Carbon Dioxide Enrichment in Greenhouses
Plant Responses to Carbon Dioxide Enrichment
Carbon dioxide is an essential component of photosynthesis (also called carbon assimilation). 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. Although atmospheric and environmental conditions like light, water, nutrition, humidity, and temperature may affect the rate of carbon dioxide utilization, the amount of carbon dioxide in the atmosphere has a greater influence.
C3 and C4 Photosynthesis
Not all forms of photosynthesis are created equal, however. There are different types of photosynthesis, including C3 photosynthesis and C4 photosynthesis. Plants with a C3 photosynthetic pathway (geranium, petunia, pansy, aster lily and most dicot species) have a 3-carbon compound as the first product in their photosynthetic pathway, thus are called C3 plants and are more responsive to a higher carbon dioxide concentration than plants having a C4 pathway (most of the grass species have a 4-carbon compound as the first product in their photosynthetic pathway, thus are called C4 plants).
Elevated Levels of Carbon Dioxide on Plant Growth
Carbon dioxide is a major feedstock as well as a regulator of plant growth. As such it is essential to autotrophic plant life. The growth of the whole plant is often stimulated when carbon dioxide levels are increased.
Effect of Carbon Dioxide Enrichment on Different Growing Factors
The rate of photosynthesis cannot be increased further after the light saturation point, which is the maximum amount of light a plant can use. However, additional carbon dioxide increases the light intensity required to obtain the light saturation point, thus increasing the rate of photosynthesis. Mostly in the winter, photosynthesis is limited by low light intensity. An additional lighting system will enhance the efficiency of carbon dioxide and increase the rate of photosynthesis and plant growth. Supplemental carbon dioxide the physiology of plants through stomatal regulation. Elevated carbon dioxide promotes the partial closure of stomatal cells and reduces stomatal conductance.
Limiting Growth Factors
The effects of elevated carbon dioxide on plants can vary depending on other environmental factors. While elevated carbon dioxide makes carbon more available, plants also require other resources including light, water, temperature, and nutrients obtained from the substrate.
Greenhouse Carbon Dioxide Levels
The ambient carbon dioxide (naturally occurring level of CO2) concentration of 340 ppm (parts per million) can occur in a properly vented greenhouse. However, the concentration is much lower than ambient concentration during the day, especially in double-glazed structures that have reduced air exchange rates, and much higher at night in sealed greenhouses especially in double-glazed structures that have reduced air exchange rates. The carbon dioxide level may drop to 150 to 200 ppm (parts per million) during the day in a sealed greenhouse, because carbon dioxide is utilized by plants for photosynthesis during daytime.
Plant Damage as a Result of Carbon Dioxide Enrichment
Do not allow excessive carbon dioxide levels in greenhouses. Levels of 5,000 ppm can cause dizziness or lack of co-ordination to humans. Higher than recommended levels can cause necrosis of old tomato and cucumber leaves. African violet leaves become very hard and brittle, show a very dark greenish-grey color and often malformed flower petals, which do not fully expand. A similar symptom with freesia flowers has been observed where the carbon dioxide burner was used to provide the majority of the heat requirements of the greenhouse, and thereby generating excessive amounts of carbon dioxide. Except in emergencies, do not use carbon dioxide burners as the prime heating system.
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