Why tomato plant wilts if too much fertilizer is applied

Question: Explain briefly why a tomato plant is likely to wilt if too much fertilizer is applied

The short answer is:

The tomato plant undergoes plamolysis - a process by which a plant or animal cell lose water by osmosis because of the cell being placed or surrounded by a solution of higher concentration than the cell sap. 

The application of too much fertilizer to the tomato plant increased the concentration of the soil solution surrounding the plant cells. This led to the flow of water from the tomato cell sap (of lower concentration) to soil solution (of higher concentration) by osmosis. 

Because of loss of water from the tomato plant, the plant begins to wilt. It is the presence of water in the plant cells that make the tomato plant turgid so once that is gone it begins to lose wilt as it loses its turgidity.
 

The long answer is:

A tomato plant contains what we call plant cells. Plant cells are different from animal cells. Plant cells have a rigid cell wall.

Plant cells have two types of environments – internal and external environments. The cytoplasm (A) together with the cell sap contained in the vacuole forms the internal environment of the cell. The atmosphere and the soil form the external environment.

Movement of fluids, water especially, occur between the two environments – between the plant cell, and the atmosphere and soil. When the flow of water between the internal environment of the tomato plant and its external environment is equal, we say there is equilibrium.

However, that equilibrium can be altered, when too much of a substance like fertilizer is applied to the tomato plant. The fertilizer changes the tomato plant’s external environment by increasing the solute concentration of the soil. This makes the soil solution hypertonic. A hypertonic solution is a solution that has far more solutes that solvent.

As a result of this osmotic gradient (that is, the difference between the concentration of the two environments), water begins to flow out of the plant cells via osmosis into the soil, leading to loss of water from the tomato plant which eventually manifested as wilting.