From a practical and theoretical standpoint, the objective in using gas exchange technology is to capture representative plant/soil physiological data, holding instrument error to a minimum. A basic goal in this is to ensure that sampling reflects real responses to natural or imposed environmental variability (treatments). Whether from an observational or experimental approach, the physiological response is not typically manipulated beyond the treatment, as this entails the risk of masking the potential treatment effect itself.
In relation to this, two gas exchange sampling parameters that receive frequent attention are i) leaf area of the cuvette and ii) the choice of flow rate during a given measurement session. This application note is intended to address some of the most common concerns related to these important parameters for which the researcher makes individual experimental design decisions. Both parameters figure prominently in the near real-time results generated by CIRAS-2’s dedicated gas exchange equations (PP Systems 2010). Therefore, both are crucial factors when considering intercomparability of data when a likely or intended change of either leaf area (LA) or flow rate (V) occurs within an experiment.
The question arises: why switch between head plates during a given measurement session or experiment? Numerous circumstances can lead to the decision to change this sampling parameter. For example, a repeated measures design characterizing single-species physiology through different phases of leaf expansion might involve changing head plates over a relatively short period (weeks). Multi-species sampling of both narrow- and broad-leaved plants might require use of different head plates during one measurement session. An alternate approach would be to avoid use of different head plates altogether, opting instead to sample unknown leaf areas (LA not defined by the inside dimensions of the head plate), and determining LA post-measurement. This also requires selection of the Energy Balance option for leaf temperature determination – this is necessary because CIRAS-2’s built-in infrared leaf temperature sensor is accurate only if the entire leaf chamber is filled, and no light is incident on the sensor itself.
Similarly, why not use a consistent flow rate throughout the experiment? Two circumstances come to mind: minimal V is advantageous when inducing or encountering very low photosynthetic rates, for example, associated with shade-tolerant species, or when sampling under marginally photosynthetic light intensities. Higher V can be used if chamber humidity is too high (>70% RH) to increase the volume flow through the chamber of the dry reference gas stream, but this is not normally needed when diverting the reference air through the desiccant columns (Envirogel). Additionally, chamber humidity settings can and sometimes should be changed to hold sampling conditions constant inside the leaf chamber, while ambient vapor pressure outside the leaf chamber fluctuates during measurements diurnally or daily.