Maximizing culinary herb production with supplemental CO2 in commercial greenhouses

Photos © Adobestock and Firefly

Lights, camera, action!

That’s an iconic phrase we associate with movies and filmmaking. What might be the cues we can use to “set the stage” for a productive day of growing plants in the greenhouse? How about “lights, CO₂, grow!”?

Plants need light and carbon dioxide (CO₂) for photosynthesis. This is a process in which plants use these inputs to create sugars that can be used for growth and development. When plants do not have enough light or CO₂, they will grow more slowly. For edible crops like fresh-cut culinary herbs that are sold by fresh weight, quickly producing a lot of harvestable biomass is important.

The rate of photosynthesis is capped by whichever resource happens to be most limiting at the time. We often think of providing more light to boost photosynthesis. Many growers, especially in northern states, provide supplemental lighting between late fall and early spring to augment low amounts of ambient sunlight.

However, the CO₂ level in a greenhouse is also an important consideration. Unlike light that we can “see” and temperature that we can “feel,” we might not think about CO₂ because it is a colorless, odorless gas that we cannot sense. But CO₂ is very important to plants.

Why might CO₂ be limiting in the greenhouse?

Ambient CO₂ levels outdoors are currently about 425 parts per million (ppm). During the day, the CO₂ concentration inside a greenhouse will fall below the CO₂ concentration outside as plants take up and use the CO₂ to photosynthesize. Greenhouse CO₂ concentrations can frequently fall below 300 ppm, especially on sunny winter days, when the greenhouse is tightly sealed to maintain temperature.

When the CO₂ concentration falls below ambient, photosynthesis will slow down, even if supplemental lights are turned on. Conversely, when additional CO₂ is provided, plants can utilize the extra CO₂ up to a point to boost photosynthesis.

One of our goals for this culinary herb project is to optimize the growing conditions for production. In this article, we will discuss how much CO₂ different culinary herbs need to sustain high rates of photosynthesis. We will also explore how adding CO₂ and light together can boost photosynthesis.

How do we measure plant response to CO₂?

We have an instrument that allows us to measure photosynthesis in real time. Keeping the leaf attached to the plant, we gently place the leaf in the measuring chamber. Inside this measuring chamber, we can monitor and adjust the light intensity, CO₂ concentration, temperature and relative humidity. It’s like a miniature growth chamber.

We can set the measuring chamber to match the conditions that the plant is currently growing in and measure its photosynthetic rate. Or, we can change one or more conditions and see by how much the photosynthetic rate changes.

For this culinary herb study, we grew basil, cilantro, mint, parsley and sage in 4.5-inch pots for three to four weeks before we started collecting measurements. Then, one plant at a time, we measured the photosynthetic rate over a range of CO₂ concentrations, from very low to very high. We did this at a “moderate” light intensity (300 µmol·m^-2·s^-1 photosynthetic photon flux density) and again at a “high” light intensity (1000 µmol·m^-2·s^-1 PPFD). Finally, we used the data and fit models of photosynthetic rate for each herb at the two light intensities (Figure 1).

What did we find?

In general, photosynthetic CO₂ response curves have a similar shape. Photosynthesis will initially increase in a straight line (linearly) as the CO₂ concentration increases. However, as more and more CO₂ is provided, plants will eventually no longer be able to use the CO₂ as efficiently.

This is because something else will have become the limiting factor, such as the amount of an input (like light) or the amount of cellular machinery (like proteins) dedicated to photosynthesis. At this point, the shape of the response will begin to curve and then flatten into an almost horizontal line.

Each herb has its own unique response curve. Specifically, the slope of the linear portion of the response, how much of a bend in the curve there is and when CO₂ saturation occurs will differ with crop and growing conditions.

How limiting is low CO₂ to photosynthesis?

As we mentioned earlier, greenhouses can have low CO₂ concentrations, especially around the plant canopy, as plants use CO₂ for photosynthesis. Based on our response curves for the culinary herbs, decreasing the CO₂ from 400 to 300 ppm decreased photosynthesis by 18% to 22% at “moderate” light (blue line in the graphs) and decreased photosynthesis by 21% to 26% at “high” light (orange line in the graphs). Cilantro was the least sensitive to the decrease in CO₂.

This highlights two findings. One, the impact of below-ambient CO₂ was more severe at high light. And two, keeping CO₂ levels as close to ambient as possible will minimize unintentionally limiting photosynthesis.

This can be achieved by using fans to increase air flow around the plant canopy. Venting the greenhouse to bring in fresh air is another good option, but it must be considered in combination with temperature and humidity management.

How beneficial is supplemental CO₂?

The “moderate” light intensity of 300 µmol·m_-2·s_-1 PPFD is realistic in the greenhouse during springtime on sunny days or with supplemental lighting on cloudy days, and the “high” light intensity (1000 µmol·m_-2·s_-1 PPFD) is realistic on sunny days in the greenhouse.

Increasing CO₂ from 400 to 600 ppm increased photosynthesis by 18% to 26% at moderate light and by 22% to 34% at high light. Doubling the CO₂ concentration from 400 to 800 ppm increased photosynthesis by 26% to 39% at moderate light and by 27% to 48% at high light.

From this, there are two pieces of good news. First, supplementing CO₂ above ambient levels increased photosynthesis. And second, the increase in photosynthesis with additional CO₂ was greater at the higher light intensity. The additional CO₂ allowed the plants to use more photons of light that otherwise would have been unusable at the high light intensity.

However, as we look at the response curves, we’d like to point out that the lines are already starting to curve as we move from 400 to 800 ppm CO₂. Thus, a 50% increase in CO₂ (from 400 to 600 ppm) provided less than a 50% increase in photosynthesis. The increase from 600 to 800 ppm was even less of a return, only boosting photosynthesis an additional 8% to 14%.

This doesn’t mean that adding CO₂ isn’t a good idea. It means that the largest benefits of adding CO₂ occur as we move from below-ambient to ambient levels. As we supplement CO₂ above ambient levels, we will see additional gains, but they will not be a 1-to-1 return. For example, doubling the CO₂ concentration will not double the harvestable biomass.

Lastly, we mentioned earlier that photosynthesis can only occur as fast as the most rate-limiting step or input. When photosynthesis is limited, adding more light or adding more CO₂ can boost it. Often, adding light and CO₂ together will provide a synergistic effect. This means that adding both at the same time can give a greater response than the sum of adding each one individually.

Using basil as an example, increasing the light intensity from moderate to high light at ambient CO₂ (400 ppm) increased photosynthesis by almost 50% (Figure 2). At moderate light, doubling the CO₂ level from 400 to 800 ppm increased photosynthesis by roughly 25%.

Thus, we might expect that adding both light and CO₂ at the same time would increase photosynthesis by about 75% if the effects were additive. Instead, adding light and CO₂ increased photosynthesis by almost 90%.

Take-home messages

Monitoring your greenhouse CO₂ levels can be a good starting point to see if they are lower or higher than expected. If greenhouse CO₂ levels are consistently below ambient, strategies that increase air movement around the plant canopy or increase the air exchange rate in the greenhouse can help minimize the CO₂ drawdown, especially during periods of rapid plant growth.

Adding supplemental CO₂ to increase levels above ambient can also increase plant growth, but the increase in photosynthesis per increase in CO₂ will incrementally decrease the further above ambient that you go. Supplementing to maintain ambient CO₂ levels or to boost CO₂ to about 600 ppm would provide the highest efficiencies.

However, if maximizing photosynthesis is the key target, then supplementing to 800 to 1,000 ppm CO₂ would be an appropriate range to target. Supplemental CO₂ can be provided through gas cylinders or tanks, CO₂ burners or CO₂ generation systems.

The five herbs that we evaluated — basil, cilantro, mint, parsley and sage — all had similarly shaped response curves. This means that one general approach to CO2 management, rather than individualized plans, will be suitable. Just remember…lights, CO₂, grow.

Jennifer Boldt is a research horticulturist and Forrest Brown was a postdoctoral research associate with the USDA’s Agricultural Research Service. Roberto Lopez is an associate professor in the Department of Horticulture at Michigan State University. The authors acknowledge funding from the USDA Specialty Crop Research Initiative, award #2022-51181-38331, for funding.