How is CO2 supplementation affected by converting to LED Lighting?

Growers working in controlled environments strive to optimize the variables in their control to produce the highest yield and best quality products. These key variables include temperature, humidity, nutrients, light, and CO2. This article examines how switching to LED technology from older lighting technologies such as high-pressure sodium (HPS) could impact CO2 supplementation.

First, some background on the critical role of CO2 in plant development. During the Calvin cycle of photosynthesis, the plant enzyme rubisco enables carbon fixation, which ultimately results in CO2 and water being converted into simple sugars (carbohydrates). The chemical reaction involving rubisco is temperature-dependent, so to optimize photosynthesis we need to understand the interplay between light, temperature and CO2 concentration.

Looking at each variable independently in figure 1, we see that increasing light, CO2 and temperature (leaf surface temperature) increases photosynthesis. There are clearly diminishing returns in the photosynthetic rate at very high levels of light and CO2.

Interestingly, increasing temperature beyond an ideal point actually decreases the photosynthetic rate. This is attributable to the temperature dependence of the rubisco reaction – see figure 2. Without supplementing, the concentration of CO2 in ambient air is roughly 300 parts per million (PPM). Under these ambient conditions, the ideal leaf temperature is about 25°C.

If we add CO2 to the environment, we can generate higher rates of photosynthesis at higher leaf surface temperatures – see figure 3.

So the key to CO2 supplementation is to achieve the concentration where you begin to experience minimal additional photosynthetic production by adding more CO2. That ideal concentration will depend on the light intensity and the leaf surface temperature – see figure 4.

The grower will ensure the leaf surface temperature is within a narrow window to achieve the required vapor pressure deficit (VPD). An optimized VPD enables maximal transpiration and photosynthesis.

The controlled environment agriculture industry is experiencing a shift in lighting technology. LEDs are rapidly displacing HPS, metal halide and fluorescent technologies. LED lighting has some unique characteristics that must be taken into account when upgrading a grow facility from older lighting technologies. LED lights typically have very little infrared energy in the beam, which reduces the leaf surface temperature. With a lower leaf temperature, the grower may choose to either decrease the relative humidity or increase the heat in the room in order to maintain the necessary VPD. And this decision is likely to influence the ideal set-point for CO2 concentration. For example, if the leaf temperature is lower, the CO2 concentration should be lowered to prevent working in the “dark-limited” phase as shown in figure 4. Conversely, if the grow facility can return the leaf surface temperature to its previous level, then an adjustment is CO2 would not be required.

Although the ideal CO2 concentration is a function of the plant species, light intensity and, leaf surface temperature, figure 5 presents typical CO2 values for the various phases of cannabis development.

Maximizing biomass production in a commercial grow facility requires a deep understanding of the critical control parameters. While the benefits of LED grow lights are obvious, growers need to consider how the new technology impacts the canopy – especially as it relates to CO2 supplementation.   

Avoid this common mistake when comparing LEDs to HPS

So, you’ve decided to take the plunge and upgrade your grow room to LEDs. Simply unplug the HPS lights and install the LED lights. Better yields and lower electric bills – you are going to be a hero. Well, you might be a hero if you take into account how the switch to LEDs impacts your leaf surface temperature and the corresponding vapor pressure deficit (VPD).

It is well-established in plant biology that leaf surface temperature must be kept within a specific window to optimize primary metabolism (photosynthesis), as well as production of secondary metabolites. The relationship between leaf surface temperature and photosynthesis is shown in the figure below. The figure consists of data from a variety of plant species.

But leaf temperature is only part of the story. The critical factor is the interplay between leaf temperature and the relative humidity in the grow room. Those two factors (temperature and humidity) determine the vapor pressure deficit (VPD), which, in turn, determines transpiration efficacy and ultimately photosynthetic rates. An example of the relationship between temperature, humidity and VPD is illustrated in the chart below. To optimize production yield, the VPD must remain in the “sweet spot” identified in the green boxes.

High pressure sodium (HPS) lighting, has long been the workhorse in many indoor grow facilities. HPS emits in a broad portion of the electromagnetic spectrum that includes infrared (IR) energy – otherwise known as heat. IR energy from HPS heats the canopy and increases the leaf surface temperature. LED grow lights typically have only a small fraction of their emission in the IR portion of the spectrum, so they do not increase leaf surface temperature like HPS. In fact, it is typical to see a 5°-10° decrease in leaf surface temperature by changing the lighting from HPS to LED. If no other action is taken, the decrease in leaf temperature may throw the VPD out of its sweet spot – thereby decreasing transpiration and photosynthesis. This will most certainly not make you a hero in the grow room.  

So how do you ensure you are still in the proper VPD range after installing LED lights? Follow the steps below:

  1. Understand your baseline. Measure the leaf surface temperature and relative humidity while you’re still using HPS. Although humidity is easily measured, measuring leaf surface temperature requires specialized equipment such as a forward-looking infrared camera. Here’s one IR camera that will do the job: Don’t assume the leaf surface temperature is the same as the ambient air; this is rarely the case. Once you’ve taken the measurements, the VPD can be determined.
  2. Repeat step #1 after switching to LED.
  3. Determine if your VPD is still in the optimal range. If it isn’t, you should:
    1. Increase the ambient air temperature to raise the leaf temperature to the target temperature that satisfies the VPD requirement.
    2. Modify the relative humidity in the room to bring the VPD into the ideal range. 

One reason LED grow lights are so efficient is that they don’t produce excess heat in the light beam like older technologies (including HPS). However, to fully achieve all the benefits of LED technology, growers must understand how the lower heat content will affect their plants and take the proper steps to achieve optimal production.