Submitted to: Photosynthetica
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/6/2012
Publication Date: 3/12/2012
Citation: Wang, Q., Fleisher, D.H., Reddy, V., Timlin, D.J., Chun, J.A. 2012. Quantifying the measurement errors in a LI-6400 gas exchange system and their effects on the parameterization of Farquhar et al. model for C3 leaves. Photosynthetica. 50(2):223-238. Interpretive Summary: Special scientific equipment is needed in order to take measurements of photosynthetic and transpiration rates of plant leaves. Such data is needed to understand how plants respond differently to their environments. However, due to the nature of the scientific equipment, it is possible for leaks and other errors to significantly affect the quality of the measured data. Mathematical models were developed to predict the size of these errors and experiments with different plants were conducted to provide data. The goal was to test if the models functioned correctly and determine how large these errors could be if uncorrected for. The results showed the developed models were accurate and indicated that substantial errors can result if the models are not used in correcting the equipment readings. The approach and results will be useful for all scientists who use similar measurement systems. The results should also help improve the accuracy of crop models which are used by scientists, crop consultants, and farmers to more effectively manage agricultural resources.
Technical Abstract: The LI-6400 gas exchange system (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 leakages of water vapor and carbon dioxide between inside and outside of the leaf chamber, and the inward dark transpiration and dark respiration released from the leaf under the gasket, can be significant. Rigorous model-based approaches were developed for estimating leakage rates of water vapor and carbon dioxide 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, carbon dioxide and dry air. Experiments were conducted using two LI-6400 systems with potato and soybean leaves. Results indicated models were reliable for estimating diffusive leakage rates of water vapor and carbon dioxide, which 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 curves for net photosynthesis versus intercellular carbon dioxide concentration or net photosynthesis versus mesophyll carbon dioxide concentration 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.