Rates of both photosynthesis and respiration in plants can be determined by measuring changes in CO2 concentration in the air passing over a leaf. Photosynthesis is measured as a decrease in CO2 concentration when light is provided to the leaf. Respiration is measured as an increase in CO2 concentration that occurs when the light is turned off. CO2 concentrations can be determined with an infra-red gas analyzer (IRGA) that detects changes in CO2 levels of a few parts per million (ppm). With this system you may also measure three other parameters in addition to changes in CO2 concentration; these are light intensity, as well as the relative humidity and temperature of the air moving through the system.
A leaf is enclosed in a leaf chamber with gas lines running to and from it. Air is moved through the system by a small air pump whose flow rate is controlled by placing flow restrictors between the pump and the next component in the pathway. The air flows over the leaf in the chamber and then flows through a humidity/temperature sensor. The air must have all water vapor removed before it enters the IRGA (water vapor interferes with the measurements) by passing it through a column containing a desiccant (a material that absorbs water). The air then passes through the IRGA where the CO2 concentration in PPM is measured.
The air that you use throughout your measurements must come from a gas bag that has previously been filled with your source gas. This could be room air or air containing different concentrations of CO2.
Before you begin measuring photosynthesis and respiration rates, you need to set a base line of the CO2 content of your source gas. To do this the gas is pumped directly into the desiccant column and then through the IRGA, by-passing the leaf chamber and humidity/temperature sensor. When you are ready to measure the CO2 uptake of the leaf, the leaf chamber and the humidity/temperature sensor are then placed in the gas-flow path, between the air pump and the desiccant column.
If you wish to review the components of the system or see the compete apparatus click here.
To test your understanding of the two gas flow paths described above, complete the quiz that follows. It will first ask you to order the components in the system for base line measurements and then again for measurements of the gas flowing through the leaf chamber.
Exercise for Baseline measurement (instrument calibration)
Exercise for experimental setup
| Support and funding provided by: |
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Andrew W. Mellon Foundation |
Instrumentation manufactured by: |
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