Atmospheres

Section 2: Carbon Dioxide

Carbon dioxide (CO₂) serves as the carbon source for photosynthesis. During photosynthesis, CO₂ and water are combined (carbon is reduced) to form carbohydrates and oxygen. Carbohydrates are compounds such as sucrose, fructose and starch. Carbohydrates serve as both energy sources as well as the building blocks of the plant.

Although the outside atmospheric (ambient) CO₂ concentration varies by location and season, it is approximately 0.034% or 345 parts-per-million (ppm) at sea level. In enclosed greenhouses, the carbon dioxide level may be significantly lower. This is because plants utilize, and thus deplete the atmosphere of CO₂ during the day. In fact, CO₂ levels in greenhouses have been reported to be as low as 200 ppm. Typically, the more tightly closed the greenhouse and the less venting that occurs, the greater is the potential for CO₂ levels to drop significantly below the outside ambient level. This is problematic because as CO₂ concentration decreases, photosynthesis, and thus plant growth, slows or even ceases. As a result, venting even in the winter, particularly for tightly sealed greenhouses, may be necessary in order to replenish the CO₂ level in the greenhouse.

In addition to the fact that plants may absorb CO₂ faster than it is replenished from the outside, many plant species have been shown to respond positively (through increased photosynthetic rates) to CO₂ levels up to 2000 ppm, and most greenhouse crops respond positively to CO₂ levels up to 1500 ppm. This is obviously much higher than even outside ambient CO₂ concentrations. Therefore, in many situations in greenhouse environments, CO₂ may be the limiting factor for photosynthesis and increasing CO₂ concentration to above the outside ambient concentration may result in increased plant growth and productivity. Some common responses of greenhouse crops to increased CO₂ concentrations are listed in the table below.

Common Responses of Some Greenhouse Crops to Increased Carbon Dioxide Concentrations
Species	Responses
Rose	Decreased bud blasting, increased stem length, increased flower weight, increased number of petals, reduced cropping time
Carnation	Increased number of flowers, increased stem strength, increased flower weight
Chrysanthemum	Thicker stems, increased stem length, reduced cropping time, increased stock production
Tomato	Increased vegetative growth and increased total fruit weight
Lettuce	Reduced crop time and increased product weight

Because of the positive responses to increased CO₂ concentrations, some greenhouse operations have found it beneficial (from plant growth and economic perspectives) to inject CO₂ into the greenhouse atmosphere (also referred to as CO₂ enrichment) to increase the atmospheric CO₂ concentration.

Increased CO₂ levels are effective only during the day. However, vents must often be opened during the day (especially during summer and in southern climates). If vents are opened more than 5% of capacity or if exhaust fans are turned on, it is difficult to maintain increased CO₂ concentrations. Therefore, the period of time over which CO₂ injection can be used effectively may be limited depending upon season, climate, and the production systems being used. Additionally, to gain the maximum effect of elevated CO₂ levels, supplemental lighting, increased temperatures (5°F to 10°F higher depending upon crop) and increased fertility levels are often required. The plant cannot fix additional carbon dioxide if other factors such as light are limiting. Injecting CO₂, as well as increasing lighting and temperature, increases production costs. The gain in production and quality must offset these increased costs. In other words, many crops may respond positively to CO₂ injection, but the response or the level of response must pay for the increased inputs.

There are numerous methods of increasing greenhouse CO₂ concentrations in a greenhouse. Carbon dioxide burners are located within the greenhouse and combust natural gas, propane, butane or kerosene to produce CO₂. Complete combustion of these hydrocarbon fuels results in the production of CO₂ and water. However, if these systems are not maintained and operated properly, they may produce harmful carbon monoxide (CO) or ethylene (C₂H₄) gases that can be injurious to plants and dangerous for people. Additionally, natural gas used in CO₂ burners should contain no more than 0.02% sulfur (w/w) or sulfur dioxide may be produced. Sulfur dioxide combines with water on plant surfaces to form damaging sulfuric acid. Kerosene should contain no more than 0.06% sulfur. Burners used to produce CO₂ also will result in the formation of nitrous oxides. High concentrations of nitrous oxides can be injurious to plants. However most researchers have found that if the CO₂ level is maintained within acceptable ranges, nitrous oxide concentrations will generally be within acceptable levels or at least the negative impact of the nitrous oxides will be significantly less than the beneficial effects of increased CO₂ concentrations.

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Some greenhouse operations burn natural gas to operate boilers used for heating. During the day, the boilers are operated and heat water that is stored in tanks for night-time heating. The flue gases are passed through a manifold to separate CO₂ from flue gases produced from burning natural gas for heating. This CO₂ is then pumped into the greenhouse during the day.

In addition to burning hydrocarbons to produce CO₂, greenhouse operations may also use systems that inject carbon dioxide into the greenhouse using compressed liquefied carbon dioxide.

The optimal CO₂ concentration will depend upon the crop and cultural conditions. However, typically, CO₂ concentrations are increased to 1,000 to 1,500 ppm for most greenhouse crops. If the CO₂ concentration is too high, plant damage may occur. For example, above 1500 ppm CO₂, damage was reported on cucumber. On gerbera, CO₂ concentrations of 1600 ppm caused foliar chlorosis and necrosis and CO₂ concentrations of 2600 and 4500 ppm caused leaf dieback. Common CO₂ toxicity systems included leaf roll and foliar chlorosis and necrosis.

When deciding on whether or not to inject CO₂ into the greenhouse, it is very important to conduct research on the crop(s) of interest to determine the optimal CO₂ concentration, the optimal light, temperature and fertility regime, as well what types of plant growth responses might be expected. It is also important to determine the cost of increasing the CO₂ concentration including the costs of the required increases in light, temperature and fertility. The increased production, reduced cropping time and/or increased plant quality achieved will need to cover the cost of the increased inputs.

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