The following article about carbon dioxide and plants comes from the Ontario, Canada, ministry of
agriculture, food, and rural affairs. To see the article in its entirety, visit:
http://www.omafra.gov.on.ca/english/crops/facts/00-077.htm
Carbon Dioxide in Greenhouses
The benefits of carbon dioxide supplementation on plant growth and production
within the greenhouse environment have been well understood for many years.
Carbon dioxide (CO2) is an essential component of photosynthesis. Photosynthesis
is a chemical process that uses light energy to convert CO 2 and water into sugars in
green plants. These sugars are then used for growth within the plant.
In greenhouse production the aim of all growers is to increase plant growth and
economically optimize crop yield. CO2 increases productivity through improved plant
growth and vigor. Some ways in which productivity is increased by CO 2 include
earlier flowering, higher fruit yields, improved stem strength and flower size.
Growers should regard CO2 as a nutrient.
For the majority of greenhouse crops, net photosynthesis increases as CO 2 levels
increase. (See graph #1.) Most crops show that increasing CO2 level to 1,000 ppm
will increase the photosynthesis by about 50% over ambient, or usual, CO2 levels.
The usual CO2 level is 340 parts per million. For some crops the economics may not
warrant supplementing to 1,000 ppm CO 2 at low light levels.
Carbon dioxide enters into the plant through the stomatal openings by the process
of diffusion. Stomata are specialized cells located mainly on the underside of the
leaves in the epidermal layer. The cells open and close allowing gas exchange to
occur. The concentration of CO2 outside the leaf strongly influences the rate of
CO2 uptake by the plant. The higher the CO2 concentration outside the leaf, the
greater the uptake of CO2 by the plant. Light levels, leaf and ambient air
temperatures, relative humidity, water stress and the CO 2 and oxygen (O2)
concentration in the air and the leaf, are many of the key factors that determine the
opening and closing of the stomata.
During particular times of the year, the carbon dioxide levels in greenhouses can
easily drop below 340 ppm. This has a significant negative effect on the crop.
Ventilation during the day can raise the CO 2 levels closer to ambient but never back
to ambient levels of 340 ppm. Supplementation of CO2 is seen as the only method
to overcome this deficiency and increasing the level above 340 ppm is beneficial for
most crops. The level to which the CO2 concentration should be raised depends on
the crop, light intensity, temperature, ventilation, stage of the crop growth and the
economics of the crop. For most crops the saturation point will be reached at about
1,000–1,300 ppm under ideal circumstances.
Relative Rate of Photosynthesis
rate of photosynthesis
140
120
100
80
60
40
20
0
0
200
400
600
800
1000
1200
1400
1600
Concentration of carbon dioxide in parts per million
Graph #1
1. What level of carbon dioxide would be considered the control group?
a. 0 ppm
b. 340 ppm
c. 600 ppm
d. 1400 ppm
2. What would be an appropriate problem statement for graph #1?
a. What is the rate of photosynthesis?
b. This experiment is designed to see how different carbon dioxide concentrations affects the rate of
photosynthesis.
c. This experiment is designed to see how different temperatures affects the rate of photosynthesis.
d. This experiment is designed to see how carbon dioxide makes plants grow.
3. What is the independent variable in graph #1?
a. the rate of photosynthesis
b. the concentration of carbon dioxide in parts per million
c. the number of stomatal openings
d. 1400 ppm
4. Which statement best describes the relative rate of photosynthesis when comparing 200 ppm carbon
dioxide with 1,100 ppm carbon dioxide?
a. The rate of photosynthesis at 1,100 ppm carbon dioxide is about half the rate of photosynthesis at
200 ppm carbon dioxide.
b. The rate of photosynthesis at 1,100 ppm carbon dioxide is about the same as the rate of
photosynthesis at 200 ppm carbon dioxide.
c. The rate of photosynthesis at 1,100 ppm carbon dioxide is about double the rate of photosynthesis at
200 ppm carbon dioxide.
d. The rate of photosynthesis at 1,100 ppm carbon dioxide is about triple the rate of photosynthesis at
200 ppm carbon dioxide.
5. What is the dependent variable in graph #1?
a. the rate of photosynthesis
b. the concentration of carbon dioxide in parts per million
c. the number of stomatal openings
d. 1400 ppm
6. How much does increasing the concentration of carbon dioxide from 1,200 ppm to 1,400 ppm affect
the rate of photosynthesis?
a. the rate of photosynthesis decreases
b. the rate of photosynthesis remains unchanged
c. the rate of photosynthesis increases
d. the change’s effects cannot the determined by the graph
7. Besides the concentration of carbon dioxide, what else affects the rate of photosynthesis?
a. air temperature
b. amount of light
c. amount of water
d. all of the above choices affect the rate of photosynthesis.
8. What is the relative rate of photosynthesis at 0 ppm of carbon dioxide?
a. 0
b. 20
c. 40
d. 60
9. Which conclusion best describes graph #1?
a. Changing carbon dioxide concentrations does not affect the rate of photosynthesis.
b. Increasing carbon dioxide concentrations from 0 ppm to 1,100 ppm decreases the rate of
photosynthesis.
c. Increasing carbon dioxide concentrations from 0 ppm to 1,100 ppm increases the rate of
photosynthesis.
d. Increasing carbon dioxide concentrations from 0 ppm to 1,400 ppm increases the rate of
photosynthesis.
10. How would the rate of photosynthesis be affected if the stomatal openings were covered with wax?
a. The rate of photosynthesis would decrease
b. the rate of photosynthesis would remain unchanged
c. the rate of photosynthesis would increase
d. the change’s effects cannot be accurately predicted