What a Plant Sees

By Kit Flynn

Previously published in the August edition of the Durham Co. EMGV newsletter

Have you ever yanked out a plant—and thought perhaps it was yelling the equivalent of “ouch!” in plant speak? Have you ever wondered what kind of world our plants inhabit? For those of us who have pets we are quite conscious that our sensory world is unlike theirs: my dogs inhabit a world of smells that I cannot even imagine, a world that is locked away forever to me. Likewise an eagle has eyesight that is six times sharper than ours—we can only imagine what an eagle’s world looks like. Therefore, it isn’t altogether farfetched to ponder on what kind of environment our plants dwell in.

David Chamowitz, Director of the Manna Center for Plant Biosciences at Tel Aviv University has written about this world.¹ This month and in the following months this newsletter will investigate the plant world from the plants’ perspective. This month we look at sight.

Phototropism is the plant’s response to light: positive phototropism has the plant bending towards the light while negative tropism has the plant leaning away from the light. As gardeners we have all observed this phenomenon in our plants: when our gardens become shadier, certain plants elongate in an effort to find more light. However, what part of the plant actually sees the light? Darwin answered this question by proving through a series of experiments that it is the tip that is sensitive to light, causing the midsection to bend toward the light [160-75].

We also know that plants respond to seasonal changes, a condition known as photoperiodism. One dramatic example of photoperiodism in seen in the case of a certain tobacco hybrid that appeared in Maryland in 1906. Known as ‘Maryland Mammoth’, this tobacco plant had one major disadvantage: it couldn’t stop growing long enough to produce flowers and seeds. Scientists experimented to see if they could break this cycle by using light: one group of tobacco-filled planters was kept in the field while a second group of planters was brought into a dark shed every afternoon. By limiting the amount of sunlight, these scientists were able to entice ‘Maryland Mammoth’ to stop growing and start producing seeds. Consequently Florida growers now grow this hybrid in the winter when daylight hours are shorter.

Interestingly enough, the important factor here is the length of the period of darkness, not the extent of daylight hours. If a poinsettia experiences flickering light at night it will forgo flowering whereas irises will start to bloom if subjected to a minute or so of light during the night. To get chrysanthemums to bloom for Mother’s Day, farmers keep the lights on in the greenhouse for a few minutes during the fall and winter only to leave them off several weeks before Mother’s Day.

However, this is not the whole story explaining how light affects a plant. The color of the light is equally important: if they are to respond to light, plants need a flash of red at night [208-29]; blue and green flashes do not work. If plants are to respond during the day to light—phototropism—they need blue light; so, it is obvious that plants can differentiate between the colors of light. Then it gets even more complicated: irises, if given a shot of red light during the dark hours, will bloom but should the red light be followed by far-red light², the whole blooming process is negated. Should the florist hit them again with just red light, the irises will proceed to flower. There is no question that plants differentiate between red light and far-red light: “We’re also not talking about lots of light; a few seconds of either color is enough” [208-89].

This ability to determine colors is called “photochrome,” which acts like a light switch: red light triggers photochrome whereas the far-red light deactivates it. This makes sense when we stop to think about it: the far-red light is the last light the plant sees before nightfall whereas red light is the first light it sees in the morning. According to Chamowitz, the photochromic mechanism instructs the plant when it last saw red light so it may adjust accordingly.

The “eye” of the plant is in the tip while the response is in the stem. Accordingly, following logic, the photochromic mechanism should be in the tip—but this is not the case. The leaves distinguish the color of the light. Strip a plant of its leaves and the plant is now oblivious to any glimpses of light [229-49].

Keep a plant in the dark and it will grow up spindly whereas the same plant grown in light will be shorter and greener. Why? “This makes sense because plants normally elongate in darkness, when they’re trying to get out of the soil into the light or when they’re in the shade and need to make their way to the unobstructed light” [229-49]. Plants have photoreceptors, which tell them when to germinate, when to flower, when to bend towards the light, and when it’s night [249-63]. Chamowitz claims that, “[P]lant vision is much more complex than human sight at the level of perception” [263-67]. Essentially light equals food; unlike animals, plants cannot move around foraging for their food because they are rooted to one spot; consequently they have to use other means to seek out light—obviously, it’s of the utmost importance for the plant to know where the light is located. Should a plant next to another plant block out much of the sun, the other plant then has two options: it can grow more quickly in pace or it might decide that now is the time to spread its seeds. Due to their photochromes, plants know when it is spring—there’s more light than in winter—and when it is fall, as the days are growing shorter.

“Sight is the ability not only to detect electromagnetic waves but also the ability to respond to those waves” [267-88]. Our retinas capture the light, transferring these images to our brains; plants absorb the light through the tips of their leaves, triggering a response in the plants, whether it should bend, form seeds, or shed leaves: “Plant phytochrome and human red photosin are not the same photoreceptor; while they both absorb red light, they are different proteins with different chemistries. What we see is mediated through photoreceptors found only in other animals. What a daffodil sees is mediated through photoreceptors found only in plants” [288-309].

Concerning sight, what we do have in common with plants is that we both have blue light receptors called cryptochromes, which control our inner clocks: “Plants, like animals, have an internal clock called the ‘circadian clock’ that is in tune with normal day-night cycles” [288-309]. Our circadian clock dictates when we’re hungry, when we’re sleepy, when we should be energetic, and when we’re lethargic. Jet lag is the classic example of an upset circadian clock, but it is easily rectified after a few days of exposure to light. By absorbing blue light, our chytochrome, the photoreceptor of blue light, is responsible for resetting our circadian clock—it’s the blue light that tells our cells when it’s daylight.

¹ Chamowitz, David. What a Plant Knows: a Field Guide to the Senses (Scientific American/Farrar, Straus and Giroux, 2012). Please note that future notations, referring to the Kindle location, will be in brackets [ ].

² This is the red light we see at dusk. Far-red light has longer wavelengths than red light. We have four photoreceptors: one that distinguishes between light and dark, the others for red, blue, and green. A fifth receptor, c cryptochrome, “regulates our internal clocks” [229-49]. Far-red light lies between red and infra-red light.