How do plants respond to different types of light?

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When growing plants with LED grow lights, one of the most important things to consider is the color and type of light produced by the light. But what color of light do plants prefer to use in photosynthesis? How do they respond to UV light and infrared? In this article we will dive into how plants respond to different types of light.

PAR Spectrum

The types of light that plants can use to perform photosynthesis can be defined as Photo-synthetically Active Radiation or PAR.

PAR encompasses light from the wavelengths of roughly 400nm to 700nm. Pigments in plant cells – primarily chlorophyll – use the energy of the photons within this wavelength range to carry out photosynthesis. 

The reason that plants are able to use sunlight to carry out photosynthesis is that sunlight contains the wavelengths of PAR in its spectrum and more.

As you can see, while a portion of sunlight is within the 400nm to 700nm there is a ton of light both greater and less than the PAR range, which includes Ultraviolet (roughly 200nm-430nm) and Infrared (roughly 750nm +).

So how do plants respond to the different types of light/radiation contained both within and outside the PAR range?

Before we answer how the plants themselves react to the different types of light, let’s quickly cover how the chlorophyll molecules and other pigments inside the plants respond to light.

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Chlorophyll absorption

Chlorophyll is the main photon-capturing, photosynthesizing molecules in plants. There are two types of Chlorophyll and they both capture light maximally at different wavelengths.

When light of a particular wavelength interacts with a chlorophyll molecule, the pigment essentially transforms the energy of that photon through photosynthesis into a lower form of energy that the plant can then use to grow.

What the chart below is depicting is that Chlorophyll a (blue line) maximally absorbs light at two different wavelengths, approximately 465nm and 665nm. Chlorophyll b (red line) maximally absorbs light at wavelengths of approximately 450nm and 640nm.

It is important to note that this does not mean that chlorophyll does not make use of the other wavelengths of light, but that these are simply the two wavelengths of light for each chlorophyll type that allows for the most efficient photosynthesis possible.

Other wavelengths of light still drive photosynthesis, but not as efficiently. You can also see that both chlorophyll molecules mostly lack absorption of the green wavelength. For a more detailed description you can check out this blog post that dives into why that is.

When you are growing plants using LED lights, many companies like to claim they have a complete spectrum of light, but there are certain wavelengths that are more important than others. Let’s dive into each type of light and what it means for you when buying an LED grow light.

Ultraviolet (UV)

UV light has a wavelength of approximately 280nm to 450nm. As we all know, UV light is produced by the sun and is the villain at hand for when you get a sunburn.

UV light’s short wavelength makes it high energy, which can cause damage to our cells, as well as those of plants. There are proponents in the growing community that UV light can cause an increase in the number of trichomes (the THC-producing structures) on cannabis plants.

The theory is that trichomes may have evolved in cannabis plants as a defense mechanism to UV light, to help protect the plant from damage as THC and other cannabinoid molecules act as antioxidants to decrease the damage from the high-energy UV photons.

The main evidence that rejects this theory is that trichomes are produced exclusively during flowering. If the trichomes are a mechanism to avoid UV-induced damage, why wouldn’t they be present throughout the life cycle

Regardless, it is likely that UV light is causing damage to plants, because of its high-energy. Plants “waste” energy in dealing with the high energy photons in order to convert it to a lower-energy usable photon within the PAR range.

In addition, UV light can produce minor mutations in plants causing irreparable damage if the cells are not able to fully cope with the UV light. LED with UV wavelength are typically inefficient at producing light at this wavelength, and it may do more damage than good to your plants. The jury is still out, but I think it’s probably best to avoid LEDs with UV.

Blue Light

Blue light has a larger wavelength than UV (roughly 450nm to 495nm) but is still produces fairly high energy photons. Plants still need to process these photons slightly to bring them down to a more manageable energy level. Blue light also has another interesting effect on plants. 

Blue light inhibits the production of the auxin molecule. Auxins are a class of hormones found in plants that have several functions, primarily to vertical promote plant growth. 

Auxin is important in early plant growth to grow a tall, healthy plant, but at consistently high levels can result in stretchy, elongated plants. Inhibiting auxin production using higher levels of blue light induces plants to invest more growth in the axillary buds, producing bushier plants. 

The auxin hormones are also important in the flowering stage, which is why flowering stages often reduce the amount of blue during flowering to inhibit auxins. What does this mean for LED grow lights? 

Blue LEDs are extremely efficient at emitting photons and are an important spectrum you definitely need to be covered in your lights.

Amber/Green Light

In a previous blog post, we addressed the misconception that plants don’t use green (495nm to 570nm) and amber light (570nm to 620nm) to perform photosynthesis based on the absorbance charts of chlorophyll. 

I won’t go into that much detail in this post, but essentially what you need to know is that in fact plants DO use green and yellow photons, but they’d rather not (that’s a pretty bad explanation so maybe check out the blog post if you’re curious). 

Obviously, they are able to grow and thrive under these conditions – HPS lights produce a ton of green/amber light – but there are several reasons why more and more people are moving from HPS to LED

So What does this means for your LED grow lights? Two things. One, although we could use green or amber LEDs for growing plants to try and target maximum efficiency, they probably wouldn’t respond well over long periods of time to that much of those types of light. And two, green and amber LEDs are both super inefficient at producing photons. 

The increase of plant photosynthesis you may get from a green/amber LED is minimal compared to the efficiency (of both energy and photosynthesis) you will get from red or blue LEDs. So, it’s just not really worth having this spectrum in an LED grow light.

Red Light

Red light is the primary photon type that drives photosynthesis since its wavelengths (620nm-660nm) are as close as possible to both the photosystem 1 and photosystem 2 pathways optimum wavelengths of 680nm and 700nm. 

Therefore, the plant only has to slightly process the photons (using less energy than processing light of smaller wavelengths) to decrease their energy slightly to the optimum 680nm and 700nm to use in photosynthesis.  

Red LEDs are extremely efficient at producing red photons and run at a way lower voltage than LEDs of other colors. Even using just red LEDs with some white light (that includes the entire spectrum) can produce big, healthy plants.

Deep Red, Far-Red & Infrared

Deep Red (660nm-700nm) light could be ideal for driving photosynthesis in plants (being so close to the optimal 680nm and 700nm), however, the LEDs are so inefficient that normal LEDs currently outweighs the benefit. Far Red light (700nm-750nm) is not useful for plants to use in photosynthesis, and it lies outside the PAR. 

Plants are unable to increase the energy of a photon, so if these photons are already lower wavelength than 700nm, they are not able to be used by the plants for photosynthesis. Infrared radiation (IR) (750nm+) is emitted as low energy photons, but is essentially just thermal energy, in other words, heat. Again, IR lies outside the PAR range. 

Typically you want to avoid excess heat in your growing space, hence one of the great benefits to cool-running LEDs, so you pretty much want to completely avoid lights producing IR. Using lights from companies that claim their LEDs emit the Far-Red wavelengths are essentially a waste of energy. And although current Deep Red LEDs aren’t as efficient as norm Red, this could change in the near future.

What does this mean for buying an LED grow light?

As you can see, plants respond differently to different kinds of light. It’s important to note that although you could possibly try to drive plant growth through the most efficient way possible, using only Red light, your plants will likely not thrive. 

As we learned earlier, plants evolved using sunlight, which contains the entire spectrum we’ve covered here. And different types of light have different effects on the plants, such as the way blue light inhibits auxin hormones. The best LED grow light will cover the entire spectrum of PAR light, with greater emission of reds and blues to maximize both photosynthetic and energy efficiency.

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