Comparative Measurements with Two Water Vapour Diffusion Porometers (Dynamic and Steady-State)

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Abstract
Two diffusion porometers of different design, dynamic and steady-state, were used to measure diffusive conductance of wheat leaves in the field. The two values of conductance measured for each leaf were compared by two statistical methods which revealed a systematic difference of between 20 and 30% between the measurements. Random deviations from this averaged around 35%. The cause of the systematic difference is not known, but was considered unlikely to be the result of rapid stomatal response to abnormal exposure within the instruments. It must therefore come from a systematic error in one or both instruments or be a consequence of the different principles of operation. The random variation was found to be a combination of the random errors inherent in the measurement of relative humidity in both instruments plus a similar contribution from the variation in leaf diffusive conductance measured at different points on the leaf.

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