What a Plant Knows: A Field Guide to the Senses

About the Author:

Daniel Chamovitz, Ph.D., is the director of the Manna Center for Plant Biosciences at Tel Aviv University. He has served as a visiting scientist at Yale University and at the Fred Hutchinson Cancer Research Center, and has lectured at universities around the world. His research has appeared in leading scientific journals. Chamovitz lives with his wife and three children in Hod HaSharon, Israel.

Excerpt. © Reprinted by permission. All rights reserved.:

What a Plant Knows
ONE What a Plant Sees She turns, always, towards the sun, though her roots hold her fast, and, altered, loves unaltered. --Ovid, Metamorphoses  
 
Think about this: plants see you. In fact, plants monitor their visible environment all the time. Plants see if you come near them; they know when you stand over them. They even know if you're wearing a blue or a red shirt. They know if you've painted your house or if you've moved their pots from one side of the living room to the other. Of course plants don't "see" in pictures as you or I do. Plants can't discern between a slightly balding middle-aged man with glasses and a smiling little girl with brown curls. But they do see light in many ways and colors that we can only imagine. Plants see the same ultraviolet light that gives us sunburns and infrared light that heats us up. Plants can tell when there's very little light, like from a candle, or when it's the middle of the day, or when the sun is about to set into the horizon. Plants know if the light is coming from the left, the right, or from above. They know if anotherplant has grown over them, blocking their light. And they know how long the lights have been on. So, can this be considered "plant vision"? Let's first examine what vision is for us. Imagine a person born blind, living in total darkness. Now imagine this person being given the ability to discriminate between light and shadow. This person could differentiate between night and day, inside and outside. These new senses would definitely be considered rudimentary sight and would enable new levels of function. Now imagine this person being able to discern color. She can see blue above and green below. Of course this would be a welcome improvement over darkness or being able to discern only white or gray. I think we can all agree that this fundamental change--from total blindness to seeing color--is definitely "vision" for this person. Merriam-Webster's defines "sight" as "the physical sense by which light stimuli received by the eye are interpreted by the brain and constructed into a representation of the position, shape, brightness, and usually color of objects in space." We see light in what we define as the "visual spectra." Light is a common, understandable synonym for the electromagnetic waves in the visible spectrum. This means that light has properties shared with all other types of electrical signals, such as micro- and radio waves. Radio waves for AM radio are very long, almost half a mile in length. That's why radio antennas are many stories tall. In contrast, X-ray waves are very, very short, one trillion times shorter than radio waves, which is why they pass so easily through our bodies. Light waves are somewhere in the middle, between 0.0000004 and 0.0000007 meter long. Blue light is the shortest, while red light is the longest, with green, yellow, and orange in the middle. (That's why the color pattern of rainbows is alwaysoriented in the same direction--from the colors with short waves, like blue, to the colors with long waves, like red.) These are the electromagnetic waves we "see" because our eyes have special proteins called photoreceptors that know how to receive this energy, to absorb it, the same way that an antenna absorbs radio waves. The retina, the layer at the back of our eyeballs, is covered with rows and rows of these receptors, sort of like the rows and rows of LEDs in flat-screen televisions or sensors in digital cameras. Each point on the retina has photoreceptors called rods, which are sensitive to all light, and photoreceptors called cones, which respond to different colors of light. Each cone or rod responds to the light focused on it. The human retina contains about 125 million rods and 6 million cones, all in an area about the size of a passport photo. That's equivalent to a digital camera with a resolution of 130 megapixels. This huge number of receptors in such a small area gives us our high visual resolution. For comparison, the highest-resolution outdoor LED displays contain only about 10,000 LEDs per square meter, and common digital cameras have a resolution of only about 8 megapixels. Rods are more sensitive to light and enable us to see at night and under low-light conditions but not in color. Cones allow us to see different colors in bright light since cones come in three flavors--red, green, and blue. The major difference between these different photoreceptors is the specific chemical they contain. These chemicals, called rhodopsin in rods and photopsins in cones, have a specific structure that enables them to absorb light of different wavelengths. Blue light is absorbed by rhodopsin and the blue photopsin; red light by rhodopsin and the red photopsin. Purple light is absorbed by rhodopsin, blue photopsin, and red photopsin, but not green photopsin, and so on. Once the rod or cone absorbs the light, it sends a signal to the brain thatprocesses all of the signals from the millions of photoreceptors into a single coherent picture. Blindness results from defects at many stages: from light perception by the retina due to a physical problem in its structure; from the inability to sense the light (because of problems in the rhodopsin and photopsins, for example); or in the ability to transfer the information to the brain. People who are color-blind for red, for example, don't have any red cones. Thus the red signals are not absorbed and passed on to the brain. Human sight involves cells that absorb the light, and the brain then processes this information, which we in turn respond to. So what happens in plants? Darwin the Botanist It's not widely known that for the twenty years following his publication of the landmark On the Origin of Species, Charles Darwin conducted a series of experiments that still influence research in plants to this day. Darwin was fascinated by the effects of light on plant growth, as was his son Francis. In his final book, The Power of Movement in Plants, Darwin wrote: "There are extremely few [plants], of which some part ... does not bend toward lateral light." Or in less verbose modern English: almost all plants bend toward light. We see that happen all the time in houseplants that bow and bend toward rays of sunshine coming in from the window. This behavior is called phototropism. In 1864 a contemporary of Darwin's, Julius von Sachs, discovered that blue light is the primary color that induces phototropism in plants, while plants are generally blind to other colors that have little effect on theirbending toward light. But no one knew at that time how or which part of a plant sees the light coming from a particular direction. In a very simple experiment, Darwin and his son showed that this bending was due not to photosynthesis, the process whereby plants turn light into energy, but rather to some inherent sensitivity to move toward light. For their experiment, the two Darwins grew a pot of canary grass in a totally dark room for several days. Then they lit a very small gas lamp twelve feet from the pot and kept it so dim that they "could not see the seedlings themselves, nor see a pencil line on paper." But after only three hours, the plants had obviously curved toward the dim light. The curving always occurred at the same part of the young plant, an inch or so below the tip. This led them to question which part of the plant saw the light. The Darwins carried out what has become a classic experimentin botany. They hypothesized that the "eyes" of the plant were found at the seedling tip and not at the part of the seedling that bends. They checked phototropism in five different seedlings, illustrated by the following diagram: a. The first seedling was untreated and shows that the conditions of the experiment are conducive to phototropism. b. The second had its tip pruned off. c. The third had its tip covered with a lightproof cap. d. The fourth had its tip covered with a clear glass cap. e. The fifth had its middle section covered by a lightproof tube. They carried out the experiment on these seedlings in the same conditions as their initial experiment, and of course the untreated seedling bent toward the light. Similarly, the seedling with the lightproof tube around its middle (see e above) bent toward the light. If they removed the tip of a seedling, however, or covered it with a lightproof cap, it went blind and couldn't bend toward the light. Then they witnessed the behavior of theplant in scenario four ( d): this seedling continued to bend toward the light even though it had a cap on its tip. The difference here was that the cap was clear. The Darwins realized that the glass still allowed the light to shine onto the tip of the plant. In one simple experiment, published in 1880, the two Darwins proved that phototropism is the result of light hitting the tip of a plant's shoot, which sees the light and transfers this information to the plant's midsection to tell it to bend in that direction. The Darwins had successfully demonstrated rudimentary sight in plants. Maryland Mammoth: The Tobacco That Just Kept Growing Several decades later, a new tobacco strain cropped up in the valleys of southern Maryland and reignited interest in the ways that plants see the world. These valleys have been home to some of America's greatest tobacco farms since the first settlers arrived at the end of the seventeenth century. Tobacco farmers, learning from the Native tribes such as the Susquehannock, who had grown tobacco for centuries, would plant their crop in the spring and harvest it in late summer. Some of the plants weren't harvested for their leaves and made flowers that provided the seed for the next year's crop. In 1906, farmers began to notice a new strain of tobacco that never seemed to stop growing. It could reach fifteen feet in height, produce almost a hundred leaves, and would only stop growing when the frosts set in. On the surface, such a robust, ever-growing plant would seem a boon to tobacco farmers. But as is so often the case, this new strain, aptly named Maryland Mammoth, was like the two-faced Roman god Janus. On the one hand, it never stopped growing; on the other,it rarely flowered, meaning farmers couldn't harvest seed for the next year's crop. In 1918, Wightman W. Garner and Harry A. Allard, two scientists at the U.S. Department of Agriculture, set out to determine why Maryland Mammoth didn't know when to stop making leaves and start making flowers and seeds instead. They planted the Maryland Mammoth in pots and left one group outside in the fields. The other group was put in the field during the day but moved to a dark shed every afternoon. Simply limiting the amount of light the plants saw was enough to cause Maryland Mammoth to stop growing and start flowering. In other words, if Maryland Mammoth was exposed to the long days of summer, itwould keep growing leaves. But if it experienced artificially shorter days, then it would flower. This phenomenon, called photoperiodism, gave us the first strong evidence that plants measure how much light they take in. Other experiments over the years have revealed that many plants, just like the Mammoth, flower only if the day is short; they are referred to as "short-day" plants. Such short-day plants include chrysanthemums and soybeans. Some plants need a long day to flower; irises and barley are considered "long-day" plants. This discovery meant that farmers could now manipulate flowering to fit their schedules by controlling the light that a plant sees. It's not surprising that farmers in Florida soon figured out that they could grow Maryland Mammoth for many months (without the effects of frost encountered in Maryland) and that the plants would eventually flower in the fields in midwinter when the days were shortest. What a Difference a (Short) Day Makes The concept of photoperiodism sparked a rush of activity among scientists who were brimming with follow-up questions: Do plants measure the length of the day or the night? And what color of light are plants seeing? Around the time of World War II, scientists discovered that they could manipulate when plants flowered simply by quickly turning the lights on and off in the middle of the night. They could take a short-day plant like the soybean and keep it from making flowers in short days if they turned on the lights for only a few minutes in the middle of the night. On the other hand, the scientists could cause a long-day plant like the iris to makeflowers even in the middle of the winter (during short days, when it shouldn't normally flower), if in the middle of the night they turned on the lights for just a few moments. These experiments proved that what a plant measures is not the length of the day but the length of the continuous period of darkness. Using this technique, flower farmers can keep chrysanthemums from flowering until just before Mother's Day, which is the optimal time to have them burst onto the spring flower scene. Chrysanthemum farmers have a problem since Mother's Day comes in the spring but the flowers normally blossom in the fall as the days get shorter. Fortunately, chrysanthemums grown in greenhouses can be kept from flowering by turning on the lights for a few minutes at night throughout the fall and winter. Then ... boom ... two weeks before Mother's Day, the farmers stop turning on the lights at night, and all the plants start to flower at once, ready for harvest and shipping. These scientists were curious about the color of light that the plants saw. What they discovered was surprising: the plants, and it didn't matter which ones were tested, only responded to a flash of red during the night. Blue or green flashes during the night wouldn't influence when the plant flowered, but only a few seconds of red would. Plants were differentiating between colors: they were using blue light to know which direction to bend in and red light to measure the length of the night. Then, in the early 1950s, Harry Borthwick and his colleagues in the USDA lab where Maryland Mammoth was first studied made the amazing discovery that far-red light--light that has wavelengths that are a bit longer than bright red and is most often seen, just barely, at dusk--could cancel the effect of the red light on plants. Let me spell this out more clearly: if you takeirises, which normally don't flower in long nights, and give them a shot of red light in the middle of the night, they'll make flowers as bright and as beautiful as any iris in a nature preserve. But if you shine far-red light on them right after the pulse of red, it's as if they never saw the red light to begin with. They won't flower. If you then shine red light on them after the far-red, they will. Hit them again with far-red light, and they won't. And so on. We're also not talking about lots of light; a few seconds of either color is enough. It's like a light-activated switch: The red light turns on flowering; the far-red light turns it off. If you flip the switch back and forth fast enough, nothing happens. On a more philosophical level, we can say that the plant remembers the last color it saw. By the time John F. Kennedy was elected president, Warren L. Butler and colleagues had demonstrated that a single photoreceptor in plants was responsible for both the red and the far-red effects. They called this receptor "phytochrome," meaning "plant color." In its simplest model, phytochrome is the light-activated switch. Red light activates phytochrome, turning it into a form primed to receive far-red light. Far-red light inactivates phyt...

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