Final answer:
It is true that auxin efflux transporters in the cap cells of the root redistribute themselves to pump auxin toward the side of the root pointing downward in response to a gravity change, leading to differential growth and gravitropism.
Step-by-step explanation:
The statement concerning auxin efflux transporters redistributing themselves in root cap cells in response to the change in gravity direction is true. When a plant’s root senses the change in gravity, its cells respond by relocating auxin transporters to pump auxin toward the side of the root that is facing downward. This hormonal redistribution leads to differential growth rates on each side of the root, exhibiting gravitropism.
In the presence of gravity, amyloplasts within the root cap cells settle at the bottom, triggering the release of calcium ions that further signal the transportation of the plant hormone indoleacetic acid (IAA), a form of auxin, toward the lower side of the root. A high concentration of IAA inhibits cell elongation at the bottom, reducing growth rate there, while the upper side continues to grow normally. This asymmetry in growth rates allows the root to curve and grow downward into the soil, showcasing positive gravitropism.
Conversely, in the shoots, higher auxin concentration at the lower side promotes cell expansion, resulting in the shoot bending upwards, which illustrates negative gravitropism. Therefore, the behavior of auxin in root cap cells aligns with the physiological plant responses to gravity as observed through root growth patterns.