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How plants respond to positive vibes: 'Talking' mechanism is a mystery


The University of Western Australia's Monica Gagliano studies how plants communicate with each other.

Studies show that basil gives a boost to chili peppers, while fennel is a real bummer. The effect has been seen even when the plants are sealed off from each other with sheets of black plastic. So does that mean that the plants are "talking" to each other through subtle vibrations? That's the kind of talk that sparks a debate — not between the plants, but between humans.

The latest study, reported in the open-access journal BMC Ecology, looked at potential communication between basil plants and chili pepper seeds. It's one of a series of experiments conducted by Monica Gagliano and Michael Renton of the University of Western Australia.

"Our results show that plants are able to positively influence growth of seeds by some as-yet unknown mechanism," Gagliano said in a news release from BioMed Central, the journal's publisher. "Bad neighbors, such as fennel, prevent chili seed germination in the same way."

Fennel plants release chemicals into the air and soil that are detrimental to most other plants, including chili peppers. Last year, Gagliano and her colleagues set up mini-gardens to study the interaction between the plants more closely. They were surprised to find that chili seeds germinated more quickly when the fennel plant was sealed off with plastic to block the transfer of those nasty chemicals. It was almost as if the baby chilis sensed that a villainous plant was nearby, and grew up faster so they'd have a better chance of fending off the fennel.

The new study looks at the flip side of plant interaction: Unlike fennel, basil is a "good neighbor" for chili plants because basil plants release chemicals that discourage weed growth. Gagliano and her colleagues found that to be the case for chili seedlings. The seeds germinated at a higher rate, even if the basil plant was sealed off with the black plastic. That led Gagliano and Renton to conclude that the seeds could still sense the presence of a friendly plant when they couldn't get the standard chemical signals.

How could this be?

"We believe that the answer may involve acoustic signals generated using nanomechanical oscillations from inside the cell which allow rapid communication between nearby plants," Gagliano said in the news release.

That surmise seems to fit with other findings on plant communication. Corn roots, for example, give off regular clicking sounds in the range of 220Hz (which corresponds to an A below middle C). Gagliano and her colleagues found that when young corn roots are suspended in water, they tend to lean toward the source of a continuous 220Hz tone transmitted through the water. The researchers suggested that acoustic signals could knit plants into an underground network of friends and foes.

But as Gagliano points out, no one has yet identified the precise mechanism by which one plant hears what another plant is saying. That's one of the reasons why other researchers haven't wholeheartedly embraced the idea that plants are talking to each other.

"Although the idea of plants communicating by sound is intriguing, there is still a long way to go before we know whether, and if so to whom, the woods sing!" the University of Leiden's Carel ten Cate wrote last December in the journal Behavioral Ecology.

Duke University's Dan Johnson, who is studying how trees respond to drought, said it's "too early to tell" whether plants truly respond to each other's sounds.

"We have been detecting these acoustic signals for almost 50 years," Johnson told NBC News. "The idea of using those signals for communication is incredibly interesting, and there is potentially some growing support for it. But we're a good ways away from strong support for acoustic signaling between plants. ... I'm glad that somebody's working on it, but I think it's too early to say."

More about plant communication:

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.