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Terpenes are among the most widely distributed secondary compounds in plants. They are responsible for the pleasant odors given off by pine trees and herbs, and they are the principal ingredients in perfumes.  But did you know that at high concentrations they can be toxic and, thus, are important defenses against herbivores and pathogens.
Terpenes are often major ingredients in so-called "essential oils" produced by plants.   These oils are segregated from the plant in containment structures specially designed for them (such things as oil tubes and glands).  These containment structures may simply be storage areas or they might also protect the plant from its own toxins. 
To explore whether these compounds are potentially autotoxic, we tested a variety of terpenes present in foliage of three species of plants--wild parsnip, rough lemon, and parsley.
The assay was simple.  We applied a droplet the size of a pin on of each compound on the surface of the leaf and then pricked the surface of the leaflet covered by the droplet with a needle.  We then monitored toxicity as the decline in photosynthesis, as measured by chlorophyll fluorescence imaging.
The animation below shows what happens when a tiny droplet of a monoterpene is allowed to enter the leaf.  The needle used to make the pinprick will appear twice.  The first time it is passed through the terpene droplet (at the center of the animation).   The second time (to the upper left) is a control prick without terpene.
citrus tox.gif (1356023 bytes) Within 240 seconds, photosynthesis in an area of about 1 square centimeter of this rough lemon leaf is destroyed by the monoterpene.  The damage is permanent, as the next day this area is brown and crispy.
A less dramatic, but still visible effect can be seen when the essential oils present inside a parsley leaf are released with a razor blade cut:

You will see two razor blade cuts.  The first one opens the pressurized oil tubes and releases the oil.  The effect of the second razor cut (above the first one) is less dramatic because the oil tubes were depressurized by the first cut.  The last frame of the animation shows an overlay of the veins in the leaf.  It is clear that the oil damage (red and orange areas) corresponds with the cutting of veins, and it is inside the veins that the oil tubes are located.

parsley autotox animation.gif (1442259 bytes)

 

Learn how caterpillars can lessen their exposure to these toxins as well as the furanocoumarins associated with them

 

last updated 9/16/2005