Wilting Point

The wilting point, also called the permanent wilting point, may be defined as the amount of water per unit weight or per unit soil bulk volume in the soil, expressed in percent, that is held so tightly by the soil matrix that roots cannot absorb this water and a plant will wilt.

Unlike field capacity, the term wilting point is associated with known scientists, Briggs and Shantz (1912). They defined the "wilting coefficient" (wilting point) as "the moisture content of the soil (expressed as a percentage of the dry weight) at the time when the leaves of the plant growing in that soil first undergo a permanent reduction in their moisture content as the result of a deficiency in the soil-moisture supply" (Briggs and Shantz, 1912, p. 9). As with field capacity, early workers felt that wilting point was a precise value.

The method of determining permanent wilting point is as follows (Taylor and Ashcroft, 1972, p. 303). An indicator plant, usually sunflower (Helianthus annuus), is put in 500 grams of soil in a metal can. The plant grows and is given adequate moisture until the third pair of true leaves is formed. Then the top of the can is sealed with wax. The sunflower grows in a greenhouse or outdoors until it wilts. Then it is transferred to a dark, humid chamber for recovery. If the plant recovers, it is put out again. The procedure is repeated until the plant remains wilted overnight (24 hours) in the humid chamber. The soil water content then is at the permanent wilting point.

For plants that have leaves that do not wilt, like cacti, Briggs and Shantz (1912) developed special procedures to determine the wilting point. For example, they put a plant with water-storage tissue in a glass container with soil. They glued a knitting needle to one side of the glass. They put the glass with knitting needle in a horizontal position by propping it between two other containers sitting on a table. The needle was free to move up and down a scale. As the cactus used water in the soil, the needle moved in one direction. Then the motion along the scale was gradually reversed, as the cactus shoot itself started to lose water. The wilting point was the point of reversal of needle movement (Briggs and Shantz, 1912, pp. 47-53).

As with field capacity, later researchers realized that the wilting point is not a unique value. It is dynamic, like field capacity. There are a range of values at which the rate of water supply to a plant is not sufficient to prevent wilting, depending on the soil profile (soil texture, compaction, stratification); the amounts of water in the soil at different depths, which affect root distribution; the transpiration rate of a plant; and the temperature (Table 8.1). One should use a water bath to determine the wilting point, to control the temperature. Also, leaves wilt differently. Usually the basal leaves wilt first (Taylor and Ashcroft, 1972, p. 303), so one can refer to the "first permanent wilting point," at which the basal leaves do not recover, and the "ultimate permanent wilting point," at which the apical leaves do not recover. The permanent wilting point depends upon plant osmotic adjustment. Therefore, we recognize that there is a range of values for permanent wilting point, and it is not a unique value (Fig. 8.2).

TABLE 8.1 Influence of temperature on the soil water percentage at which sunflowers will wilt permanently. (From Taylor, S.A., and Ashcroft, G.L., Physical Edaphology: The Physics of Irrigated and Nonirrigated Soils, p. 303, ©1972 by W.H. Freeman and Company. Used with permission.)

Permanent wilting percentage for three soils

TABLE 8.1 Influence of temperature on the soil water percentage at which sunflowers will wilt permanently. (From Taylor, S.A., and Ashcroft, G.L., Physical Edaphology: The Physics of Irrigated and Nonirrigated Soils, p. 303, ©1972 by W.H. Freeman and Company. Used with permission.)

Permanent wilting percentage for three soils

Temperature

Millville silt loam

Benjamin silty clay loam

Yolo fine sandy loam

°C

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Responses

  • ralf
    What is ultimate wilting point?
    3 years ago
  • ferdinanda
    Which is the indicator plant for permanant wiliting point?
    3 years ago
  • hannele
    What are the effect of permanent wilting point?
    2 years ago
  • vanessa kortig
    Why the sunflower is used as indicator plant for wilting?
    2 years ago

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