Determining Soil‐Induced Plant Water Potential Depressions in Alfalfa by Means of Infrared Thermometry1

Abstract

To ascertain the ability of a remote sensing technique to evaluate the soil‐induced component of plant water potential depressions in forage crops, field experiments were conducted to measure the air vapor pressure deficit and foliage‐air temperature differentials of well‐watered alfalfa (Medicago saliva L.) at five different sites throughout the United States. At two of these sites, data were also acquired for various degrees of stressed alfalfa along with concurrent measurements of total plant water potential. The temperature and vapor pressure data were used to construct a plant water stress index which was related to the plant water potential. Different results were obtained for the two sites studied; but after a procedure was developed to remove variable atmospheric effects, the remaining soil‐induced plant water potential depressions were found to be well described by a single function dependent upon the plant water stress index. These results demonstrate the ability of surface temperature measurement by means of infrared thermometry to rapidly assess large areas of cropped land for plant water potential depressions arising from shortage of soil moisture.