A Krypton Diffusion Porometer for the Direct Field Measurement of Stomatal Resistance

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Abstract
A stomatal diffusion porometer is described which measures directly the diffusion of radioactive krypton through amphistomatous leaves. The porometer is relatively small and portable and is easily used under field conditions. It consists of a miniature diffusion chamber above an acrylic plastic reservoir which contains 1200 cm3 of air enriched with 85Kr. Geiger tubes in the diffusion chamber and in the reservoir monitor the relative concentrations of 85Kr. Krypton is allowed to diffuse from the larger reservoir through the leaf into the diffusion chamber and the time for its concentration in relation to that in the reservoir to change between two fixed values is recorded. When this time lapse was calibrated against known resistances a linear relationship, independent of temperature was found. Sources of error are analysed and some experiments are described in which the porometer was used to measure diurnal changes in stomatal resistance. Resistances of potted sunflower plants (Helianthus annuus L.) grown in a greenhouse were measured with both the krypton diffusion porometer and a condensation-type porometer and the results were used to calculate both cuticular and stomatal resistances. Demonstration of field measurements with the porometer include data from eucalyptus trees {Eucalyptus camaldulensis Dehn) and from an unirrigated cotton crop {Gossypium hirsutum var. SJ 2) growing under semi-arid conditions. Stomatal conductance of the cotton crop during the opening phase was linearly related to solar radiation.

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