Answer:
The vacuole doesn't have enough water in it
Step-by-step explanation:
If you look at a diagram of a plant cell, you will see that it has a quite large (in proportion to a cell, of course) oblong shape called a vacuole. A vacuole is full of water and the cell's waste. When the plant is lacking water, the vacuole is the first to lose its contents. Therefore, it "deflates" and wilts.