The CO₂ Analysis laboratory package may be used to measure photosynthesis, respiration and photorespiration in attached or detached leaves maintained in a leaf chamber attached to an open flow gas exchange system. Initially, the CO₂ concentration in the gas provided to the chamber is measured with an infra red gas analyzer (IRGA), and then the gas is pumped through the chamber at a known flow rate. The concentration in the effluent gas from the chamber is measured using the IRGA, and the difference between this and the input gas is used to measure photosynthetic rate (or respiration rate if a greater CO₂ concentration is measured in the effluent gas stream).

Photosynthesis requires both light and CO₂. Light is provided by a halogen light source, the intensity of which may be varied by a sliding dimmer control. Alternatively, a variable LED light source, available from Qubit Systems, may be used. The amount of light passing through the leaf is measured by a photosensor placed beneath the transparent leaf chamber. When using the LED light source, the light output from the source is measured directly without need of a light sensor. CO₂ is supplied from the air, or from a gas bag, the latter method allowing gases of different CO₂ concentrations to be supplied to the leaf chamber. Measuring photosynthesis in a leaf over a range of CO₂ concentrations allows a photosynthetic CO₂ response curve to be plotted, from which can be derived such physiologically important parameters as carboxylation efficiency, CO₂ compensation point, CO₂ saturation point and apparent photorespiratory rate.

Many of the experiments described in this manual involve measuring photosynthesis in air, or constructing photosynthetic CO₂ response curves. However, there is great scope for measuring the effects on photosynthesis of numerous environmental parameters such as light quantity, light quality, and O₂ concentration. In addition, plants may be pre-treated in various ways that affect their photosynthetic rate, and the effects of these treatments may be measured under conditions of light saturation at both saturating and limiting CO₂.

Finally, the IRGA package may also be used to measure both photorespiration and dark respiration. Dark respiration is measured as the rate of CO₂ production from the leaf in the absence of light, whereas photorespiration may be measured by at least 3 different methods:

1. CO₂-efflux into CO₂ free air in the light
2. post-illumination CO₂ burst, and
3. extrapolation of photosynthetic CO₂ response curves.

QUBIT SYSTEMS Inc.

CO₂ Analysis Lab

Overview