Interactions of Light, CO2 and Temperature on Photosynthesis
Light and temperature are two of the most commonly manipulated environmental factors in greenhouse crop production. Supplemental lighting is useful during light-limiting conditions, while during high-light conditions, shading is often used to help prevent excessively high greenhouse temperatures. Often times, we simplistically think about light as the driver of photosynthesis and temperature as the key to control crop timing. Although that's generally true, other cultural and environmental factors influence photosynthesis and thus, crop growth and quality.
Light provides the energy for photosynthetic pigments to convert carbon dioxide (CO2) and water into sugars and oxygen. As light intensity increases Ð until a point Ð the amount of sugars increases and thus, more energy is available for plant growth and maintenance. However, the concentration of CO2 and temperature also influence photosynthesis in a potentially dramatic way. Cultural factors such as watering and fertility also influence photosynthesis; when these are limiting, photosynthesis is also limited.
Effects of CO2. The CO2 concentration outdoors continues to increase and is now 400 ppm — and even higher near urban areas. While this increase has negative effects on the environment, it is a main ingredient for photosynthesis and thus subtly increases plant growth. However, the CO2 concentration inside a greenhouse is often not at 400 ppm. For example, when greenhouses are closed during the winter and filled with crops, CO2 is used by plants and the concentration becomes low, perhaps as low as 200 ppm. As Figure 1 illustrates, a low CO2 concentration has two consequences: photosynthesis is reduced and the light saturation point is decreased. (The light saturation point is the intensity at which additional increases in light do not increase photosynthesis.) This means the value of supplemental lighting is marginalized at a low CO2 concentration.
Photosynthesis increases as CO2 increases until some saturating concentration, which is typically around 1,000 ppm. Enriching the air with CO2 enables plants to more effectively utilize light, resulting in an increase in the light saturation point. Just as with supplemental lighting, the law of diminishing returns applies to CO2 supplementation. Increasing the CO2 concentration from 300 to 500 ppm causes a much greater increase in photosynthesis than increasing the CO2 from 800 to 1,000 ppm. In the United States, few growers of ornamentals use supplemental CO2, but it is commonly used in greenhouse production of vegetables, especially for tomatoes.
Effects of temperature. The rate of most biological processes increases with temperature and that's also the case with photosynthesis. However, the "optimum" temperature for photosynthesis depends on the concentration of CO2, as illustrated by Figure 2. When the CO2 concentration is low, the rate of photosynthesis peaks at a moderate temperature, which varies from one crop to the next. If a greenhouse is enriched with CO2, then the rate of photosynthesis increases much more dramatically with increases in temperature, resulting in a higher "optimum" temperature for photosynthesis.
To maximize plant responses to light, consider bringing in fresh outdoor air during the day when the greenhouse is closed (during the winter) to avoid CO2 depletions. In addition, consider the costs/benefits of CO2 supplementation during periods of limited ventilation, especially when supplemental lighting is used. Remember that the benefits of CO2 enrichment are greater under high light levels and at warmer temperatures. Finally, during the summer, don't excessively shade plants since that can limit photosynthesis.