I recently finished building a one plant hydro planter (prototype) that includes a built-in air pump and an LED light.  My plan is to write some posts documenting what I learned about building an LED lamp in the process.  The posts I plan to write include:

  • Stuff about the quality of light I need to know in order to choose the LEDs. (This post)
  • Stuff about the quantity of light I need to know.
  • Design of the prototype LED grow lamp including LED chip choice and other implementation implications.
  • Testing the prototype planter by tracking the growth of a basil plant from seed to making pesto.

The Goal

The goal of this post is to get a strong feel for how what light quality means to plants and how that affects the choice of LEDs.  Specifically, gain a firm understanding of the two light enabled processes:  photosynthesis and photomorphogenesis.  Up until this point, I had a fair knowledge about how plants use light for photosynthesis.  However, I didn’t know much about photomorphogenesis.  Folks had kindly (and slowly) tried to explain photomorphogenesis to me…but…the explanations just didn’t stick.  I continued to attribute stuff that happened because of photomorphogenesis to photosynthesis.  Besides, there are a lot of syllables in the word photomorphogenesis.  All those syllables made me think photomorphogenesis was intimidating as a concept to grasp.  If it wasn’t, then wouldn’t it have a more approachable/friendly name?  Most likely many of you are wondering (perhaps in disbelief?) how I could be so challenged in grasping what you consider simple concepts.  If that is the case, this post is a waste of your time!

Thanks to Those That Went Before

I just finished the “Greenhouse and Horticultural Lighting Summer 2016” extension course offered by Michigan State University.  I will be used the knowledge about LED lighting for plants that I learned in the course to build the LED lamp.  THANK YOU to Heidi Wollaege – our teacher.  The content, Heidi’s delivery, and her exceptional answers to my questions made the course well worth the price.

Lighting for Plant Growth (for Dummies Like Me)

Without light, no plants.  Without basil, no pesto.  As I mentioned earlier, the two light processes I needed to have a firm grasp of where light quality is important are photosynthesis and photomorphogenesis.  

But what is light quality?

Light Quality

As noted in Dr. Erik Runkle’s article Light Quality Defined: “…light quality refers to the spectral distribution of light, or the number of photons of blue, green, red, far red and other portions of the light spectrum emitted from a light source.”

now onto two key light enabled processes that go on in a plant – photosynthesis and photomorphogenesis.

Photosynthesis

I thought this video – Williams Basics Plant Growth Development 4A  1 – does a great job in simply explaining photosynthesis.  Before I found this video, I wrote out my findings on what photosynthesis is.  Like I’ve said before, writing stuff out helps me know what I don’t know.

Since I appreciate stuff like Ohm’s law to simply explain important electronics concepts, I’m starting with the equation that simply explains photosynthesis:

I liked this description:  Chemically speaking, the inputs to photosynthesis are six carbon atoms, 12 hydrogen atoms and 18 oxygen atoms. Glucose uses six carbon, 12 hydrogen, and six oxygen molecules. Simple math shows 12 leftover oxygen atoms, or six oxygen molecules. Interestingly, and not coincidentally, the process of respiration breaks apart the glucose molecule. Respiration occurs in the cells of nearly all living things…” …respiration releases the energy the plant uses to grow.

Photons are absorbed by the plant leaves through the green pigment – chlorophyll.  

Carbon dioxide enters the plant through the stomata (which are on the “underbelly” side of a leaf). 

Water comes into the plant through a plant’s roots.

and there it is..easy schmeazy.

Most Efficient for Photosynthesis

Since chlorophyll is the molecule that absorbs photons,

 Microscopic Image of Chlorophyll

 the most efficient LED color wavelengths for photosynthesis are red and blue.  Dr. Runkle’s article, “Light Wavebands & Their Effects on Plants” provided a nice summary:

  • “…Blue light (400 to 500 nm)… Chlorophyll in plants highly absorbs blue light that is used for photosynthesis.  It also helps regulate the opening of stomata, which are tiny openings in the leaves that regulate the uptake of carbon dioxide…”
  • “…Green light (500 to 600 nm)…Plants appear green because they reflect and transmit slightly more green light than they do blue or red light…. generally, green light is less efficient at stimulating photosynthesis than blue or red light…
  • …Red light (600 to 700 nm)… Most LED arrays  [for plant growth] emit a high percentage (often 75-90 percent) of red light because it is absorbed well by chlorophyll, and the electrical efficiency of red LEDs is high.    

 Notice under the rationale for red light the mention of electrical efficiency.  Looking at a graph from Cree’s XP-E2 data sheet:

voltageDropAcrossRedLED

the voltage needed to power red LEDs is much less than the voltage needed to power blue or green LEDs.  Since P=IV, the cost of electricity (e.g.: in KW/hour) will be less if there is more red LEDs than blue (or green).

Besides photosynthesis, research has shown red and blue wavelengths of light have a big effect on photomorphogenesis.

Photomorphogenesis 

Now this just plain out amazes me…plants have light sensors.  Who knew?  Not me…So many times I have built some sort of prototype that uses a light sensor like this one:

PhotoResistorLightSensor

I use it to detect simple stuff.  Like how light or dark the light sensor thinks it is.  Here is link to an Instructable that gives an example of an Arduino project that uses this type of simple light sensor.

Plants use of their much more sophisticated light sensors to – as explained in this article to “…manipulate the shape and characteristics of a plant, such as leaf size, branching, and stem length.”  Saying the same thing in a slightly different way, plants use light sensors to detect the light conditions under which they are growing.  The plants use the readings they get from the light sensors to manipulate their size and shape. By manipulating size and shape, the plant can optimize for the light conditions it finds it self under.

Light Sensors Are Detecting Red and Blue Light

Phytochrome and cryptochrome are the pigments found in a plant acting as light sensors.  Phytochrome detects red (and far-red) light.  Cryptochrome detects blue light.  It makes sense there is a close relationship between optimizing the plant’s shape to the light source since light is providing a necessary ingredient to make food.  For more info on these pigments, I found this article interesting.

The Ratio of Blue to Red Light

Given a bunch of LEDs mounted on a PCB, what percentage should be red and what percentage should be blue? The stuff about photosynthesis and photomorphogenesis prepares me to make an informed (first) attempt at setting this percentage.  The factors I use in deciding the percentage include:

  • Red light is more efficient for both photosynthesis and photomorphogenesis
    • photomorphogenesis: from this abstract: “Plants with as little as 10 µmol·m−2·s−1 of B(blue) light were 23% to 50% shorter and had 17% to 50% smaller leaves than plants under only R(red) light.”
    • photosynthesis: Back to  Dr. Runkle’s article, “Light Wavebands & Their Effects on Plants”: “Most LED arrays emit ahigh percentage (often 75-90 percent) of red light because it is absorbed well by chlorophyll”
  • Red light takes less electricity than blue light.  
  • Blue light will help keep the plant stocky.
    • Heidi  Wollaeger – our teacher for Michigan State’s lighting course and co-author of the peer-reviewed article “Growth and acclimation of impatiens, salvia, petunia, and tomato seedlings to blue and red light.”  pointed out to me in an email exchange: “…in a sole-source environment (no sunlight), blue light activates cryptochrome (a photoreceptor in plants) which triggers a change in hormones in the plant which results in reduced stem extension and leaf expansion. You will see that 100% R light (without sunlight) leads to the most elongated plants with the largest leaves. A minimum of 10% B inhibits that excessive stem extension and leaf expansion. What is “ideal” is all a matter of perspective. If you (are) an ornamental plant plug producer and want small compact rooted plants – you will want some B or green in the spectrum. If you a basil grower indoors, you would want those large leaves and might want to have a 100% R.

Blue to Red Ratio for the Prototype

Given I will be growing basil, I will go with 100% Red LEDs

What I’ve Learned After Writing this Post

  • I have a much better feel for photosynthesis and photomorphogenesis.
  • I understand what light quality is.
  • I understand the importance of red and blue light to photosynthesis and photomorphogenesis.
  • I have made an informed choice on the Blue:Red ratio – 0:100 – for prototyping growing basil.

What’s Next

Now that I have a better feel for light quality (the Blue:Red ratio and what it means for photosynthesis and photomorphogenesis), I need a better understanding of light quantity.  Then onto design and implementation of the LED portion of the planter.

 

 

Thanks for reading this far.  Please find many things to smile about.

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