Correct Option: D. Concentration of auxins in the region of curvature Explanation of the Correct Answer: In plants, phototropism is the growth response to light, where parts of the plant grow towards the light source. The coleoptile, which is the protective sheath covering the emerging shoot of a seedling, exhibits this response prominently. The mechanism behind this response involves the plant hormone auxin, which plays a crucial role in regulating growth.

1. Auxin Distribution: When light shines on one side of the coleoptile, auxins are redistributed. They move away from the light and accumulate on the shaded side. This uneven distribution of auxins is key to the phototropic response.
2. Growth Rate: Auxins promote cell elongation. Therefore, the side of the coleoptile that has a higher concentration of auxins (the shaded side) will grow faster than the side exposed to light (the illuminated side). This differential growth causes the coleoptile to bend towards the light source.
3. Curvature Formation: The region of greatest curvature occurs where the concentration of auxins is highest, which is on the shaded side. As the cells on this side elongate more than those on the light-exposed side, the coleoptile bends towards the light.

Why Other Options Are Incorrect:

• Option A: Movement of auxins away from the region of curvature This option suggests that auxins move away from the area where the curvature occurs. However, this is incorrect because the curvature is actually caused by the accumulation of auxins on the shaded side, not their movement away from it. If auxins were to move away, there would be less growth on that side, leading to less curvature, not more.
• Option B: Even distribution of auxins in all parts of the coleoptile An even distribution of auxins would not result in curvature. If auxins were evenly distributed, both sides of the coleoptile would grow at the same rate, and there would be no bending towards the light. The differential growth caused by uneven auxin distribution is what leads to the bending.
• Option C: Inhibition of growth by auxins in the region of smaller curvature This option implies that auxins inhibit growth on the side with less curvature. While auxins can have inhibitory effects in certain contexts, in the case of phototropism, they primarily promote growth on the shaded side. The side with less curvature (the light-exposed side) does not experience inhibition but rather a lower concentration of auxins, leading to slower growth compared to the shaded side.

Summary of Key Points:

• Phototropism is the growth of plants towards light, mediated by auxins.
• Auxins accumulate on the shaded side of the coleoptile, leading to greater cell elongation there.
• Curvature occurs due to the differential growth rates between the shaded and light-exposed sides.
• Correct answer is D: Concentration of auxins in the region of curvature, as this leads to the bending of the coleoptile towards the light source.