Final answer:
Adding a substance that makes the membrane more permeable to hydrogen ions would likely increase the rate of sucrose transport into a plant cell by enhancing the proton gradient used for sucrose co-transport.
Step-by-step explanation:
The student's question involves understanding how different factors affect sucrose transport into plant cells. Based on the given figures and knowledge of plant cell physiology, the experimental treatment that would increase the rate of sucrose transport into a plant cell would likely be adding a substance that makes the membrane more permeable to hydrogen ions.
This is because the active transport of sucrose into plant cells often relies on proton gradients (hydrogen ion concentrations), where the movement of protons (H+) can facilitate the movement of sucrose against its concentration gradient, a process known as co-transport. Making the membrane more permeable to hydrogen ions would enhance the proton gradient, which could in turn increase the uptake of sucrose into the cell.