Measurement of leaf and canopy photosynthetic CO2exchange in the field

The principles and limitations of leaf gas exchange measurements in portable gas exchange systems are described. Attention is given to the design and developments in infrared gas analysers used in portable systems, and the basic structure of single and dual beam instruments is presented. The significance of flow measurement in these systems and the principles of thermal mass flow measurement are illustrated. Considerations of leaf area measurement, chamber design and choice of materials are outlined. Two specific developments in field gas exchange systems are described and their significance in field measurements is illustrated with examples. (1) An integrating sphere leaf chamber for the determination of the quantum yield of photosynthesis, on the basis of absorbed light, is explained and equations for its use are developed. The significance of this approach is illustrated by a comparison of data for contrasting leaves plotted on an absorbed and incident light basis. This measurement of light-limited photosynthesis is also critical in understanding the contribution of shaded leaves to canopy photosynthesis. (2) A system for the measurement of canopy photosynthesis from arable crops and low stature natural vegetation is described. Results from a season-long study of wheat CO2 exchange are shown to illustrate its application.