Why are plants green? The great misconception

There is an age-old misconception about the green color of plants that we are going to address today, and what it means when choosing the right LED grow light for your operation. To start, let’s talk about what most people learn in school: some basics about light and why we see certain colours.

We have all learned at some point in our lives that the reason we see certain colors is due to whether or not the material you are looking at reflects or absorbs wavelengths of the color you are seeing. For example, something that is pure white looks white to our eyes because it reflecting all types of visible light. Something that is black is absorbing all the different colors of visible light.

reflection

So, things that appear green to our eyes (like the leaves of most plants) are reflecting green light. This very basic approach and understanding to light and is mostly right. Mostly. But not completely, which has produced a very deep-seeded misconception about the types of light that plants use to perform photosynthesis. You’ve probably heard it before or believe it yourself:

Because plants are green, they reflect green light and therefore they don’t use it for photosynthesis.

This is wrong actually, and we’re going to learn why, right now. The real explanation is slightly more complicated (but there is a whole Magic School Bus episode that explains most of it so it’s obviously not THAT complicated). If you’re bored or even just baked, I’d highly recommend watching the episode, it’s a classic.

The Magic School Bus Gets Planted from Melissa Erwin on Vimeo.

Chlorophyll

Seat belts everyone! Now that you’re back from your photosynthetic adventure with Ms. Frizzle you know that the functional photosynthetic unit of a plant is the chlorophyll. Chlorophyll use light to convert light into sugars in order to grow, be happy and eventually produce fruits or THC or whatever other reproductive/protective compounds plants like to make when they’re happy.

Besides the green color of plants and our basic idea of green things reflecting green light, another reason for the grand misconception that plants don’t use green (and yellow) light is because of the way chlorophyll molecules absorb different types of light. It’s also important to note here that I just dropped a bomb on you in that plants also reflect a large amount of yellow light, but until now we’ve just been simplifying it down to green light because it’s easier to explain. I’m also going to mostly continue to do so for simplicity’s sake.

As you can see in the chart below, both types of chlorophyll maximally absorb light in the blue and red wavelengths. This is why most LED grow lights you see on the market today typically have a red or blue hue while turned on, because they are targeting chlorophyll using those types of light to induce growth.

Spectrum

But that’s still not the whole story. Up until recently, grow lights being used were mostly HPS and HID lights (link to other blog post) that produce a yellowish light, and can still produce great yields – because yellow and green light are still within the range of Photosynthetically Active Radiation (PAR) as you can see below. And while many people are huge proponents of blue and red light being the best light for plant growth, due to the absorbance of chlorophyll it’s simply just not true. Plants can still use green and yellow light, even though chlorophyll doesn’t absorb it as much as the others.

In fact, a study by Terashima et al. (2009) describes how green light is actually a much more efficient type of light for performing photosynthesis than red and blue, and plants do in fact use that green light to perform photosynthesis, albeit in smaller amounts. However, plants and their chlorophyll have evolved to avoid absorbing large amounts of green light, reflecting it to appear green to our eyes. Why is that?

PAR

Why don’t plants make use of green light?

So why would plants evolve chlorophyll that avoid absorbing green light, if it actually drives photosynthesis better than other types of light? Well here’s my theory. Throughout the day, the most intensely produced light from the sun lies between the range of roughly 450nm to 600nm. And as we’ve already looked at from the absorption of chlorophyll, plants reflect light in the exact same range from 450nm to 600nm.

greenlight2

Starting to figure it out? Think about it. Most plants (with the exception of a select few) are stuck in the same spot each and every day, all day. They don’t have the option to say “I think I’ve had enough sun today” and go inside like we can. Being constantly bombarded by high energy radiation (light) all day can cause damage to the plant tissues and for plants to try and harness all the energy is unrealistic and would cause problems by trying to harness all that energy. So, by evolving to contain chlorophyll molecules that poorly absorb the highest intensity form of daylight (450-600nm) plants can still perform photosynthesis at a healthy rate using the other, lower intensity wavelengths and also survive up to 18 hours of daylight each and every day. These videos can also help illustrate some of the same points I’ve been making in this blog post if you’re interested, as well as another nice theory of how green chlorophyll evolved based on evolution from green archaea.

So why we don’t we use green LEDs?

So we now know that green/yellow light (450-600nm) is more effective than other wavelengths at driving photosynthesis in plants. But we also just learned that too much of this type of light can be harmful to plants. So you may be asking why don’t we use green LEDs when growing indoors and just keep the lights on for less time? Well, the answer is a little underwhelming. It turns out that green LEDs are just extremely inefficient at producing green photons than red LEDs. Even white LEDs can sometimes produce more green light per watt than green LEDs. Red and blue LEDs on the other hand are extremely efficient, with red LEDs producing up to 75% more light than green LEDs at the same wattage. We know that plants can handle way more red and blue light wavelengths than green, and we can produce red and blue light using LED grow lights much more efficiently than green light, so its kind of a no-brainer.

So that’s our blog post for today – why plants are actually green, why chlorophyll avoids absorbing green light, and we we don’t use green LEDs for grow lights.

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