What causes sunflowers to bend toward the sunlight?

Mature sunflowers do not track the Sun.

Immature sunflower buds exhibit heliotopism. The heliotropic motion of the bud is performed by the pulvinus, a flexible segment just below the bud, due to reversible changes in turgor pressure (no growth).

Many plants are known for their ability to track the movements of the sun by a process called solar tracking or heliotropism. This is sometimes controlled by water movement in and out of cells and plant hormones.

However, cultivated sunflowers will track the sun only when young or in bud. Once mature, the flowers tend to face east. Wild sunflowers do not track the sun.

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As to why they face east, no one knows at this time. It is thought it may be to protect young developing seeds from the hot rays of the afternoon sun.

Leaves of many plants track the sun to collect the most light energy for photosynthesis, but photosynthesis has nothing to do with the leaf movements.