This exodus of water vapor from the plant through transpiration must be balanced by root water uptake in order for the plant to maintain a healthy water status. When the rate of transpiration exceeds the rate of root water uptake, then the water stored in the plant tissues begins to be depleted, and the plant begins to shrink or wilt. If this depletion is prolonged, the water potential inside the plant also decreases and the plant reduces the size of the stomatal openings in an effort to reduce its transpiration rate. You may have seen these processes displayed through rolled or wilted leaves of plants exposed to high evaporative demands. These are telltale signs of plant water stress.
When the evaporative demand is reduced, when the sun goes down, for example, the root water uptake rate may exceed the transpiration rate. Then, the water stores in the wilted plant are replenished, the water potential inside the plant rises, and the plant turgor is restored. The same plant recovery processes may occur, even with ongoing evaporative demand, if the soil water potential is increased due to new water inputs from rainfall or irrigation. However, under prolonged drying conditions, the plant water stress may reach a critical level of severity at which the plant cannot recover even if the evaporative demand is removed or the soil is rewetted. At this point, the plant is permanently wilted, and the soil volumetric water content corresponding to this condition is called the permanent wilting point. The exact soil moisture state when permanent wilting point occurs depends on the plant species and the evaporative demand, but the soil water content at a matric potential of -1500 kPa is often used as an estimate of permanent wilting point.