Author: Michael Forster, PhD.
Growers can artificially increase the amount of atmospheric carbon dioxide (CO2) available to plants to improve growth and yield. Scientists call it “atmospheric fertilization” and growers have known for a long time that increasing CO2 can improve plant productivity. Adding additional CO2 around plants is similar to adding additional nitrogen to the soil – it gives plants a boost to grow more rapidly and increase the size and number of fruits.
Low cost technology is now available to growers to automatically detect, monitor and control CO2 levels in grow rooms, greenhouses and glasshouses. Broadly, the technology available today allows growers to take one of two approaches to artificially increase atmospheric CO2 around their plants: the continuous approach or the day/night approach.
The continuous approach constantly maintains CO2 at a level during both the day and night. For example, a CO2 level controller sets CO2 to a pre-defined minimum and maximum level (e.g. 1,200 to 1,500 parts per million [ppm]) and opens or closes a valve to a CO2 gas cylinder when these pre-defined levels are reached.
The day/night approach only provides additional CO2 from a gas cylinder during daylight hours. For example, a day/night CO2 level controller acts as a continuous CO2 level controller, as described above, during the day. But at night, the valve to the CO2 cylinder is always turned off. A day/night CO2 level controller has an inbuilt light sensor that can detect whether it is day or night and switches on/off accordingly.
The justification for not providing additional CO2 for plants during the night is that plants only require CO2 during the day for photosynthesis. At night, there is little or no photosynthesis and, therefore, there is no reason why CO2 should be maintained at artificially elevated levels. This is particularly the case for the group of plants known as C3 plants.
In this article, we outline why you may consider the day/night approach. We are not recommending the day/night approach over the continuous approach; rather we outline 5 reasons why adopting the day/night approach may actually reduce your costs while increasing productivity and yield when compared to the continuous approach.
Disclaimer: Whether the continuous or day/night approach is suitable for you depends on a range of factors. If you are unsure whether the continuous or day/night approach is better suited to your particular situation, we highly recommend you seek the advice of a local expert agronomist. You may even consider conducting your own experiments to determine which approach is more suitable for your particular situation and crop. Results can vary between crops (for example, Bunce  found total biomass varied significantly between five different plant species in response to continuous or day/night approach) and can even significantly vary within a crop species with numerous cultivars (for example, see Ainsworth et al  for a review of how elevated CO2 can influence the growth and yield of different soybean [Glycine max] cultivars).
1. decrease CO2 costs
One of the most obvious and instant advantages to turning off CO2 at night is to save on costs. Injecting CO2 gas into a grow room can be expensive and a calculation can be used to determine how much gas you need for a given room size and how long a gas cylinder will last. By turning the valve for the CO2 gas cylinder off at night, almost instantly your CO2 gas cylinder costs may decrease by up to one half. The cost savings will depend on the length of the night time in your particular geographic growing region and the time of the growing season. In the tropics, this could be up to 12 hours per day consistently throughout the year. In temperate regions, daylight is longer during the peak growing season so the cost savings may not be as high. However, if a growing season can be achieved during the colder months, for example with supplementary light and increased temperature, then the cost savings of switching off CO2 at night may be higher.
The need to manage relative humidity levels in greenhouses and glasshouses can also be at odds with maintaining night-time CO2 enrichment. There is a continuous need to manage the internal greenhouse climate even at night with respect to humidity levels that can impact on increasing disease pressures like encouraging fungal pathogens. Varying greenhouse temperature is one strategy to manage rising humidity but this has significant associated energy costs. The Mollier diagram, or psychometric chart, can be used to show that it is typically easier and cheaper to open the roof ventilators to achieve the same result with less energy used, however this would mean significant loss of CO2 to outside that would require extra input to maintain their target levels. Turning off CO2 at night simply overrides the need to maintain an enhanced CO2 level.
Many commercial growers recognise that the supply of carbon dioxide at night is for C3 vegetable crops not needed. The crop raises the carbon dioxide concentration during the night due to the respiration of the crop. So a high concentration will be present when the greenhouse is not ventilated. The fact that the crop releases carbon dioxide instead of using it, indicates an additional supply of carbon dioxide at night has little utility. However, for plants undertaking other forms of photosynthesis, such as CAM plants, a supply of CO2 at night is a different story.
2. decrease energy costs
Similarly to reason number 1, turning off CO2 at night saves on energy costs. Electricity is required to run CO2 detectors, flow meters, valves and cylinders. Therefore, if energy can be saved in any way possible then this will add a cost advantage to the grower. Furthermore, significant energy usage during the day can be offset by investments in a solar panel system. This option is obviously not available during the night and investing in battery storage is still largely prohibitive. Although, using electricity during off-peak hours, such as the night, usually has a lower associated per kilowatt cost than during the day, or peak, period, there is still a cost that can be eliminated or reduced.
As mentioned under reason #1, there is also a need to manage relative humidity levels at night. Increasing temperature can do this but there is significant energy costs associated with this management strategy. It is easier and cheaper to open the roof ventilators instead but then this will lead to a loss in CO2. Turning off CO2 at night will overcome this cost.
3. increase plant growth
It is generally common knowledge amongst plant physiologists, horticulturalists and agronomists that increasing atmospheric CO2 significantly increases plant growth rates and final biomass. For example, Poorter and Navas (2003) found that at elevated CO2 the biomass enhancement rate in plants increased by as much as 48%.
Griffin et al (1999) conducted the first experiment to determine whether switching off CO2 at night had any effect on plant growth. Their experiment was conducted on soybean (Glycine max) cultivar Williams grown in environmentally controlled growth chambers. Griffin et al had four experimental treatments: 250 ppm CO2 during the day and at night (250/250 treatment); 250 ppm CO2 during the day and 1000 ppm during the night (250/1000 treatment); 1000 ppm CO2 during the day and 250 ppm during the night (1000/250 treatment); and 1000 ppm CO2 during the day and night (1000/1000 treatment). For the purposes of this article, we are interested in comparing the results of the 1000/250 treatment (day/night approach) against the 1000/1000 treatment (continuous approach) but we will also discuss the results from the 250/250 treatment (or not artificially increasing CO2 for plant growth). By the end of the experiment, Griffin et al found that total biomass in the 1000/250 treatment was 24.69 grams versus 19.38 grams in the 1000/1000 treatment (and versus 8.91 grams in the 250/250 treatment). Therefore, by turning off CO2 at night, Griffin et al actually increased total plant biomass when compared with leaving CO2 on continuously.
However, Bunce (2003) conducted a similar experiment and found variable results. Bunce found that the day/night approach significantly increased plant growth in two species (Amaranthus retroflexus, a C4 herbaceous perennial species; and Medicago sativa, a C3 herbaceous perennial species), but the continuous approach significantly increased growth in two other species (Acer rubrum, a tree species; and Glycine max, an herbaceous annual species and contrary to the results found by Griffin et al for G. max).
Adding nitrogen to the soil can also affect the growth response of plant species to the day/night or continuous approach. For example, Asensio et al (2015) conducted an experiment with Arabidopsis thaliana and Triticum aestivum (better known as wheat) plant species and added nitrogen fertilizer in the form of either nitrate (NO3-) or ammonium (NH4+). Asensio et al found no difference in plant growth in either species when ammonium was added, yet there was a significantly greater increase in growth in A. thaliana when nitrate was added to the soil under the day/night approach.
These scientific experiments highlight the variability across growth forms (trees versus herbs), species, and even with species (for example, the variable results with soybean). There is not a single solution for all situations and care should be taken to apply either the day/night or continuous approach to your particular crop and circumstances.
4. increase seed output or plant yield
Seed output, or yield, is perhaps even more important than plant growth rate for growers. A plant that grows rapidly is next to useless if it produced no or poor quality yield. Generally, atmospheric CO2 enhancement experiments increase the yield in economically important crop species. For example, relative yield in rice (Oryza sativa) increased between 13% and 44%; relative yield in soybean (G. max) increased between 27% and 40%; and relative yield in wheat (T. aestivum) increased between 19% and 47% (Ziska and Bunce 2007). Kimball et al (2007) grew sour orange trees (Citrus aurantium) at elevated CO2 from seed for 17 years. Harvest of oranges in year 17 of the experiment was 32.9 kg per tree in the elevated CO2 treatment versus 10.9 kg per tree in the ambient CO2 treatment. Over the entire 17-year period of the experiment, total cumulative fruit harvest for the elevated CO2 treatment orange trees was 518.2 kg per tree versus 280.8 kg per tree for the ambient CO2 treatment.
As an increase in atmospheric CO2 leads to larger plants, perhaps it is not surprising that the total amount of fruit and seed on the plant also increases. However, does the day/night or continuous approach have any effect on plant yield?
Bunce (2005) provided the results from a 4-year experiment on soybean grown in the field under ambient, day/night and continuous CO2 treatments. Year to year yield varied with natural variations in seasonal rainfall, temperature and evaporative demand. Across all years, the continuous treatment yielded significantly more seeds than the ambient treatment. During tough growing seasons the day/night treatment had a similar seed output to the ambient treatment. However, during good to excellent growing seasons the day/night treatment had similar seed output to the continuous treatment. There was no growing season where the day/night treatment had greater seed output than the continuous treatment. Bunce (2014) also found a similar result in an experiment with common garden bean (Phaseolus vulgaris).
Earlier, we mentioned the experiment on soybeans by Griffin et al (1999 – see reason number 1, above) and how the day/night approach significantly increase plant growth and biomass. However, this experiment also found that there was no yield, or seed set, at all, in the soybeans grown under the day/night approach. The authors could not offer a reason for this outcome.
These few experiments indicate that growers should carefully consider whether the day/night approach is appropriate for their particular situation. We highly recommend you undertake your own experiments with both approaches to determine whether the day/night or continuous approach increases seed output and yield for your particular crop.
5. improve the water use efficiency (WUE) of plants
Stomata are pores on the leaves of plants that open to allow water to escape the plant in exchange for the uptake of CO2 for photosynthesis. When there is more CO2 in the atmosphere, of CO2 has been enhanced or elevated, the pores on the leaves do not need to open as wide in order to uptake the same amount of CO2 for photosynthesis. Therefore, the amount of water that is lost from a plant decreases when the amount of CO2 in the atmosphere increases. When water is lost through the stomata on the leaves, scientists call this stomatal conductance. In European forest trees, the rate of stomatal conductance decreases by 21% when atmospheric CO2 approximately doubles (Medlyn et al 2001).
On the other hand, photosynthesis is stimulated, or increases, with elevated CO2. That is, the more CO2 in the atmosphere increases the rate of photosynthesis and the growth rate and overall plant biomass also increases, as discussed above in this article. Ainsworth and Rogers (2007) found that photosynthesis, on average, increases by about 30% when atmospheric CO2 approximately doubles.
Water use efficiency (WUE) is defined as the amount of carbon assimilated in photosynthesis per unit of water loss through the stomata. Or, in other words, the photosynthetic rate divided by stomatal conductance. Under enhanced CO2, photosynthesis increases by about 30% and stomatal conductance decreases by about 20% and, therefore, the WUE of plants increases.
Growers can therefore increase the amount of plant biomass and yield by using relatively less water when they increase atmospheric CO2 in their grow rooms. But does this relationship still hold when the day/night approach is taken?
Bunce’s (2014) experiment on the common garden bean tested this hypothesis. Bunce found that there was no difference in stomatal conductance between the continuous treatment and the day/night treatment. There was also no difference in the photosynthetic rate between the continuous and day/night treatments. Therefore, switching CO2 off at night does change the WUE of the common garden bean.
However, as we now know, increasing atmospheric CO2 leads to bigger plants which also means plants have a larger amount of leaves and a larger amount of leaf area. So, even though stomatal conductance decreases in elevated CO2 environments, the overall water use of the plant, or transpiration, can actually increase because the plants are significantly larger. In Bunce’s experiment with the bean, leaf area in the continuous CO2 treatment was significantly larger than the day/night treatment. Although Bunce did not measure total plant water use or transpiration across the experimental treatments, it is possible that plants in the continuous treatment had a higher overall demand for water than the plants in the day/night treatment.
Controlling CO2 in the grow room, greenhouse or glasshouse via the continuous approach or day/night approach have their advantages and limitations. The day/night approach logically wins out on the cost side by using less CO2 and electricity (and possibly water). On the production and yield side, there is some evidence to suggest the day/night approach is better, similar or slightly worse than the continuous approach. The results vary widely across species and even cultivars within species and there is no one single solution for all situations. For your particular circumstances and crop, we highly recommend consulting a local, expert agronomist or conducting your own experiments with the continuous and day/night approaches.