Lighting Series: Red Light
Red light: if you’re driving a car in a hurry, it’s the bane of your existence. If you’re a leafy green plant in a controlled-environment agricultural setting — or for that matter, any plant in any setting — the tiniest exposure to red light is enough to make your chlorophyll shake with joy.
What we’re trying to say is that plants love red light. Which, much like our dependence on a plant’s oxygen waste product to breathe, seems complementary to our natural human aversion to the color red. For us, red is a warning — an evolutionary alert system symbolic of toxic berries, emotions that elicit blushing and anger, or even blood. For plants, it is quite the opposite. But before we discuss why that is, what do we mean when we talk about ‘red light?’
What is red light?
On the visible spectrum of light (aka the rainbow), we perceive the color red when we look at light with a wavelength of about 600–699 nanometers. Interestingly, the color we perceive an object to be is actually the color it reflects the most. For plants, what this means is that red hits them right in their sweet spot. They absorb it easily, say thank you, and ask for second helpings. What they don’t absorb as well, as you may recall from middle school science class, is green light. Which is precisely why they tend to appear lush and verdant to our eyes.
It is important to note that while we detect light through our vision, plants — with the exception of those in the Christopher Walken ‘Googly Eyes Gardener’ SNL sketch — lack the gift of eyesight, and have therefore adapted to detect light in a different manner: with pigments.
Let’s talk about pigments
Pigments are how plants absorb light. Specifically, when we’re talking about red light, chlorophyll a and chlorophyll b are the main pigments utilized to identify and collect it. Both of these are located within the protein complexes called photosystem I and photosystem II. At their most efficient points, photosystem I absorbs light at 700 nanometers and photosystem II at 680 nanometers. It just so happens that light at both of those wavelengths is red, as stated earlier.
Caption: Note the bi-modal distribution pictured. While this article focuses on the importance of the peak in absorbance between 600–700 nm, the other peak, between 400–500 nm is a topic for a separate discussion.
We can make this more complex and talk thylakoid structures and chloroplasts until the cacti come home, but for now let’s keep it simple: light shines on a plant’s leaf, the light is absorbed best by the photosystems where it occurs as red light, the green light is reflected, and the process of creating energy for the plant begins.
To better visualize this energy creation process: take a look at figures a, b and c below:
From bottom to top, figure c demonstrates a spike in available light for the leaf as a whole at that perfect red light range of 600–699 nanometers. Figure b then shows us that in that same range, the pigments within the photosystems exhibit nearly full absorption of available light for that leaf, and finally, figure a shows that very same red range providing maximum energy production by the photosystems within our lucky leaf.
Essentially, red light is a plant’s best friend. Nearly all red light a plant is exposed to is absorbed, and of that absorbed light, nearly all of it is converted to energy. It’s an incredibly efficient and satisfying process. Now, let’s talk about the effects.
How does red light measure up?
When we measure plant growth, what we’re looking for are changes in both fresh weight and compounds. Fresh weight is an easily visualized result, and what we typically think of as being observable proof of healthy energy production within a plant. Think increases in weight and size of the plant. That giant head of lettuce winning the blue ribbon at the county fair? That’s a lotta fresh weight. Compounds, though less easily discernible to the naked eye, are also important, as a quality growing environment can produce a great deal of rich and complex chemical compounds within a plant.
We already know that red light is great (apologies to the rest of ROY G BIV), but for a more full picture, we also need to know how much of it is needed for healthy plants.
Photosynthetic Photon Flux Density, or PPFD, is a measurement of the intensity of light, equivalent to 1 micromole of photons/m2/second. Surely it follows that the more intense the PPFD of red light, the more plant growth we will observe, right? Well, actually, not quite.
Observe this figure:
These are the results of an experiment where Catharanthus roseus, or Madagascar periwinkle, was grown in a greenhouse under red light. As the intensity of red light increased, you can see along the X-axis that the plant’s production of compounds (vindoline (VDL) and catharanthine (CAT) measured along the Y-axis increased, hit a stride at 150 PPFD, and then tapered off. What does this mean? Plants should not be inundated by red light, but they should receive a healthy dosage of it.
Furthermore, we’ve done a lot of talking about the importance of red light, and while it shoulders quite the load in plant growth, it’s not the only color providing value in agriculture. Take this study of different brands of lettuce grown under a consistent PPFD but with exposure from across the visible spectrum of light:
What the results tell us is that if red light were the only color that mattered, Fionia lettuce would have experienced the greatest increase in fresh weight, as it underwent the greatest exposure to red light (reminder: 600–699 nm). But, despite Fionia’s impressive performance, that isn’t the case. So, we can surmise that the rest of the spectrum does play a role in maximizing plant growth.
Conclusions
We know it may seem like we are firmly in the pocket of “Big Corporate Red Light,” but what we’ve learned is that, like all things, there’s virtue in moderation. Red light is extremely efficient. The pigments essential to energy production deep within plant cells love it. But too much of it, both in intensity and exclusivity, can lead to a reduction in potential fresh weight growth and compound production.
The trick, it seems, is to tinker, on a plant-by-plant basis to discover the nuances of what sort of light exposure makes for maximum growth. Red is all the rage this growing season, but it’s up to you to discover just how much you need.
