What causes the roots to grow downward? Light exerts a downward force on the roots. Light exerts a downward force on the roots.
Air in the soil applies a downward pushing force on the roots. Air in the soil applies a downward pushing force on the roots.

Gravity exerts a downward force on the roots. Gravity exerts a downward force on the roots.

Minerals in the soil apply a downward pushing force on the roots.