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

The need for measurement of stomatal apertures arises in many kinds of investigations, including studies of gas diffusion through stomates, physiology of stomatal control, and penetration of applied solutes into the leaf. Sometimes the measurements need to be made rapidly on many leaves or repeatedly on selected leaves without inflicting injury or seriously altering stomatal functions. iXiethods that involve stripping the epidermis, making impressions with collodion, infiltration with liquids, or direct microscopic observation are often not acceptable. Since Desai (3) reviewed the methods available in 1936, considerable improvement has been made in the porometer method (4, 7), while Glover (6) and Gaastra (5) demonstrated the usefulness of precise hygrometric measurements for determining rate of transpiration and stomatal diffusion resistance. Either method can be adapted for portable use. This paper describes a portable hygrometer and its use for measuring comparative rates of water vapor transfer from leaf surfaces, and evaluates the interpretation of such measurements in terms of relative stomatal apertures. Materials and Methods Description of IiistruimenCt. Two major components are involved-a sensor whose electrical resistance (resistance is the principal component of the electrical impedence of the sensor unit) varies inversely with humidity, and a portable resistance meter. Ten sensing elements are available, covering the range from about 1.6 to 99% RH (relative humidity). Four units were used in the present study, having the following nominal sensitivity ranges: 18 to 30%, 29 to 43%, 41 to 59%, and 54 to 72%. For use in a particular test, the unit is selected that is sensitive at the ambient humidity of the atmosphere, thus minimizing the humidity change imposed on the leaf surface. The sensor, as supplied commercially (Hygrodynamics, Inc., 949 Selim Road, Silver Spring, 1\Jaryland), is protected by a perforated metal shell. This is replaced by a tube of clear acrylic plastic 12.5 mm inside diameter by 38 mm long (fig 1). A gasket for making an airtight seal with the leaf is provided by cementing to the open end of the plastic tube a piece of soft rubber tubing 12.5 mm I.D. by 8 mm long, tapered to a thin edge at the distal end. A side opening consisting of a short length of 3-mm copper tubing is inserted in the outer wall of the