Effect of Dew on Leaf Water Potentials and Crop Resistances in a Paspalum Pasture¹

Abstract

Abstract Opportunities for comparing the water status of crops when the presence or absence of dew is the main environmental variable are rare. This paper reports the results of such a comparison made on successive days on a Paspalum dilatatum (Poir) pasture. Leaf water potentials (Ψ 1 ) were measured with a pressure chamber, and the energy balance method was used to determine evapotranspiration. Crop resistances (r c ) were calculated from the micrometeorological data. The presence of dew halved the early morning crop resistances (1.7 compared to 0.9 sec cm −1 ). The paspalum pasture did not appear to exercise stomatal control over evapotranspiration until most of the dew had evaporated. At ca 1000 hours r c was 1.6 sec cm −1 on both days. Leaf water potentials measured before sunrise were −1.3 bars and at noon were ca −13.0 bars on both days, but the pattern of change in Ψ 1 between these times varied. In the absence of dew Ψ 1 decreased rapidly to reach −12.0 bars at 0900 hours; whereas with dew present, relatively high Ψ 1 were maintained until most of the dew had evaporated. Dew, which is a significant feature of many agricultural regions, may thus have an important effect on the water balance of pastures and crops and thereby enhance production.