What a Plant Smells

by Kit Flynn, reprinted from Extension Master Gardener Durham County Newsletter, September 2012

Can plants smell? The question is not as silly as it would seem to be at first blush. According to David Chamowitz, plants definitely smell.3 “Plants obviously emit odors that attract both animals and human beings, but they also sense their own odors and those of neighboring plants” [315-34].

All fruits emit ethylene. Plants can smell ethylene, which we use to our advantage: place a hard avocado and a ripe banana in the same paper bag and the avocado will gradually ripen. Working on this same principle, the ancient Egyptians would slash open several figs, thereby releasing the ethylene, which helped the remaining figs to ripen. Ripe fruits emit a lot of ethylene; consequently the ethylene released by ripening apples on an apple tree will cause other apples on neighboring trees to follow suit.

Cuscuta pentagona, aka dodder, is an interesting plant. Lacking chlorophyll, this is a plant that has no leaves and is entirely dependent upon other plants for its food. If it is to survive, this plant is parasitical out of necessity. When the seed first germinates, it acts as though it were a typical plant with a new shoot sprouting in the air and roots burrowing through the soil. However, without an available plant host it will soon die. As the seedling grows it “moves its shoot tip in small circles, probing the surroundings the way we do with our hands when we’re blindfolded or searching for the kitchen light in the middle of the night” [374-96]. What the dodder is doing is, while searching for a plant, rotating itself in an effort to get near one. Once the seedling has found a leaf of another plant, it quickly moves down until it can attach itself to the stem where it will send “microprojections” into the stem’s phloem allowing it to start tapping into the sugary sap. As the dodder thrives, the host plant will wilt.

Is it just luck that the dodder seedling has found a suitable host plant? Scientists have demonstrated that the dodder will never grow towards an empty container but will faithfully grow towards another plant regardless of whether that plant is in the sun or the shade. Through intricate testing scientists at Penn State have determined that the dodder actually smells the neighboring plant.4

Since 1983 scientists have known that plant communication does exist. Two scientists from the University of Washington noticed that caterpillars “were less likely to forage on leaves from willow trees if these trees neighbored other willows already infested with tent caterpillars.” The difference was that the resistant tree leaves, unlike the leaves on the infested trees, emitted phenolic and tannic chemicals, thereby repelling the caterpillars [422-38]. The trees did not touch one another, nor did they share common root systems, so the two scientists deduced that somehow the infested trees were managing to send out warnings. Subsequent tests came up with the same results in different plant species. It became apparent that the trees and plants were able to send “airborne chemical signals,” a theory now fully accepted by the scientific community.

What does this have to do with the sense of smell? Surprisingly the lima bean supplied scientists with the answers. When a beetle eats a lima bean plant, the plant has two responses: (1) it releases chemicals that repel the beetle and (2) the flowers begin to produce nectar to entice beetle-eating anthropods [456-77]. When beetle-attacked lima bean plants are placed next to beetle-isolated plants, the healthy plants began emitting the same volatile chemicals into the air. Scientists deduced that the isolated plants detected the chemicals on the infested plants—sort of an olfactory eavesdropping. However, when an infected plant, encased in a plastic bag, is placed next to an uninfected plant, the infected plant continued to produce the chemical whereas the uninfected plant did not. As soon as the plastic bag came off, the uninfected plant began giving off the chemical.

So what are these chemicals thrown off by plants when under attack? Those plants under bacterial attack emit methyl salicylate through their leaves while leaves undergoing insecticidal infestation will throw off methyl jasmonate. The former, methyl salicylate, a volatile form of salicylic acid, the precursor of aspirin—which we derive from willows—builds up the plants’ immune systems. “One way to understand the difference between salicylic acid and methyl salicylate is this: plants taste salicylic acid, and they smell methyl salicylate” [535-56]. Remember, the senses of taste and smell are interconnected.

We gardeners live in a cornucopia of plant aromas: lilacs beckon us in the spring while many lilies fill the garden with scents in the early summer. We value those roses that smell like roses, as so many of the hybrid teas have lost their scent. These odors have a purpose as they act as enticements to pollinators. Now scientists know that not only do plants smell but they can also smell other plants.

Pheromones play a role with plants just as they do with animals and humans. In humans pheromones can trigger social responses: it is common knowledge that women living in close proximity with one another tend to share the same menstrual cycle—and we now know that this is triggered by pheromonal cues in perspiration. By the same token “plants detect a volatile chemical in the air, and they convert this signal into a physiological response. Surely this could be considered olfaction” [564-76].
In answer to the question, “can plants smell?” it appears that plants inhabit a world of aromas and, al- though they lack noses, they definitely do smell.

3Chamowitz, David. What a Plant Knows: a Field Guide to the Senses (Scientific American/Farrar, Straus and Giroux, 2012).
Kindle edition, Chapter Two. Future references will be in brackets [ ].

4Unconvinced? Enclose a dodder in one box and a tomato plant in another one, connecting the two covered boxes with a
tube. The dodder will begin growing towards the tube as it senses the nearby presence of the tomato plant. Given a choice
between a tomato plant and wheat, dodder will always choose the tomato plant, even though from a chemical standpoint the two plants have similar odors. Take away the tube and the dodder no longer tries to grow towards the tomato [401-22].