Wednesday, July 2, 2008

Poison oak and's all relative.

I am a botanist by education (and work experience), and so I have always enjoyed studying the relationship of one plant to another. In trying to get non-botanists to be interested in plants, a great teaching tool is to relate plants commonly used by people (food, landscaping, etc.) to native plants that one will see in natural areas. Poison oak is a great example. Hopefully those of you that are frequent enough visitors to the Puente Hills and other natural areas will recognize the attached photo as the oily, reaction-causing leaves of poison oak (leaves of three…leave it be!).

Here is a bit of background on plant taxonomy.....two closely related species of plants occur in the same genus (genera). For example, white sage, black sage, and purple sage are all in the genus Salvia. Similarly related genera are placed in the same plant family. Plants of the genus Salvia, along with other “mints”, are placed in the plant family “Lamiaceae”. Getting back to poison oak....this plant is placed in the family “Anacardiaceae”. Other representatives of this family that occur in the Puente Hills include native plants such as lemonadeberry, laurel sumac, and skunkbrush, and non-native plants such as the Peruvian pepper tree. Many species in this plant family contain a type of oil called “urushiol”, which is particularly abundant in poison oak (genus Toxicodendron). This is the oil, which for people that are sensitized to, will cause the characteristic delayed allergic reaction. Up to 30% of people have no allergic reaction when in contact with the oils from poison oak; however most will become sensitized to it with repeated exposure.

What many people find interesting is to know what common food items occur in the same plant family, and are therefore more closely related to poison oak. These include the pistachio, cashew, and mango. The urushiol oil is found in the nut shell of the cashew fruit (though not in the cashew fruit itself), and in the mango plant sap as well as the skin of the mango fruit. With the respect to the mango plant, breaking the vine of a green mango fruit will produce a fine aerosol of sap that causes severe reactions in previously sensitized individuals. The mango fruit skin has a lesser concentration of urushiol, though still with some potential for the delayed skin rash with more sensitive people. However, the edible part of the mango fruit itself does not cause these reactions, and is one of the most popularly used fruits worldwide; used for food, juice, flavoring, fragrance, and color. The mango is an excellent overall nutrient source, including dietary fiber, carbohydrates, and antioxidant vitamins.

Here is a great game to play. The next time you are eating a meal, think about each plant product that you are eating, where it comes from, and what it might be related to. Is it a fruit? If not, what part of the plant does it come from? Is it a root? A stem? A leaf? Dinner will never be the same again!

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