Given a choice between a tomato and some wheat, the dodder will choose the tomato. If you grow your dodder in a spot that is equidistant between two pots—one containing wheat, the other containing tomato—the dodder will go for the tomato.
At the basic chemical level, eau de tomato and eau de wheat are rather similar. Both contain beta-myrcene, a volatile compound (one of the hundreds of unique chemical smells known) that on its own can induce Cuscuta to grow toward it. So why the preference? One clear hypothesis is the complexity of the bouquet. In addition to beta-myrcene, the tomato gives off two other volatile chemicals that the dodder is attracted to, making for an overall irresistible dodder-attracting fragrance. Wheat, however, only contains one dodder-enticing odor, the beta-myrcene, and not the other two found in the tomato. What’s more, wheat not only makes fewer attractants but also makes (Z)-3-Hexenyl acetate, which repels the dodder more than the beta-myrcene attracts it. In fact, the Cuscuta grows away from (Z)-3-Hexenyl acetate, finding the wheat simply repulsive.
in 1983 two teams of scientists published astonishing findings related to plant communication that revolutionized our understanding of everything from the willow tree to the lima bean. The scientists claimed that trees warn one another of an imminent leaf-eating-insect attack. News of their work soon spread to popular culture, with the idea of “talking trees” found in the pages not only of Science but of mainstream newspapers around the world.
David Rhoades and Gordon Orians, two scientists at the University of Washington, noticed that caterpillars were less likely to forage on leaves from willow trees if these trees were neighbors of other willows already infested with tent caterpillars. The healthy trees growing near the infested trees were resistant to the caterpillars because, as Rhoades discovered, the leaves of the resistant trees—but not of susceptible ones isolated from the infested trees—contained phenolic and tannin chemicals that made them unpalatable to the insects. Because the scientists could detect no physical connections between the damaged trees and their healthy neighbors—they did not share common roots, and their branches did not touch—Rhoades proposed that the attacked trees must be sending an airborne pheromonal message to the healthy trees. In other words, the infested trees signaled to the neighboring healthy trees, “Beware! Defend yourselves!”
Just three months later Dartmouth College researchers Ian Baldwin and Jack Schultz published a seminal paper that supported the Rhoades report. They studied poplar and sugar maple seedlings (about a foot tall) grown in airtight Plexiglas cages. They used two cages for their experiment. The first contained two populations of trees: 15 trees that had two leaves torn in half and 15 trees that were not damaged. The second cage contained the control trees, which of course were not damaged. Two days later the remaining leaves on the damaged trees contained increased levels of a number of chemicals that are known to inhibit the growth of caterpillars. The trees in the control cage did not show increases in any of these compounds. Baldwin and Schultz proposed that the damaged leaves, whether by tearing as in their experiments or by insect feeding as in Rhoades’s observations of the willow trees, emitted a gaseous signal that enabled the damaged trees to communicate with the undamaged ones, which resulted in the latter defending themselves against imminent insect attack.
These early reports of plant signaling were often dismissed by other individuals in the scientific community as lacking the correct controls or as having correct results but exaggerated implications. But over the past decade the phenomenon of plant communication through smell has been shown again and again for a large number of plants, including barley, sagebrush and alder. While the phenomenon of plants being influenced by their neighbors through airborne chemical signals is now an accepted scientific paradigm, the question remains: Are plants truly communicating with one another (in other words, purposely warning of approaching danger), or are the healthy ones just eavesdropping on a soliloquy by the infested plants, not intended to be heard?