Modification and Use of an Electric Hygrometer for Estimating Relative Stomatal Apertures

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Abstract
Commercial humidity sensors (Hygrodynamics, Silver Spring, Md.), used with a portable a-c resistance meter, permit measurement of time required (tL) for a predetermined change in humidity in the sensor exposed to a transpiring leaf surface. A similar measurement over a free water surface at the same temperature provides a standard reference value (tW). tW/tL is a measure of the relative rate of vapor transfer from the leaf surface and is also a measure of relative stomatal aperture when comparing leaves with equal stomatal densities. The sensor subtends 1.2 cm2 of leaf surface. Choice of a sensor that is sensitive at the ambient humidity establishes a humidity gradient comparable to natural condition of the leaf.