BSCI 1510L Literature and Stats Guide: Infrared gas analysis

Rates of photosynthesis in plants can be monitored in several ways, but the most effective is using an infrared gas analyzer (IRGA).  An IRGA takes advantage of the absorbance of CO₂ molecules at a wavelength of 4260 nm caused by the stretching vibrations of the C=O double bonds. 

Fig. 1  Leaf chamber of a portable IRGA

Portable IRGAs have been engineered to take relatively rapid readings from air that has passed over a leaf surface (Fig. 1).  Thus the rate of gas exchange can be measured on an intact leaf under field conditions.  A sophisticated IRGA can measure (and even control) the concentration of CO₂ and humidity entering the leaf chamber, the intensity of the incident light and the CO₂ concentration exiting the chamber.  At the same time, it can calculate and record photosynthesis rates using an on-board computer.  With such an IRGA, one can determine photosynthetic light response curves for a plant species under specific temperatures and CO₂ concentrations (Fig. 2).  (It might interest you to know that a portable IRGA can cost more than a new car!)

Fig. 2 Photosynthetic light response curves for Echinacea tennesseensis at 25 ºC.  PAR=photosynthetically active radiation, i.e. light intensity; H and L refer to high and low temperature and moisture pretreatments.  From Baskauf and Eickmeier (1994).

Fig. 3. IRGA sensor

In this experiment, we will be using an IRGA system (Fig. 3) that can interface with a microcomputer.  Although the apparatus is less sophisticated (and less expensive!), its principles of operation are the same as that discussed above.  The shaft of the sensor is placed in a closed container.  A hot metal filament produces the incident infrared radiation (and the glowing light that you observe winking on and off as the unit operates).  Holes in the shaft allow air to diffuse into the sensor.  At the end of the shaft, an infrared sensor measures the intensity of the radiation after it passes through the air sample.  The higher the concentration of CO₂, the less radiation that reaches the sensor.  The sensor takes a measurement about once per second.  Because air must diffuse into the sensor, there is a short lag time before changes brought about by leaf activity cause a change to occur in the readings. Note that unlike the IRGA shown in Fig. 1, this is a closed system and that the CO₂ concentration impinging on the leaf cannot be controlled.  Thus the sensor measures changes in CO₂ in the container over time, rather than measuring the difference in CO₂ before and after air passes over the leaf.  Nevertheless, we can measure the rate of gas exchange by observing the rate of change of CO₂ in the container.  The slope of the CO₂ concentration vs. time graph represents the rate of gas exchange.