The relationship between rate of transpiration and depression of water potential of the leaves (δψ1) was investigated with Gossypium hirsutum and Heliantlnis annum grown in water culture. (δψ1) was measured with a thermocouple psychrometer over a wide range of transpiration rates. The different transpiration rates were obtained by altering the environmental conditions in a climatological wind tunnel. At zero transpiration rate δψ1 was found to be 5.5 atm in H. annuus and 6.0 atm in Gossypium hirsutum. As the transpiration rate was raised, δψ1 rose steeply to a plateau in both species and remained at near to 9 atm within the transpiration range 0.4–2.4 g/dm2/hour. Thus within this range resistance in the plant declined in proportion to the transpiration rate. With transpirational fluxes greater than 2.4 g/dm2/hour, δψ1 rose, indicating little further decline in resistance.
That the variable plant resistance is located in the roots was confirmed since plants with killed roots had a constant resistance, δψ1 exhibiting a linear relationship with transpiration. The resistance of the dead root system was much lower than that of living roots at low transpiration rates, but at high transpiration rates the resistance of living roots fell to about the same level as the dead roots. The implications of these findings are discussed.