Measurement of photosynthetic CO2 assimilation by Infra Red Gas Analyzer (IRGA)

CO2 assimilation

Because CO2 makes up only 0.3% of our atmosphere. It’s difficult to assess photosynthetic C02 intake. To achieve acceptable estimations of photosynthesis, early researchers depended on vast amounts of gas transferred between a plant and the surrounding environment. The absorption of CO2 into an alkaline solution or the interaction of CO2 with an alkali metal oxide such as CaO were used in these approaches. The change in volume of the enclosed air space surrounding a plant or group of plants was used in manometric procedures. Gravimetric approaches, on the other hand, relied only on the chemical reactant’s weight change. More recent experiments have used radioactively tagged 14c02 to detect how quickly this substrate is removed from the air around a plant.

Infrared gas analysis, which permits the measurement of parts-per-million fluxes of CO2 in an airstream travelling through a transparent container containing a leaf, is being used to quantify photosynthesis. This approach eliminates the logistic challenges associated with radioactive chemicals and may be used in the field, in addition to allowing the investigator to swiftly assess modest CO2 fluxes.


Heteroatomic gases absorb certain wavebands of radiation. In the middle infrared wavebands, C02 absorbs heavily. As CO2 levels in the atmosphere rise, this is projected to generate the global “greenhouse effect.” The presence of a gas between the radiation source and the detector causes a decrease in infrared waveband transmission, which is measured by infrared gas analyzers. The drop in transmission is proportional to the gas concentration.

Dispersive infrared analyzers use monochromatic light that is delivered successively to assess the concentration of numerous components in complicated gas mixtures. Photosynthesis systems, on the other hand, do not use dispersive analyzers to measure the concentration of a specific gas species. These utilize infrared light with a broad spectrum that is selective for CO2 through the application of filters in the optical path.

Detectors built for CO2 typically have cross-sensitivity to the water vapour absorption spectrum. Although filters reduce this interference, if there is a substantial amount of water vapour in the airstream, it is required to adjust the apparent C02 concentration. Alternatively, right before the airstream enters the analyser, water vapour can be condensed or chemically eliminated.


Accurate photosynthetic measurements need the coordinated functioning of all photosynthesis system components. A leaf chamber, flow metre, and some method of producing and regulating the air flow across the leaf are included in all gas-exchange systems, in addition to the infrared.

gas analyzer (IRGA).A mass flow controller, which provides a set velocity by heating or cooling a tiny fraction of the airstream, is used in some systems to precisely maintain flow. A humidity sensor may be installed in leaf chambers to monitor transpiration. Irradiance is measured with a light sensor. as well as a blower to guarantee proper air mixing in the leaf chamber

An artificial light source with a spectral composition comparable to that of sunshine is required for laboratory operation. but with infrared wavebands that produce the least amount of heat (e.g.. metal halide). Although any terrestrial plant’s photosynthesis may be studied, cactus and other plants with highly modified leaves may require specific leaf chambers. Filters and heat exchangers may also be used in leaf chambers to reduce infrared leaf healing.

Photosynthesis systems can be set up in an open or closed configuration. The concentration of CO2 in the airstream flowing over a leaf is measured in open systems (also known as differential systems) in comparison to air that has not been exposed to the leaf. Closed systems (depletion systems) circulate air continuously through the leaf chamber and measure the quantity of CO extracted from a set volume of air when the leaf is closed. Both configurations can produce adequate photosynthetic data, but the circumstances in which each should be employed are determined by variations in mobility, measurement speed, precision, and environmental control in the leaf chamber. 

Open System: Open systems are configured to allow air from a single source to enter both the analysis and reference lines. After passing through the leaf chamber, the analysis air is pumped to the IRGA, while the reference air is pumped directly from the source to the IRGA. Most IRGA’s use two cells to assay the reference and analysis gas simultaneously, whereas some assay the analysis and reference air sequentially with a single cell by switching the airstreams with a solenoid. Absolute concentrations of CO2 in the analysis and reference lines are determined by comparison with air in an internal loop in which CO2 has been chemically removed. Air may come from a pressurized tank or from the atmosphere so long as the CO2 concentration is stable. For most studies, the CO2 concentration should be close to the global mean of about 340 pm . Leaf chambers used with open systems enclose small volumes. usually only a portion of a leaf, so the small CO2 fluxes will significantly alter that concentration of the analysis air. Accordingly. a single measurement may require only a few seconds. Each system’s properties are shown below.

Closed Svstems: Two closed gas flow loops run a closed gas exchange system. To keep the CO concentration in the reference loop at zero, a 2 2 reference cell is connected in series with a chemical CO2 scrubber. The sample cell is part of a closed loop that comprises a leaf chamber and a desiccant circuit through which all or part of the air sample from the leaf chamber can flow. The humidity in the confined chamber may be controlled by the use of a desiccant, which continues to rise as the plant transpires.

To get an absolute measurement of CO2 concentration, the signal from the sample cell is compared to the zero gas reference signal. In an IRGA system that is closed. A leaf is placed in a container that is sealed to prevent gas exchange with the environment, and the rate at which the CO2 content in the chamber varies is recorded, usually for 10 to 20 seconds. The rate of change in CO2 concentration is then used to determine net photosynthesis.

There are two major disadvantages to using a closed IRGA system: photosynthesis measurements must be made within a few seconds after closing the leaf chamber and the operator has limited control over environmental conditions within the chamber. Once the leaf is sealed in the chamber, CO2 concentration in the leaf chamber- is continually decreasing. Consequently. if the leaf has a high photosynthetic rate, resulting in a rapid reduction of the chamber CO2 concentration, measurements must be made quickly to avoid the possibility of a direct effect of low CO2 concentration on photosynthesis.

This restricts the amount of time a leaf may adjust to a given experimental setting (light intensity, temperature, etc.). In certain closed systems, this problem can be partially solved by utilising an external flow valve, which allows the operator to open the system and bring outside air into the chamber while the leaf acclimates before starting measurements.

The regulation of temperature and relative humidity within the chamber during measurement is the second restriction. Because the closed system was created with portability in mind. Heat-exchange systems for maintaining stable air temperatures within the chamber are usually not included. Furthermore, the air stream cannot be humidified to the necessary amount on a continuous basis. Open systems, on the other hand, frequently provide steady state humidity management.

The closed IRGA system, on the other hand, has at least three advantages: it is compact and light-weight, reasonably inexpensive, and relatively easy to calibrate and operate. As a result, it is suitable for use in secondary and undergraduate field courses.

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