Question

Why the opening and closing of sunflowers is growth movement?

Answer 1

if cell growth on the east and west sides are cyclical and matches the sun's movement, sunflower stems would appear to 'follow' the sun. The other is that of water movement within the plant, known as turgor pressure.

Answer 2

Auxin moves to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant. This produces a curving of the plant stem tip toward the light, a plant movement known as phototropism.