MEASUREMENT OF LEAF AND CANOPY PHOTOSYNTHETIC CO2 EXCHANGE IN THE FIELD

The principles and limitations of leaf gas exchange measurements in po rtable gas exchange systems are described. Attention is given to the d esign and developments in infrared gas analysers used in portable syst ems, and the basic structure of single and dual beam instruments is pr esented. The significance of flow measurement in these systems and the principles of thermal mass flow measurement are illustrated, Consider ations of leaf area measurement, chamber design and choice of material s are outlined, Two specific developments in field gas exchange system s are described and their significance in field measurements is illust rated with examples. (1) An integrating sphere leaf chamber for the de termination of the quantum yield of photosynthesis, on the basis of ab sorbed light, is explained and equations for its use are developed. Th e significance of this approach is illustrated by a comparison of data for contrasting leaves plotted on an absorbed and incident light basi s. This measurement of light-limited photosynthesis is also critical i n understanding the contribution of shaded leaves to canopy photosynth esis. (2) A system for the measurement of canopy photosynthesis from a rable crops and low stature natural vegetation is described, Results f rom a season-long study of wheat CO2 exchange are shown to illustrate its application.