Impacts of soil type and crop species on permanent wilting of plants

The lack of a continuous network of water-filled pores necessary for the flow of water in the soil has been related to the wilting of plants as well as to the limitations of soil sample equilibrium in pressure plates. The objectives of this study were: (i) to quantify deviations of the soil water content measured in pressure plates and dew point measurements at a matric potential of -15,000 hPa (commonly defined as the permanent wilting point), (ii) to determine the soil matric potential at which the physiological wilting of different plant species occurs in soil with contrasting texture, and (iii) to evaluate if the wilting of plants occurs at the hydraulic cut-off in soil. An experiment was conducted to determine the soil matric potential at which wilting of sunflower (Helianthus annuus L.), maize (Zea mays L.) and soybean (Glycine max L.) occurs in four soils with clay contents ranging from 7 to 57%. Soil water retention characteristics were determined by the pressure chamber and the dew point technique. Soil water retention data were fitted to empirical models for estimates of soil matric potential and water content at the hydraulic cut-off. The water contents at -15,000 hPa in samples equilibrated in pressure plates was similar to those obtained from the dew point method in our soils. Our results show that physiological wilting of plants is both plant species and soil dependent. In soils with low clay contents, different plant species wilted at similar water contents. However, in the clayey soils, the different crops showed different abilities in the uptake of water from the soil, where soybean wilted at significantly lower water contents than the other two species. The values of matric potential at the permanent wilting point found in our study were considerably lower than -15,000 hPa, suggesting that the commonly used definition of permanent wilting point as water content at -15,000 hPa is incorrect.