Theory and practice of a portable photosynthesis instrument

Theory and practice of non‐steady‐state portable photosynthesis instruments (LI‐6000 and 6200, LI‐COR Inc., Nebraska, U.S.A.) are presented. Mass balance equations for the time dependence of H 2 O and CO 2 mol fractions within the leaf chamber were used to describe instrument function. Measurements for each run were fitted to an exponential function to estimate average rates of CO 2 assimilation and transpiration during the measurement period. Stomatal conductances and intercellular CO 2 mol fractions were also computed. Linear data analysis used in the LI‐6200 produced similar results for assimilation rates, stomatal conductances and intercellular CO 2 concentrations compared to a more rigorous nonlinear analysis, provided humidity within the chamber was kept constant during the measurement period. Instrument performance for CO 2 fluxes was confirmed by injecting pure CO 2 at steady rates from a microsyringe into the chamber. Miniature evaporimeters were designed to check H 2 O flux measurements. Significant discrepancies were observed between LI‐6200 estimates of H 2 O fluxes and direct measurement and errors were attributed to adsorption desorption of water vapour on chamber walls or to leaks. The leaf chamber should be stored at humidities and temperatures similar to those during measurement conditions for maximum reliability of results.