Plants produce volatile signals that attract insects that prey upon herbivores.

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How does N. attenuata attract its insect allies? A hint comes from the smell of a newly mown lawn. This distinctive smell originates from chemicals released from the cut blades of grass. Some of the chemicals released when plants are damaged represent nothing more than the exposure of cell interiors to the air. Others, however, are produced specifically in response to damage. When N. attenuata is attacked by M. sexta caterpillars, it produces volatile signals that attract insects that prey on the eggs and larvae of M. sexta. The adage “The enemy of my enemy is my friend” seems to hold true when it comes to protecting plants from herbivore damage.

Studies first conducted in the 1980s showed that undamaged plants increase their synthesis of chemical defenses when neighboring plants are attacked by herbivores (Fig. 32.16). Although these plants were described in the popular press as “talking trees,” the plants under attack are not altruistically warning their neighbors. Instead, subsequent studies suggest that the undamaged plants are detecting the signals released by their neighbors that are directed at other insects, then ramping up their own defenses. Recent work demonstrates that even the sound of a neighboring plant being chewed on by insects can trigger defensive responses in a nearby plant.

HOW DO WE KNOW?

FIG. 32.16

Can plants communicate?

BACKGROUND Plants respond to herbivore damage by producing chemicals that make their tissues less palatable. Field experiments in which caterpillars were added to target plants suggested that undamaged neighboring plants increased their production of defensive chemicals, whereas plants farther away did not.

HYPOTHESIS Volatile chemicals released from plants that have been attacked by herbivores elicit a defensive response in undamaged plants.

EXPERIMENT One set of plants was placed downwind of other plants in which two leaves were torn, simulating herbivore damage, and a set of control plants was placed downwind of an empty chamber. The concentrations of defensive compounds in all sets of plants were measured and compared.

RESULTS When measured 52 hours after the initial damage, both the damaged plants and the undamaged experimental plants exhibited elevated levels of defensive compounds in their leaves compared to control plants.

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FIG. 32.16

CONCLUSION Volatile chemicals released from damaged plant tissues can trigger the production of defensive chemicals in undamaged plants.

FOLLOW-UP WORK The volatile chemicals released from damaged plants have been identified and shown to attract predatory animals that feed on the herbivores or parasitize their eggs or both. Thus, when plants respond to damaged neighbors, they are “eavesdropping” on signals that likely evolved to attract predatory insects, rather than to warn neighboring plants.

SOURCE Baldwin, I. T., and J. C. Schulze. 1983. “Rapid Changes in Tree Leaf Chemistry Induced by Damage: Evidence for Communication Between Plants.” Science 221:277–279.