Permanent wilting point is the "point of no return" for a plant.
Try this: place two lettuce plants in containers (one containing Water Crystals, the other without). Use the same amount of soil, and water them equally until fully grown. Next, stop watering, and observe the results.
The plant with Water Crystals will normally live 6 days longer than the other plant. This little test works the same for all plants. In your home. In your garden. On your lawn. And, especially on farmland.