The water requirement for corn root expansion growth was studied by directly relating root extension to tissue water potential. Root growth rate was accurately measured over a short period (5–15 min) with a radial variable displacement transformer. Root tissue total and osmotic water potential was then immediately determined with an isopiestic thermocouple psychrometer. Roots experiencing a single water deficit of short duration (0–20 min) exhibited a nearly linear proportional response of growth rate (0 to 1.6 mm hr‐1) to tissue water potential over the range of about ‐1 to ‐10 MPa. Expansion ceased at at ‐10 MPa, defining this as the root's permanent wilting point. Roots experiencing longer periods of water deficit (6–7 hr) also exhibited the linear relationship but their osmotic potentials shifted to lower values in the process (osmoregulated).