|Color shows a change in biological activity in plants on the right, caused by a gas emitted by cut plant on the left.|
To see this effect in a magical-looking (but scientific) way, take a look at this short BBC video.
(The image above is from the video, and I took it from the BBC website. Thanks to Las Pilitas Nursery for sharing this BBC video on FaceBook, sending me off on this tangent!)
People have also demonstrated this effect using California sagebrush cuttings. I read about this experiment, done at the University of California at Davis in 2009, in this Science Daily article These scientists also established that herbivores did indeed react to the changes in the neighboring plants (and not directly to the gas given off): "Choice and no-choice experiments indicated that herbivores responded to changes in plant characteristics and were not being repelled directly by airborne cues released by clipped individuals."
They also found that " Plants with root contact between neighbors, but not air contact, failed to show this response." - I wondered why not?
And I found the answer in this abstract of a scholarly article, Plant-to-Plant Communication Mediating In-Flight Orientation of Aphidius ervi, in the Journal of Chemical Ecology. These folk found that when a broad bean is infested with aphids, it changes its chemistry to attract a specific aphid parasite, A. ervi. - This in itself is amazing. Also -- the plants' non-infested parts send out a volatile chemical signal that stimulates nearby bean plants to produce the same attractant. Wow!
And about the root contact: They also found out that "This change was not observed when root contact was prevented among plants that had their aerial parts in close proximity, suggesting that an exudate from the roots of the infested plant may cause the induction of the attractive volatiles in uninfested plants." And they confirmed this by another experiment - So that's why. The plants switch to another type of signal via the roots if that is available -- aren't plants amazing!
BTW, I'm sorry not to list the actual scientists responsible in each case - But that's science for you. It's all about the results, not who got them! You can see all the worthy pioneers' names if you click the links.