The turgor pressure is very important for the plants. The loss of the turgor pressure can cause wilting of the plant. The turgor pressure is maintained with the help of osmosis.
The central vacuoles play a very important role in the maintenance of the osmosis flow. In case, the central vacuole is damaged, the cell cannot regulate the osmotic flow. This ultimately cause loss of turgidity and rigidity.
Hence, by rupturing the membrane of central vacuole, the herbicide can kill the plant.