Quantifying the Diffusion Leakages in a Li-6400 Gas Exchange System and Their Effects On the Parameterization of the Coupled Photosynthesis-Stomatal Conductance-Transpiration Model.

The portable IRGA (infra-red gas analyzer) gas exchange system Li-6400 (Li-Cor Bioscience, Inc., Lincoln, NE) has become the standard equipment for accurate measurements of leaf photosynthesis and transpiration. However, diffusion leakages of water vapor and CO₂ will occur through the chamber gasket and leaf, and the absolute and/or relative magnitude of the diffusion rate can be significant. Though the diffusion problems have been known for a while and the Li-6400 has provided a method to calculate the carbon dioxide diffusion leakage, most previous studies considered the diffusion leakages to be negligible. A few studies have quantified the diffusion leakages but there is not a consensus about the results. The objectives of this study were to describe the theoretical considerations of estimating diffusion leakages, quantify the diffusion rates of carbon dioxide and water vapor using potato leaves and analyze the leakage effects on a biochemically-based coupled photosynthesis-stomatal conductance-transpiration model. Three series of experiments were conducted using two Li-6400 gas exchange systems to estimate the leakage rates. The diffusion leakages were estimated with an empty chamber and the chamber enclosing a dead leaf with and without water vapor leakage at six different flow rates. The intercellular compensation point and the mitochondrial respiration were performed using the Laisk method and Kok method. The results suggested that the calculation equations used in previous studies and the Li-6400 manual may be theoretically incorrect. The leakage rates from empty chambers were larger than the rates from chambers that enclosed dead leaves. The large diffusion leakage of CO₂ at high gradient may suggest that the response of plant photosynthesis, CO₂ compensation point and respiration to CO₂ enrichment might be overestimated before applying the leakage corrections. The relative error due to leakage was large when photosynthesis rate was low. The effects of water vapor and CO₂ on the parameterization of the coupled photosynthsis-stomatal conductance-transpiration model were analyzed using A-C_i curves and A-C_c curves. The results indicated that both water vapor and CO₂ diffusion leakages need to be considered in measurements using Li-6400 gas exchange system. It is important to estimate the magnitudes of leakage for the leaves of any particular species under the given experimental conditions.