Which organelle, if empty, would cause the plant to wilt? Why is this?

The vacuole

Vacuole is an important organelle present in the cells of plants. Vacuoles contain water, enzymes and other substances. Part of the functions of the vacuole is that it maintains the turgor and proper pressure within the plant cells to provide structure and support for the growing plant. If the vacuole is empty (without water), there would be lack of turgor pressure which is needed for the vacuole to push against the cell wall and this would result in the wilting of the plant.