What is the principal substance that causes phototropism in stems or leaves?

Question

Quizplus · Accepted Answer

Phototropism in stems or leaves is primarily caused by the hormone auxin, which moves from the light side of the stem or leaf to the shaded side and then causes elongation of cells on that side, leading to bending towards the light source. Gibberellin, abscisic acid, and ethylene are also involved in different aspects of plant growth and development, but not in the regulation of phototropism.

What Is the Principal Substance That Causes Phototropism in Stems