Measurement of transpiration and leaf conductance

Measurements of leaf transpiration and calculations of leaf conductance to water vapor are important in almost all investigations of plant water relations. Transpiration is a primary determinant of leaf energy balance (Chapter 7) and plant water status (Chapter 9). Together with the exchange of CO2 it determines the water use efficiency. The close linkage between CO2 uptake and H2O via the stomatal pore has allowed for separation of stomatal and biochemical limitations to photosynthesis through calculation of intercellular CO2 concentrations. In this chapter we will cover the principles and instruments necessary for measurement of leaf transpiration and the calculation of leaf conductances to water vapor exchange. We will also consider the methodology and problems involved in determining whole-plant and canopy transpiration rates. Emphasis is placed on methods and equipment that have as their primary purpose, the direct measurement of transpiration rates or leaf conductance to water vapor loss. It should be noted that in many research problems, knowledge of both CO2 and H2O exchange are required. In the past, porometers that measure only leaf conductance to water vapor have sometimes been used to infer more general environmental response of gas exchange including CO2 uptake. While a general correlation is expected, direct measurements of CO2 exchange, which are much more feasible than even a few years ago, are clearly more appropriate. Field equipment designed for simultaneous measurements is covered primarily in Chapter 11. In this chapter we will, however, cover the water vapor sensors and the theory and procedures necessary to measure transpiration and calculate stomatal conductances in these systems.

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