How do auxin distributions cause stems to bend toward light or grow upright after leaning?

Auxins are plant hormones that regulate growth by stimulating cell elongation in stems. They facilitate this by acting on the chemical bonds of carbohydrates in plant cell walls, allowing cells to deform and expand as they take in water.12

The movement of stems is driven by the unequal distribution of these hormones:

• Phototropism: When a stem grows toward light, greater quantities of auxin are distributed to the shaded side than to the side facing the light. This causes the shaded side to elongate more rapidly, forcing the stem to curve toward the light source.12
• Geotropism: When a stem is leaning, more auxin accumulates along the lower side of the stem than the upper side. This increased concentration on the bottom causes the lower side to elongate more, generating an upward curvature that allows the plant to grow upright against the force of gravity.12

While auxins promote stem elongation, they have an inhibitory effect on root growth, which explains why roots respond differently to auxin accumulation.12