Quantifying the measurement errors in a portable open gas-exchange system and their effects on the parameterization of Farquhar et al. model for C3 leaves

A portable open gas-exchange system (Li-6400, Li-Cor, Inc., Lincoln, NE, USA) has been widely used for the measurement of net gas exchanges and calibration/parameterization of leaf models. Measurement errors due to diffusive leakage rates of water vapor (L-W) and CO2 (L-C) between inside and outside of the leaf chamber, and the inward dark transpiration rate (D-W) and dark respiration rate (D-C) released from the leaf under the gasket, can be significant. Rigorous model-based approaches were developed for estimating leakage coefficients of water vapor (K-W) and CO2 (K-C) and correcting for the combination of these errors. Models were based on mass balance equations and the Dusty Gas Model for a ternary gas mixture of water vapor, CO2, and dry air. Experiments were conducted using two Li-6400 systems with potato and soybean leaves. Results indicated that models were reliable for estimating K-W and K-C, and the values varied with instrument, chamber size, gasket condition, and leaf structure. A thermally killed leaf should be used for this determination. Measurement error effects on parameterization of the Farquhar et al. (1980) model as determined by P-N/C-i curves were substantial and each parameter had its own sensitivity to measurement errors. Results also indicated that all four error sources should be accounted for when correcting measurements.