Plants & Shadows

Plants live in a world of constantly changing light. Researchers have identified that plants actually can detect shadows using a specific tool, which helps them maximize efficiency for capturing sunlight.

CHAPEL HILL — It’s not easy being a plant—and not because humans and animals can step on you, or some errant weed whacker could chop you. All of those are detrimental, of course, but at the most basic level, being a plant is tough because your world is constantly changing in providing one of the most important things you need to survive: sunlight. 

Think about it: the sun rises, clouds come by, shadows move, sunlight flickers and then sun sets. Essentially, the sun isn’t always there and plants need sunlight to make food. 

“You can really think of sunlight as a nutrient,” explains Alan Jones, Ph.D., a biology professor at the University of North Carolina Chapel Hill. “Plants need water, and plants need nutrients like nitrogen and potassium—that’s why we add fertilizers to our garden. They also need carbon dioxide, and plants are competing for these things, and plants are competing for sunlight.” 

Plants need three basic things to live: water from the soil, carbon dioxide from the air and energy from the sun. Plants combine those ingredients to make food in a process called photosynthesis. 

It all happens in the plant’s cells. Plants capture sunlight using a compound called chlorophyll. Chlorophyll is found inside a structure called a chloroplast. Photosynthesis converts sunlight into chemical energy, which is used to make glucose, or sugar, along with oxygen. Plants use glucose to live and grow, and they release oxygen. In fact, most of the oxygen on earth that we breathe comes from plants. 

Scientists have discovered that a plant’s light detection system is tied to the efficiency of photosynthesis. 

“If you’re a plant, you need to know right now: Is it sunny? Is it dark? Is that a shadow? Is that flickering light important to me?” says Jones, snapping his fingers for emphasis. “So in a way, plants are pretty smart!” 

The professor then shows me a growth chamber, which looks something like a giant refrigerator. He opens it up to show me rows and rows of plants. The light inside was set to imitate sunlight. Researchers at UNC-Chapel Hill used the chamber to mimic the light changes a plant experiences throughout the day. That led to the discovery of a protein in the plant, called RGS1. The protein detects changes in light and measures changes in glucose to control how efficiently photosynthesis works. 

“Because of this protein, the plant is able to know when something is a shadow, a flicker of light or the end of the day,” says Jones. “Of course I’m using the word know in an anthropomorphic way, but a plant has to be able to figure that out because it can’t move. It has to stay where it is and become inefficient in collecting the light because if it wasn’t it would literally burn up.” 

In short, when light changes a plant needs to show restraint. If a shadow forms and the plant increases the efficiency of photosynthesis, when the sun comes out of the shadows quickly, the plant will burn up. The plant needs to determine: Is this a shadow or the end of the day? Protein RSG1 helps plants do this. 

The biologists turned to mathematics to understand just what the protein’s role is.

“So we have a plant at some height and we let the sun shine a lot of light on the plant, so we’ll enter all of that into the equation,” says Patrick McCarter, a Ph.D. candidate in bio-informatics and computational biology. He writes a blizzard of numbers and symbols on a dry erase board. “So we have the plant height at the beginning of the experiment, then light into the plant and then we measure how the height will change so we can plot out plant height as a function of time.” 

So what does a plant consider to be a shadow? The study found any change in light that was longer than four minutes was determined to be more than a flicker of light and so the efficiency of photosynthesis was increased. 

“My model shows that, if the duration is very short, which would be shorter than four minutes, the plant won’t react at all,” says Kang-Ling Liau, a post doctoral student in biology. 

It turns out that sugar, and photosynthesis, were the signal for what and when a change in light was determined to be. And ultimately, what the plant should do about it. 

“We think of sugar as something that is important as a nutrient, but it’s also a signal," says Jones. “Just like a hormone is a signal, or light is a signal, or a horn is a signal, the amount of sugar and how it changes in time is also a signal.”

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