Do Plants Have Senses?

Do plants have senses?

Yes, kind of. To make things interesting let us invoke the five traditional senses we normally think about when talking about people: Sight, hearing, taste, smell, and touch. How do plants compare?

While plants can sense their environment, it is still quite different than how people and animals perceive the world. According to the American Psychology Association, sensation is defined as “The process by which stimulation of a sensory receptor gives rise to neural impulses that result in an experience, or awareness of, conditions inside or outside the body.” Plants do not have neurons and therefore do not experience the world as we and other animals do. Instead, plants have evolved rudimentary sensing mechanisms that in some ways are similar to how animals sense the environment. Here, we will explore some of those similarities.

Sight

We see light in the form of a small portion of the electromagnetic spectrum (image below) as a result of radiation from the sun and space coming in contact to the surface of the earth. The visible spectrum, one tiny portion of the electromagnetic spectrum that our eyes can see, is composed of wavelengths that range from about 400-700 nm (Figure 1). As it turns out, plants use light in the visible spectrum for photosynthesis and can detect slightly higher wavelengths in the 710-740 nm range.

Humans and other animals can see because of an organ that is sensitive to light. In our case, the eye is incredibly complex and can perceive a multitude of colors. The lens in our eyes focuses light to the back of the eye where the wavelengths of light in the visible spectrum are perceived by specialized cells called rods and cones. These rods and cones contain protein pigments which absorb certain wavelengths (colors) and eventually transmit this information to the brain.

Obviously plants don’t have eyes. However, they are able to “sense” and physiologically respond specifically to the ratio of red light and far-red light, and blue light. Plants can sense these colors because they themselves have protein pigments, called photoreceptors, that are sensitive to the wavelengths that correspond to those colors. These photoreceptors transmit information about the light quality of the environment surrounding the plant and elicit a change in growth habit or development. 

Plants can “see” their neighbors because light reflected or transmitted from nearby plants has a lower ratio of red light to far-red light. The ability of plants to detect red light, far-red light, and their changing closeness to their surroundings is probably one of the most important ways in which plants can sense. 

Plants can also sense blue light. One of the most commonly studied blue light responses of plants is the directional growth that is seen in response to the direction of a light source. 

There are other important blue light responses of plants. For example, circadian rhythms require both phytochromes and blue light receptors. This means plants can sense day and night cycles. Another critical role of photoreceptors is in the flowering of plants which exhibit photoperiodism. Plants that exhibit photoperiodism are sensitive to the duration of night length and begin flowering when the nighttime is short or when the nighttime is long depending on the species.

Hearing

Have plants evolved tiny ears that allow them to hear you? It was often said that plants grow “better” if you talk or sing to them. Does this mean plants are listening? Should we be concerned they know all our secrets? Well, carbon dioxide is the carbon source plants use to form sugars via photosynthesis. When we talk or sing to plants we are doubling or tripling the concentration of carbon dioxide around the leaves of a plant so in some cases they can grow “better.” Don’t worry, they do not have tiny ears or any sort of auditory sensory mechanism and therefore cannot hear us.

Smell

Nope.

Taste

They cannot.

Feel

Some plants can actually detect when they are being touched.  This might be most obvious in the Venus’ flytrap. Hairs on the trap lobes can detect the movement of an insect. When an insect moves within a window of time and activates the trigger hairs, the trap rapidly closes ensnaring the insect.  

Sense of Balance

We don’t often think of balance as a sense, but we actually have an entire organ called the vestibular system in the inner ear that helps us stay balanced. Plants don’t necessarily need to balance themselves because they are anchored to the ground by their root system. But how do roots “know” to grow downward into the soil and not up? The tip of roots contains specialized cells with a gravity-sensing mechanism. Amyloplasts, parts of those specialized cells, are organelles which function to synthesize and store starch. When a root is oriented horizontally, the amyloplasts in these cells settle to the bottom of the cells by gravity. A hormonal signal is sent to the growing points of the root where the hormone is asymmetrically distributed causing differential growth resulting in a downward bend.

So while plants aren’t listening in on our private conversations, they are certainly more perceptive beings than previously considered.

This piece was written by Pablo Rosas-Anderson, an agricultural scientist at Bowery Farming.

Sources:

• Ballare, C.L., Scopel, A.L. and Sanchez, R.A., 1990. Far-red radiation reflected from adjacent leaves: an early signal of competition in plant canopies. Science, 247(4940), p.329.
• Glossary of psychological terms.  Retrieved from http://www.apa.org/research/action/glossary.aspx
• Kasperbauer, M.J., 1971. Spectral distribution of light in a tobacco canopy and effects of end-of-day light quality on growth and development. Plant physiology, 47(6), pp.775-778.
• Simons, P.J., 1981. The role of electricity in plant movements. New Phytologist, 87(1), pp.11-37.
• Swarup, R., Kramer, E.M., Perry, P., Knox, K., Leyser, H.O., Haseloff, J., Beemster, G.T., Bhalerao, R. and Bennett, M.J., 2005. Root gravitropism requires lateral root cap and epidermal cells for transport and response to a mobile auxin signal. Nature cell biology, 7(11), pp.1057-1065.