Final answer:
When the sun is directly above, auxin hormones are evenly distributed in the plant's stem, causing the plant to grow upright. There is no phototropic curvature towards the light because the light intensity doesn't differ from one side to another. Thus, IAA does not accumulate on one side and balanced growth is promoted.
Step-by-step explanation:
When the sun is directly above a plant, the auxin hormones are evenly distributed throughout the plant's stem because there is no difference in light intensity on either side of the plant. This uniform distribution of auxins results in consistent growth along all sides of the stem, causing the plant to grow upright rather than bending towards a light source.
Under normal conditions with the sun at an angle, auxins tend to accumulate on the shaded side of the plant, causing the cells there to elongate more and the stem to bend towards the light, a well-known phenomenon called phototropism. However, when the sun is directly above, this directional growth cue is minimized or absent. As a result, IAA, the naturally occurring auxin, does not accumulate on one side, and the phototropic response does not occur, preventing any curvature towards a light source and promoting balanced growth.