I have a grapefruit tree growing inside our apartment. It is approaching its 30th year since, on a whim, I threw two seeds from a grapefruit I was eating into a pot on our kitchen windowsill. Whatever was supposed to be in that pot had failed to germinate. The grapefruit seeds, however, sprouted and began to grow to become an entangled bush that I moved outside in the summer and back indoors every winter. It got blown over in storms and was often neglected. It survived and began flowering near its 20th year. In its 29th it produced its first grapefruit.
The one constant through all the years of tending to this 2.2 metre (over 7-feet tall) tree is that I have talked to it when I pull up the blind in our den each morning, or when I water and prune it. I do this because, once upon a time, a green thumb friend of mine told me that plants like it when you talk to them.
My friend's notion is backed up by research. A 2008 article written by a researcher at Penn State University quotes King Charles III, who, in 1986, before becoming king, discussed his penchant for gardening and admitted, "I just come and talk to the plants, really. Very important to talk to them; they respond."
Charles, ruler of the United Kingdom and the Commonwealth, apparently isn't alone in talking to plants. A book published in 1848, authored by Gustav Fechner, a professor at the University of Leipzig in Germany. Fechner, regarded as the founder of the field of psychophysics, the study of how physical stimuli influence perception and sensation, described how this works in plants. His theory led to a popular belief that plants can hear and respond to sound.
Rich Marini, the head of the Horticulture Department at Penn State, suggests there is sufficient evidence to support Fechner. He notes that "Wind or vibration will induce changes in plant growth. Since sound is essentially vibration, my guess is that vibration is causing a response."
Other researchers have discovered that plants exposed to windy conditions produce ethylene, a hormone that causes slower growth and a thickening of the trunk, stems and branches to withstand damage.
Additional research has demonstrated that plants respond to music when played at 70 decibels, about the level of a normal conversation. So, maybe talking to my grapefruit tree is a positive plant influencer.
Different research notes that seeds germinate faster when exposed to sound levels of 92 decibels, as loud as standing next to someone blowing a car horn. In other words, shouting loud enough can get the seeds you just planted to start sprouting faster.
There is even research that shows pollinators, such as bees, when flitting around flowers, stimulate nectar production.
Other generated sounds can accelerate root growth and budding.
All these observations, however, do not explain how a grapefruit tree is listening to or understanding me. After all, trees don't have ears or a nervous system. So, what do they have to detect sound?
Sound is vibration, and vibration is the key.
Plant cells possess mechanoreceptors with mechanosensitive ion channels that respond to touch, pressure and vibrations. The best demonstration of how mechanoreceptors work is seen in a Venus Flytrap, with mechanosensory hairs that trigger a rapid response to touch.
What about airborne sounds? As sound travels through air, it causes molecules to vibrate, leading to subtle pressure changes. Louder sounds create larger pressure changes. These changes are subtle, but plant mechanoreceptors can detect them. Leaves, flowers and roots can act as receiving antennas. So, it's not the conversation to which the plant responds; it is the vibration. That means you can literally play the Beach Boys song, "Good Vibrations," to help your house plants grow while fulfilling a need to hear one of my all-time favourite rock songs.
The lack of a nervous system doesn't stop plants from feeling things. Instead, through changes to its cell chemistry, it can both detect and send messages in response to external stimuli.
This ability has led researchers at Simon Fraser University in Vancouver, British Columbia, to invent a robot with artificial intelligence (AI) that is mobile and capable of listening to the subtle physiological changes emanating from tomato plants that indicate water stress. The robot is sensing plants asking for water. It detects and monitors plant electrophysiology, or EP, the tiny electrical signals emanating from plants when stressed.
EPs are electronic pulses generated by physiological changes. As the robot moves through a greenhouse of tomato plants, its onboard EP detector, contained within a Faraday cage to shield it from electrical signal interference, can detect whether a plant needs water or not.
The technology can tell the difference in water stress levels using a built-in scalogram that turns the EP into a visual map of the greenhouse, spotting plants that are overwatered or dry. The robot can detect a plant's need for water even when there is no visible sign of wilting. The technology is likened to a Holter monitor for plants. Holter monitors are portable medical ECGs used to detect arrhythmias, heart rhythm disturbances in patients. In this case, the robot is detecting the EP heartbeat and the voice of plants asking for a drink.
On a macro level, this technological advance using robots and AI could revolutionize irrigation systems and optimize water distribution in drought-prone areas and greenhouses.