Plant wilting can be caused either by the plant or by the soil

In this paper, plant wilting is re-analysed and re-interpreted on the basis of previously published work. Wilting is considered only in terms of the stress caused by the matric suction of the soil water. Other factors that can induce wilting, such as salinity and plant pathogens, are not considered. It is found that there is confusion around the subject for two main reasons. First, it is usually assumed that the matric suction of the pore water that exists in soil samples when they are removed from pressure plate extractors is equal to the air pressure that was applied. Second (and this is a special case of the first reason), because the soil water content when most plants wilt is very close to that remaining in soil samples on pressure plates operating with an air pressure of 1.5 MPa, it is assumed that plants wilt at a pore water suction of 1.5 MPa. These assumptions are examined here, and it is shown that neither of them is true. Published results are used for the wilting condition. The recently described double-exponential (DE) equation for soil water retention is used for cases where the water is non-equilibrated because of hydraulic cut-off. The non-equilibrated condition is appropriate for plant roots because they, like pressure plate extractors, extract water from soil by immiscible displacement. The DE equation is used to illustrate the conditions under which plant wilting can be either a plant or a soil property. It is shown how this approach can be used to estimate the pore water suction at which plants would wilt because the soil is no longer able to supply water to their roots. It is demonstrated that the commonly used, but often erroneous, value for the wilting-point suction of h = 1.5 MPa is a consequence of the fact that this is the largest value of air pressure used in pressure cell extractors. It is therefore neither a plant nor a soil property, but is an artefact of the experimental procedure. The use of the DE equation for soil water retention shows that we know only that h ≤1.5 MPa, and that h can be as small as 0.2 or 0.3 MPa. Implications for estimation of plant water availability in soils, for plant breeding, and for soil microbial activity are discussed.