Influence of Water Stress on the Diurnal Exchange of Mass and Energy between the Atmosphere and a Soybean Canopy¹

Abstract A micrometeorological‐physiological study was conducted at Mead, Nebr., during the summer of 1980 to examine the diurnal exchanges of mass and energy of well‐watered and water‐stressed soybean ( Glycine max (L.) Merr. cv. Harosoy) canopies and to relate these exchanges to environmental and physiological variables. Data are presented for 2 clear days when the canopy was fully‐developed. Measurements of CO 2 , latent heat and sensible heat flux were made using the Bowen‐ratio energy balance technique. The soil of the area is a Sharpsburg silty clay loam (a fine, montmorillonitic, mesic Typic Argiudoll). Water stress greatly influenced the partitioning of available energy between latent and sensible heat flux. When the crop was well‐watered, sensible heat (H) was directed toward the crop and caused latent heat exchange (LE) to exceed net radiation (Rn). When the crop was water stressed, only two‐thirds of Rn was consumed as LE; the remainder was converted into sensible heat. Since both Rn and vapor pressure deficit were greater on the day when the crop was water‐stressed, stomatal closure appears to have been the primary cause of the reduction in LE. Carbon dioxide exchange was not sensitive to water stress in the morning but was severely limited by such stress during midday. The midday reduction in CO 2 exchange appears to have been caused by a combination of high stomatal resistance limiting CO 2 diffusion to the cell chloroplasts and low leaf water potential coupled with high air temperature affecting the enzymatic reactions associated with photosynthesis. Water use efficiency (defined in terms of the CO 2 ‐water flux ratio) was greater when the crop was well‐watered than when it was stressed for water. A combination of water stress, a large vapor pressure deficit, and high air temperature reduced the CO 2 ‐water flux ratio.