The measurement and control of stomatal resistance in the field.
The rate of evaporation from a crop is expressed in terms of weather parameters and a quantity rS derived from profiles of temperature, humidity, and wind. In transfer equations, rS is formally similar to the diffusion resistance of the stomata in a single leaf, and measurements in a field of barley supported the hypothesis that rS is the diffusion resistance of the complete crop canopy. rS was correlated (regression given) with stomatal resistance Tp measured on individual leaves with a porometer. Because Tp was measured on upper, sunlit leaves, which were more porous than lower, shaded leaves, the ratio Tp: rS was expected, and was found, to be less than the LAI. Stomata were closed and Tp increased by spraying the top of the canopy with NSA (methyl ester of nonenyl succinic acid). The increase of rS calculated from the decrease of transpiration rate was consistent with the change of rp.-R.B.