Chapter 7

Carbon Dioxide in Greenhouses

Carbon Dioxide and Plants

Carbon dioxide is an essential component of photosynthesis. Photosynthesis is a chemical process that uses light energy to convert carbon dioxide and water into sugars in green plants. These sugars are then used for growth within the plant, through respiration. The difference between the rate of photosynthesis and the rate of respiration is the basis for dry-matter accumulation (growth) in the plant. In greenhouse production the aim of all growers is to increase dry-matter content and optimize crop yield. Carbon dioxide increases productivity through improved plant growth and vigor.

Carbon Dioxide Concentration

The concentration of carbon dioxide in ambient outside air commonly varies from 300 to 500 parts per million (ppm) or more by volume depending on the season, time of day and the proximity of carbon dioxide producers such as combustion or composting, or carbon dioxide absorbers such as plants or bodies of water. Carbon dioxide levels can drop quite rapidly after sunrise as the crop’s photosynthetically-driven consumption of carbon dioxide exceeds the basic rate of respiration.

Carbon Dioxide Depletion

In a greenhouse filled with plants, carbon dioxide concentration will closely follow ambient outside concentrations during the day as long as ventilation is needed. Carbon dioxide concentrations rise during the dark period because plants are not using carbon dioxide for photosynthesis. During light periods in which ventilation is not required, carbon dioxide levels may fall below ambient. However, in the winter, greenhouses may be closed during the day to conserve heat. This situation may occur for several consecutive days during periods of inclement weather in the cooler temperate and frigid zones.

Carbon Dioxide Effect on Plant Growth

Many studies have shown that carbon dioxide concentrations well above ambient can benefit plant growth. Typically, a three- to four-fold increase in carbon dioxide concentration yields a 10 to 25 percent increase in plant growth. Supplemental carbon dioxide increases leaf area, dry weight, lateral branching, and in some cases decreases time to flower.

Carbon Dioxide and Light

As light increases from a very low level, photosynthesis increases up to the light saturation point. However, if supplemental carbon dioxide is added to the greenhouse atmosphere, the light saturation point is reached at a higher light intensity and at a higher photosynthetic rate.

Carbon Dioxide and Nutrition

Rapid plant growth under supplemental carbon dioxide and bright condition also means an increase in the rate of nutrient uptake and utilization by plants.

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