What is the cause of phototropism in stems and roots?

Phototropism is the growth or movement of a plant in response to light.

In stems, phototropism is caused by the differential distribution of the plant hormone auxin, which is responsible for a plant's ability to grow toward the light. It accumulates on the shaded side of the stem and promotes the elongation of cells on that side, causing the stem to bend towards the light source. 

In roots, phototropism is caused by a similar differential distribution of auxin, where the auxin accumulates on the side of the root away from the light source, causing cells on that side to elongate less and the root to bend away from the light source.

Epinasty and hyponasty refer to the upward or downward bending of leaves in response to environmental factors.

Graviperception refers to the ability of plants to sense and respond to gravity, not to light.