John Evans and Louis Santiago
Photosynthesis or respiration and transpiration can be calculated by enclosing the leaf in a chamber and measuring the fluxes of water and CO2 being exchanged between the leaf and the atmosphere. The rates of these processes depend on environmental parameters. The most important parameters are irradiance, temperature, CO2 concentration and humidity. These environmental parameters can be controlled and varied, depending on the system.
Most commonly, gas exchange systems are open and the concentrations of water and CO2 entering and exiting the leaf chamber are measured with infra red gas analysers. Leaf temperature, the flow rate of air entering the chamber, atmospheric pressure, irradiance and leaf area enclosed in the chamber are also needed for the calculations. For large flat leaves, clamp on cuvettes which use soft gaskets to seal to the leaf surface are the most convenient. However, a variety of specialised chambers exist for plants with needles or small leaves.
Gas exchange measurements usually involve surveying different leaves or plants under a common condition, or making repeated measurements on a given leaf under different environmental conditions by varying, for example, irradiance or CO2.
Given the dependence of gas exchange rates on environmental parameters, a survey can either try to reflect actual rates, or by controlling one or more parameters, measure photosynthetic rate under a specified condition, such as at high irradiance and ambient CO2. The length of time a leaf is within the chamber before the measurement is taken can influence the result as stomatal conductance can change or photosynthetic induction can occur. The degree of environmental control depends on the system being used and the amount of power available.
If the system has an artificial light source, then irradiance response curves can be measured. The spectral composition of the light should be specified as it influences stomatal opening, the energy balance of the leaf, quantum yield and the profile of absorption through the leaf. Key parameters that can be derived are the maximum photosynthetic rate, respiration rate (dark and day) and apparent quantum yield.
If CO2 concentration can be controlled, then CO2 response curves can be measured. Usually these are expressed with respect to intercellular CO2 concentration as this allows analysis of underlying biochemical limitations e.g. the Farquhar von Caemmerer and Berry (1980) C3 photosynthesis model. Parameters that can be derived are Rubisco activity, electron transport rate, day respiration and triose phosphate use.
Some instruments allow the chamber temperature to be varied which is used to control leaf temperature. The range depends on the system and how much power is available. Alternatively, natural diurnal variation in air temperature can be used.
The ambient humidity can be varied by controlling the humidity of air entering the chamber and/or the flow rate. It is critical that humidity is kept below the dewpoint temperature as any condensation invalidates gas exchange measurements and takes considerable time to remove before valid measurements can again be taken.