Infrared measurement of canopy temperature and detection of plant water stress

Abstract

Leaf temperature is directly determined by leaf energy and water balance. If diminished water availability decreases latent heat flux at the leaf surface, a complementary increase of sensible heat will occur and create a larger temperature difference between foliage and air. Radiation, air temperature, humidity and wind speed modify leaf temperature and may mask indications of water stress. The position, inclination and orientation of leaves within the canopy also produce considerable variation of leaf temperature. These factors were incorporated in a linear transpiration model, using physical and physiological characteristics of cotton. Water stress was simulated by imposing a limit value for stomatal conductance. The energy balance equation was solved as a function of angle between leaf and solar beam, to determine leaf temperature frequency distribution. The results show that stress induced temperature rise occurs over a small percentage of the total leaf area. Detection of moderate stress requires a normalizing procedure which takes into account meteorological conditions. The leaf temperature distribution is a better indicator of stress than the average value.