Combined leaf gas-exchange system for model assessment

Abstract

Leaf gas-exchange measurements are useful in assessing environmental responses of plants. However, uncertainties in the leaf gas-exchange model potentially limit its application. The main challenge in the model-dependent calculations is to detect violations of assumptions. Here, we developed a system that integrates the direct measurement of leaf intercellular CO2 concentration and the standard open-flow (OF) and novel open-diffusion (OD) systems for flux measurement into one instrument. In the OD system, a gas-permeable membrane between the leaf ambient and outside air creates CO2 and H2O differentials, rather than the air flow in the OF chamber. We measured hypostomatous and amphistomatous leaves of several species with different photosynthetic capacities. The CO2 and H2O differentials in the OD system strictly depend on the flux measured by the OF system. The lower permeability of the membrane resulted in a larger differential per flux, indicating that the OD system can increase the resolution for a small flux. An analysis of conductance model and observations suggested that cuticle and leaf intercellular conductances and the unsaturation of leaf humidity contributed to discrepancies between the direct measurement and standard calculation. The developed combined system provides an opportunity to address these overlooked concepts in leaf gas exchange.

Keywords: conductance; cuticle; diffusion; intercellular CO2; leaf; method; photosynthesis; stomata; transpiration; unsaturation.