Permanent wilting point moisture content for a crop represents the

Permanent Wilting Point (PWP)
In Permanent Wilting Point (PWP) plants can be no longer obtains enough moisture to meet their requirements and remained wilted until water is added into the soil.
 It is a soil characteristic.
It is the lower limit of available water to plant.
Hygroscopic water: The water that is sandwiched or adsorbed or tightly held between the colloidal of soil particles is called hygroscopic water. This water is difficult to extract even by the plant as a force of attraction between soil and water may reach as high as 1000 atm.
🛑Important Points: 
Free water or Gravitational water: The water that can flow freely under gravity and available for drainage is called free water.
Capillary water: The water retained by soil with its moisture-holding capacity under tension (Surface tension) is called capillary water. Plants use this water for their growth.
Chemically combined water: After all the hygroscopic water gone by heating soil to 150 o C, the remained water in the soil is called chemically combined water. It is irrelevant to extract water from this soil as it changes the physical and chemical properties of soil.