Explain why a plant wilts if it lacks water

The vacuole doesn't have enough water in it

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.