The scent of botanical pain

I have a cousin who, when she was a little girl, never stopped talking. Her mother, who had a generous ear but naturally had to attend to other things, sometimes had to resort to a few alternatives. One of them was asking her only child to talk to their house plants. My cousin readily did that. 

We generally treat plants as the silent witness to the melodramatic lives of humans. Most of us really don’t know or don’t care about their secret lives. But accepted evidence in science should change that. In particular, we now have evidence that plants can smell. Of course, the most obvious question would be, “what do plants have that serve as noses?” Well, it is not the same as plants as with animals so we rely on scientists who spend a good portion of their lives investigating if plants can smell and how they do it.

Daniel Chamovitz summarized the major studies that started in 1983 that seemed to indicate that plants release gases as a warning when they are injured and other plants around them sense it too and protect themselves. He wrote an article in the May 2012 issue of the Scientific American entitled “What a Plant Knows.” Chamovitz is director of the Manna Center for Plant Biosciences in Tel Aviv University. 

One study he cited that fascinated me was on a parasitic vine called a dodder (Cuscuta). This vine wraps itself around tomato plants sucking out its juices. The study was done by Consuelo de Moraes, an entomologist in Pennsylvania State University. She observed that the dodder vine never grows toward a fake plant or an empty spot but always toward tomato plants regardless of whether she placed the tomato plants in the shade or in the sun. So she devised an experiment where she put a tomato plant in one box and a dodder in another box. She connected the two boxes with a tube. As she guessed, the dodder grew toward the direction of the tube. She even tried to deceive the dodder by placing the extract of tomato on cotton and a solvent of the extract (mostly wheat odor) on sticks. Again, the dodder preferred to squiggle its way toward the tomato extract. This was evidence that at the very least, the dodder could sense the odor of the tomato extract.

Chamovitz cited another landmark study on how plants smell and this time, it was on wild lima beans (Phaseolus lunatus). The researchers were led by Martin Heil in the Center for Research and Advanced Studies in Mexico. It was already known that lima beans, when attacked by beetles, release chemicals in the air and the flower of the lima plant produces nectar that attracts arthropods that attack beetles. So Heil and his team investigated on whether the plants released these odors to warn other plants.

After wrapping attacked leaves in plastic, they opened the wrap in the direction of other leaves within the same plant, toward neighboring plants or away from itself. It turns out that the lima plant releases the gases mainly to protect its sibling leaves but depending on the gas emitted (which could travel in differing distances), other leaves of other plants “leavesdrop” (a term used by Chamovitz) and in effect, respond to the danger as well. When gases reach the leaves of the same plant or neighboring plants, they give a physiological response which could mean other kinds of odors to protect themselves. It is different from the kind of “smelling” the way we humans detect scent. I think that is such a cool experiment. Because I know this now, I pass plants and smell their odors and say “so you fellas must be having a good smelly chat today.”

I still have not done my investigation on the effect of chatty kids on plant growth. My cousin has long left for college and the old house has been sold. My aunt has bigger problems now than worrying about house plants. But until then, we know now that plants can smell in their own way, for their own protection. When you pass them and smell them, you now know it is not for little old you.  

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