Plant response to differences in water availability in the field was measured in grain sorghum during July in centra1 Arizona. During two 24‐hours periods, systematic measurements were made of meteorological factors, transpiration resistance, leaf thickness, leaf temperature, and evaporation, first in dry soil and then in wet soil, after irrigation.Limited soil water availability partially counteracted the normal, light‐induced stomatal opening and led to increased daytime values of leaf diffusion resistance, decreased evapotranspiration, and leaf temperatures that exceeded air temperature, in contrast to plant reactions with water freely available.On the other hand, leaf thickness, as measured by a beta‐ray gauge, decreased negligibly from the dawn reference value, even when the soil water content approached the 15‐bar percentage. The absence of significant leaf response in this regard appears to be a failure of the beta‐ray gauge technique to assess the water content of sorghum leaves.