Sucrose loading drives phloem pressure flow Steps:

• Sucrose is actively transported from leaf mesophyll into phloem sieve tubes.
• Sucrose accumulation raises solute concentration inside the tubes.
• Increased solutes decrease water potential (more negative ψ).
• Lower water potential creates a gradient, pulling water from xylem into sieve tubes via osmosis. Why D is correct:
• Water potential lowers due to solute potential formula (ψ_s = -iCRT), increasing osmotic pressure that draws in water and raises solution volume. Why the others are wrong:
• A: Water potential does not increase; sucrose loading makes it more negative.
• B: Water potential does not increase; it decreases while volume rises.
• C: Volume does not decrease; water influx expands the solution.

Final answer: D