Question

Why the integuments of an ovule harden and the water content is highly reduced as the seed matures?

Answer 1

Integuments of ovules harden to protect the seed as a tough coat.
Water content is reduced to make the seed dry and keep it in a dormant state.

More to know:

The micropyle facilitates entry of oxygen and water into the seed during germination. As the seed matures, its water content is reduced and seeds become relatively dry and the general metabolic activity of the embryo slows down. The embryo may enter a state of inactivity called dormancy.
If favourable conditions are available they germinate.