Carbon Dioxide: It's What Plants Crave (pt 1)

Carbon dioxide is essential for plants to live and grow. Plants are autotrophs, meaning they can generate their own energy to live and grow by using the simple substances around them. People, on the other hand, are heterotrophs; we need to consume external sources of energy (food), like meat and vegetables.  
The simple substance plants use to generate energy: Carbon Dioxide.
During the photosynthesis process, plants use light energy to break apart the molecular bonds of the CO2 compound, shake it up with some H2O, and wahla! They’ve created hydrocarbons (CH-) and oxygen (O2). The hydrocarbons (aka sugars, carbohydrates) are used as the source of energy for metabolic processes, such as photosynthesis and evapotranspiration, and are the building blocks for cell growth and development. Oxygen is the waste byproduct of photosynthesis, and our symbiotic relationship with plants is rooted. 
When the environmental conditions are right – plentiful water, balance of nutrients, good weather, and lots of sunshine – plants will maximize stomatal opening in their leaves and gulp up as much CO2 as possible. The more CO2 under these optimal conditions, the more they’ll consume and photosynthesize, and the faster they will grow. This plant response is what drives many indoor farmers and greenhouse growers to enrich their plant environments with CO2.

Although increasing the CO2 levels alone can have a marked increase on photosynthetic rates, the effect can be even greater when other variables, such as air temperature and light intensity, are optimized together. The optimization point for CO2, air temperature, and light intensity is different for every crop. As shown in the graphs below, at a constant PPF of 2000 umol/m2/s, the optimal CO2 level is around 700-800 ppm under all air temperatures. But increasing air temperature from 18° to 32°C results in a marked increase in photosynthetic activity. At a constant temperature of 25°C, the maximum amount of CO2 the plant will use is between 400-500 ppm, but is nearly 2x greater at 2000 umol/m2/s than 500 umol/m2/s. These graphs demonstrate that all three of these variables affect photosynthesis and growth rates TOGETHER. Additionally, there is a limit to which these variables can be increased while still seeing a benefit to growth rates. 

Another important observation in these graphs is that there is an exponential increase from near zero CO2 concentrations up to about 300 ppm, regardless of the temperature and light level. Therefore, at minimum, it is recommended that growers maintain ambient CO2 levels (400 ppm) in their production rooms to make the most of lighting and temperature inputs. This is just one reason why I always recommend growers have access to outside air (ventilation), whether or not they plan to enrich their rooms with CO2 or not. See my video on more reasons for outside air access here.

Figures courtesy of Stanghellini, C, JA Bunce. Response of photosynthesis and conductance to light, CO2, temperature, and humidity in tomato plants acclimated to ambient and elevated CO2. Photosynthetica, 29(4): 487-497 (1993)

Notably, many of the studies to date have focused on traditional horticulture and floriculture crops grown in greenhouses. New research is starting to examine new indoor crops, such as leafy greens and culinary herbs grown in vertical farms. One such USDA-funded project, OptimIA, is specifically researching the co-optimization of multiple environmental factors – light, temperature, humidity, airflow, and CO2 – to help make the industry more profitable and sustainable. 
Not surprisingly, cannabis plants have yet to receive the attention required to optimize the inputs and environmental variables. We see a lot of cannabis growers enriching their rooms with 1200-1500 ppm CO2 under high pressure sodium lights, but then maintain temperatures down at 75°F. Based on the evidence presented above, it is possible that growers are pushing the accelerator on two variables (light and CO2), while hitting the brake on the third (temperature). What would be possible if air temperature was 80°F instead? 
Ultimately, carbon dioxide is the simple substance that plants crave and need to photosynthesize, to create their own energy, and to build cells and grow. But is there such thing as too much CO2? Could high levels of CO2 be detrimental to plant growth or product quality? I’ll be asking these questions in Part 2 of this blog.  

Growers: We’d love to hear from you! If you’ve tinkered with CO2 levels and these other environmental factors, have you found a sweet a spot? Have you noticed big differences when changing one factor over another?