Gas exchange measurements in the unsteady state

Abstract Leaf level gas exchange is a widely used technique that provides real‐time measurement of leaf physiological properties, including CO 2 assimilation ( A ), stomatal conductance to water vapour ( g sw ) and intercellular CO 2 ( C i ). Modern open‐path gas exchange systems offer greater portability than the laboratory‐built systems of the past and take advantage of high‐precision infrared gas analyzers and optimized system design. However, the basic measurement paradigm has long required steady‐state conditions for accurate measurement. For CO 2 response curves, this requirement has meant that each point on the curve needs 1–3 min and a full response curve generally requires 20–35 min to obtain a sufficient number of points to estimate parameters such as the maximum velocity of carboxylation ( V c, max ) and the maximum rate of electron transport ( J max ). For survey measurements, the steady‐state requirement has meant that accurate measurement of assimilation has required about 1–2 min. However, steady‐state conditions are not a strict prerequisite for accurate gas exchange measurements. Here, we present a new method, termed dynamic assimilation, that is based on first principles and allows for more rapid gas exchange measurements, helping to make the technique more useful for high throughput applications.