Water potential, transpiration rate, and stomatal resistance of the various parts of the onion (Allium cepa L.) plant were measured under field and controlled conditions. Water potentials of florets and pedicels were always lower than leaf water potentials. Water potential difference between the florets and the soil of as much as —9 bars were found. The greatest drop in water potential was found between the flowers and the upper part of the seed stalk. These differences in water potential were found even when plants were growing in soil well supplied with water. Transpiration rates of the leaves were the greatest of all plant parts, the seed stalk next (about 15 to 20% of the flux density from the leaves), and the umbel lowest, representing a very small fraction of the total. Stomatal resistance of the leaves was at all times lower than seed stalk stomatal resistances. An almost on‐off stomatal behavior by which the onion plant regulates its transpiration was found to operate in a narrow range of leaf water potential, —3 to —7 bars. The relation between flux density and water potential drop was found to be highly nonlinear in plants growing in different soils and under a wide range of soil water potential and environmental conditions. Impedance to water flow in the seed stalk was 5 to 8 times greater than in the leaves. For the same conditions potential drop between soil and root surface was less than 5 cm of water.

Total water potential of barley (Hordeum vulgare L.) leaves from plants grown under greenhouse and growth chamber conditions was divided into pressure and osmotic potential components, and their relationship to leaf relative water content was determined. Pressure potential approached zero at a water potential of about −32 bars, and a relative water content of about 65%. A change in the elasticity of leaves occurred at about 2 bars pressure potential and about −12 bars water potential. First visible wilting was observed between 75 and 80% relative water content. Transpiration decreased as leaf relative water content decreased but transpiration was independent of soil water content until about 16% (0.6 bar soil suction). First visible wilting of barley leaves was observed at soil water content between 9 and 13% (1–5 bars soil suction). Water potential and leaf relative water content decreased as the soil matric potential decreased. There was a shift to lower relative water content and water potential values as plants became older when the soil matric potential decreased.

The need for greater plant yields and more efficient use of water makes it essential that the relations between soil‐water content, soil‐water potential, transpiration rate, and plant response be made ever more quantitative. The dry matter production rate of snap beans (Phaseolus vulgaris L., var. Bush Blue Lake) growing under field conditions on a sandy soil is analyzed during a drying period. Measurements of plant‐ and soil‐water potentials, dry matter accumulation, and stomatal resistance were made as soil‐water was depleted, while the transpiration rates were obtained by a model for a loosely structured canopy. The transpiration and dry matter production rates decreased curvilinearly with soil‐water potential. When the soil‐water potential decreased from −0.28 to −0.40 bar, there was 47% reduction in the dry matter production rate. This is related to the turgor pressure‐operated stomatal mechanism. The adaxial stomatal resistances increased at leaf‐water potentials lower than −8 bars, which coincided with a rapid decrease in the dry matter production rate. It was found that stomatal closure due to water stress resulted in a greater reduction of growth rate than in transpiration.

Transpiration, evaporation, and drainage from a sparsely populated onion field were measured separately by a method which should be applicable to many row crops. Transpiration computed from measured evapotranspiration, potential evaporation, and stomatal resistances agreed well with values calculated independently from raeasured stomatal resistance, air vapor pressure, plant‐temperature and wind. Transpiration was only about 20 percent of total evapotranspiration. When the population was increased threefold to near maximum commercial density, evapotranspiration increased only about 15 percent because as transpiration increased, evaporation decreased. The portion of transpiration accounted for by each plant part was 3 percent for umbels, 42 percent for leaves and 55 percent for scapes. Drainage from this sandy soil exceeded evapotranspiration.

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