Increasing Carbon Dioxide for More Productive Plants?

Musings of a Cell Biologist on the Way to Becoming a Colorado Master Gardener

Posted by Kristin Moore, Master Gardener Apprentice  

As gardeners we spend a lot of time trying to figure out how to make our plants grow as productively as possible. However, if we really boil down the major constituents that make up a plant, it comes down to a small number of chemical molecules and elements. Water, which is generally taken up through the root system of a plant, can make up 80-90% of a live plant’s mass. However, if we take water out of the equation, the chemical elements that make up the vast majority of the dry mass of the plant (the mass recorded after water has been dried out) are carbon and oxygen, which combined make up approximately 90% of the dry mass of a plant. Another 6% dry mass is made up by hydrogen. The remaining 4% of plant dry mass is made up of the other 17 essential nutrients that we spend lots of time (and money) supplementing our soil with (examples: nitrogen, potassium, and phosphorus). While those other 17 nutrients are absolutely essential to plant growth and productivity, I am going to focus on the 90% of the plant that is carbon and oxygen for this article.

Have you ever stopped to think about where all the carbon and oxygen that make up most of what we’d really consider to be a plant (leaves, stems, bark, etc.) come from? Your initial response may be that they come from the soil and get incorporated into the plant through the roots. If so, you are not alone in this assumption (See this video that asks graduating MIT students this same question). In reality, all of the carbon and most of the oxygen come out of the air. Photosynthesis, the unique trait of all green plants (and some bacteria and algae), is the process of utilizing energy from sunlight to convert gaseous carbon dioxide (CO₂ - a carbon atom bonded to two oxygen atoms) into the sugars and structural molecules such cellulose that makes up the energy storage and fibrous tissue of most plants. Plants do not actually “suck up” carbon from the ground. That 90% of the dry mass of a plant came from converting the CO₂ in the air into carbon-based molecules in the plant.

This fact then begs the question: in an era of increasing CO₂ emissions and concentrations in our atmosphere, might plants actually benefit from increased concentrations of carbon in their environment? Could they grow bigger faster since one of their major nutrients is now in excess? As it turns out, the answer to this question is a lot more complex than it may initially appear. There is strong evidence that increasing atmospheric CO₂levels does increase the photosynthetic rate of plants. This results in larger, faster growing plants. Higher CO₂concentrations also results in less water loss for many types of plants. This is because plants have to open up pores on their leaves called stomata to let CO₂ from the air into the plant. When stomata are open, not only can CO₂ come in, but water can also escape out into the air. With higher CO₂ levels these pores are open for less time resulting in less water loss from the plant.

That all sounds pretty great! Bigger plants with lower water consumption! However, the increased rate of carbon incorporation into plant tissues results in increased amounts of sugar (mostly made of carbon and oxygen) compared to protein (lots of nitrogen) in a plant. Thus, plants grown in elevated CO₂ may not have the same nutritional value (more sugars and less protein) as those grown under current CO₂ levels. There is evidence to indicate that insects feeding on these plants have poorer outcomes and cause more plant damage as they try to make up for these nutrient shortcomings. One way to fix this imbalance may be to increase nitrogen supplementation through fertilization, which can be costly and have other impacts. Additionally, the impact of having extra sugar molecules accumulating within the plant is unknown. While sugars provide energy and structure to a plant cell, they can also signal plants to make changes in the way they grow. We are just starting to gain an understanding of how the increases in sugar accumulation due to increased CO₂concentration would impact this facet of plant physiology. Lastly, the overall increase in temperatures that accompany increased CO2 production as well as the changes in weather patterns will ultimately have a huge impact on plant growth and productivity. Many of the current predictions indicate that these will not increase plant yields.

Schematic from 'Preserving the nutritional quality of crop plants under a changing climate: importance and strategies' (Soares et. al., 2019). GHG = greenhouse gas; eCO2 = elevated carbon dioxide; Single letter chemical element abbreviations outlined in red boxes; OM = organic matter. 

Combined, all of these data indicate that the impacts of changing levels of atmospheric CO₂ on plant physiology, production, and value remain…complicated at best. Although increased plant growth and decreased water consumption are consistent results in many experiments run under increased atmospheric CO₂ concentrations, there is an upper limit to this increase. Furthermore, how these plants will adapt to their new environments (water availability, temperature, growth season length, etc.), and whether they will have the same value as those grown today is uncertain. What is certain, is that our changing environment will ultimately results in changes to plant productivity and their critical role in the global carbon cycle.