Experimental Studies of the Factors Controlling Transpiration

Abstract

Apparatus and experimental techniques are discussed for use in the investigation of transpiration rate of leaves as controlled by stomatal aperture and leaf water content. The leaf chambers used and the methods adopted for the estimation of the water transpired are described. The designs of the porometer cups used for the different types of leaves ( Pelargonium and wheat) employed in the work are described. To obviate the difficulty that stomata within the cup behave abnormally, the design employed makes possible a removal of the cup from the leaf except during the short periods required to estimate stomatal resistance to air flow at intervals during the course of an experiment. In these experiments the water content of the leaf is changed at will by interrupting the water-supply and re-establishing it to permit recovery from wilting. The methods used to carry out this cycle of operations are fully dealt with. Determinations of the transpiration and absorption rates during the experiment and of the final leaf water content make it possible to follow changes in leaf water content throughout the experiment. An account is given of the methods used for varying the speed of flow, the humidity, and the CO ₂ concentration of the air streams.