Previous Blog Posts
Blog 1 - Challenge Overview + Plant Growth Factors
Blog 2 - Plant Growth Chamber Design
Blog 3 - The Project Plan + Prototype Materials
Blog 4 - Automation Using The Challenger Kit
Enclosure Prototype
I found a 0.5m3 cardboard shipping box in the garage and repurposed it into my enclosure prototype. While the enclosure can certainly accommodate 32 pot systems, I chose to prototype with only 8 so I could focus on quality experimentation over quantity. More on the plant pot system in the next blog.
Light
Lighting is pretty straightforward. Put plants under any adequate light, and they're sure to grow right? Wrong. I had that misconception before and it cost me weeks of head scratching and confusion. The reality is that there's science behind plant lighting and (fancy vocabulary aside) it's not too difficult to learn. First, let's familiarize ourselves with basic vocabulary and fundamental concepts:
PPFD (Photosynthetic Photon Flux Density) umolm2·s
PPFD is the rate at which photons hit plant leaves. This is affected by how far grow lights are positioned above the plants as well as the grow light intensity. If we were to compare light to rain, PPFD would be the rate of rainfall.
DLI (Daily Light Integral) molm2·day
As the name suggests, daily light integral is the total amount of photons that hit plant leaves over the period of a day. Back to our rain analogy, DLI would be the amount of rainfall.
Achieving a specific DLI is what really matters for plant growth. Having a 3000 PPFD and a 6-hour light photoperiod yields the same DLI and growth results as having a 1500 PPFD and a 12-hour light photoperiod.
Here's an amazing video with Dr. Bruce Bugbee explaining the science behind plant lighting. He is the Director of the Crop Physiology Laboratory at Utah State University and his work includes research into space farming with NASA. I gained most of my fundamental plant lighting knowledge through watching 5:30 - 19:30.
Summary of Spectral Effects
Another consideration is the spectrum of light. While all spectrums of visible light contribute to photosynthesis, each type yields a particular effect:
Blue photons inhibit cell expansion
Green photons facilitate human vision
Red photons efficient photosynthesis
Far red photons enhance cell expansion
Crops grown under mostly blue light will be more compact while those grown under mostly red light will be more elongated.
Chlorophyll, the key pigment responsible for photosynthesis, primarily absorbs red and blue photons. So red/blue LEDs, which exclude green light, are going to be the most effective right? Not particularly. Research conducted by Dr. Bruce Bugbee shows that green photons are just as good at photosynthesis as other photons across the spectrum as demonstrated by the following video from start to 9:30.
So are red/blue LEDs or full spectrum white LEDs a better choice? Well, considering that both yield similar growth results and that green photons in white LEDs add color to plants, white is the clear choice. I found 12V LED light strips that are 0.5m long, the perfect length for a snug fit in the enclosure, and I got a 12V power supply to accompany the LEDs. For a future iteration of this project, customizable colored LEDs with far red light would be great to implement because plants can benefit from customized light recipes. The Automation HAT could catalogue ideal light recipes per plant so that when the user tells the system what plants are in the chamber, the system would automatically activate the custom light recipe for those plants. For now, white LEDs are sufficient and within my tight budget.
Red leaf lettuce thrives in a DLI of around 10. My LEDs produce a PPFD of a little over 200 as indicated by my light meter. Choosing a 12-hour light photoperiod yields my desired DLI. I used this PPFD to DLI calculator to quickly do the math. For the time being, I'll use an outlet timer to automate the 12-hour light photoperiod. The same calculations were used used to determine the ideal lighting conditions for tomato, onion, and cilantro.
Air/Temperature
With all that time spent on lighting, it is easy to forget about other factors for plant growth. One crucial factor for plant growth that is easily overlooked is air. Natural convection currents on Earth facilitate the flow of oxygen and carbon dioxide, which we pretty much take for granted. Without natural convection currents in space, plants could suffocate in bubbles of oxygen. To ensure that carbon dioxide circulates to the leaves, I installed a small fan in the interior of the chamber.
The fan not only creates convection currents but also helps regulate temperature to a small extent. Temperature and air flow impact the rate of transpiration, the evaporation of water from plant leaves. This flow of water enables nutrients to travel from the roots up the stem and ultimately arrive at the leaves. Controlling the speed of the fan through a power supply with power control, we can regulate two key factors that impact transpiration.
In the next blog, I'll describe my plant pot system and demonstrate how it innovates upon existing methods! Until then, keep learning and have fun! Click the arrow to navigate to the next blog.
