4x Faster Than Sunlight?! The Secret of Plant Factories Growing Lettuce Under Red Light (Plot Twist: It Was Purple Light!)

Ken Kuwako, Science Trainer, here. Every day is an experiment!

Lettuce Grown Under Nothing but Red Light? The Magic of Plant Factories

Growing lettuce with no sunlight at all, just red light? Sounds like science fiction, right?
Believe it or not, this is happening in plant factories right now. NHK for School (a Japanese educational broadcaster) has a video showing how it works. Take a look!

https://www2.nhk.or.jp/school/movie/clip.cgi?das_id=D0005403217_00000

Pretty cool, huh? According to the video, lettuce grown under red light grows four times faster than lettuce grown in sunlight. That raises a few questions:

– Why red, of all colors?
– What about green light?
– Do plants need blue light, too?

Which Colors of Light Do Plants Actually Use?

The clue is in the leaves themselves. Leaves look green because they reflect green light instead of absorbing it. In other words, plants don’t use green light as much as they use red or blue.

Inside the chloroplasts, where photosynthesis happens, chlorophyll absorbs red and blue light especially well.
So if you shine plenty of strong red light, which is easy for plants to use, they can grow very efficiently. And in a plant factory, you can deliver exactly the light you want all year round, no matter the weather or the season.

So Does That Mean Blue Light Is Useless?

This is where it gets interesting. Blue light is used for photosynthesis too, but that’s not all it does. It also helps shape the leaves, controls how big they get and how the plant stretches upward, and even opens the stomata, the tiny pores that let air in and out.

Under red light alone, plants grow quickly, but they can end up tall and spindly, and their leaves can look a little different. That’s why plant factories often use a mix of red and blue. For a plant, light isn’t just fuel. It’s also a signal that tells it, “Here’s how you should grow right now.”

Is Green Light a Total Waste?

Green light gets reflected a lot, but it isn’t useless. It penetrates deeper into the leaf, so it may help carry light down to the lower leaves that are shaded by the ones above. A color that seems “hard to use” might have an important job after all.

Try Exploring Light Colors in the Science Lab!

This is a light source that gives off purple light, used in a lab. In this experiment, we shine it on Elodea (a common aquarium plant) to see photosynthesis in action. When photosynthesis is happening, the plant gives off oxygen, so if you watch the tiny bubbles rising from the leaves, you can actually see it working.

曇りの日でも成功！オオカナダモ・イヌムギを使った光合成実験の準備から手順まで（光合成で使われる気体）

What happens to the bubbles if you switch the light to red, blue, or green? You might just uncover the “secrets of light” used in plant factories with your own eyes. Next time you see a plant, ask yourself: “I wonder which colors of light this one is using?”

Contact and Requests

Science is full of wonder and fun, and it’s closer than you think! This site is packed with fun experiments you can do at home, along with tips to make them work. Try searching around!
– The content of “Kagaku no Neta Cho” (Science Idea Notebook) has been published as a book! Find out more here.
– Learn more about the author, Ken Kuwako, here.
– For requests of any kind (writing, talks, science workshops, TV supervision or appearances, and more), please click here.
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