has been harvested for many centuries to make a high quality varnish for wooden furniture. The urishiol in this sap is a major constituent of the lacquer and reacts with water and air to form a hard, protective encasing material. It is hypothesized that the urishiol in all Toxicodendrons carry out this function. Toxicondendron leaves are quite delicate in structure and are, thus, easily damaged by even light physical trauma. The urishiol in the damaged leaves could react on air exposure to form a resealing lacquer that helps to reestablish the structural and protective integrity of the leaf.   The plant benefits ecologically and evolutionarily via this urishiol repair system because it is able to put less of its total system energy into the construction of its leaves. It is thus able to make more leaves and grow more rapidly on a specific amount of available energy.
 
And, by the way, the lacquers on these pieces of Chinese and Japanese furniture are capable of generating a “poison ivy” reaction in a sensitized individual.
Science answers questions like “what” and “how” very well. Our original question “why,” though, is less clearly dealt with via scientific thought. Why do humans have such a powerful (and unique) reaction to urishiol? There is only one answer: I don’t know.  
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2 Responses to Poison Ivy and Berries

  1. Daniel Sullivan says:

    If 1-2 days after picking wild blackberries, one breaks out in a bad case of poison ivy/oak then it likely got on the berries too. Does the cooking part of jamming neutralize the urushiol oils in poison ivy/oak that cause allergic reactions people or do those berries and/or jam have to be thrown out?

  2. Mike Mosall says:

    Perhaps there is no evolutionary significance to the human response. Modern chemistry has produced many compounds that can cause contact dermatitis. In most cases, this result was not the purpose of the compound. Having said this, it is not difficult to believe that the great compound producing world of plants could not do the same. Certainly, the offending oils of the poison ivy plant confer survival advantages to the plant but these advantages may have nothing to do with humans. In short, the reaction in humans may be purely incidental.
    Thinking in the other direction may produce a more plausible explanation. Perhaps the human reaction to urushiol may confer an evolutionary advantage to humans.

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