Sucrose unloading raises phloem water potential and reduces volume

Steps:

• Sucrose moves out of sieve tube to shoot tip, lowering solute concentration inside.
• Lower solute concentration makes solute potential less negative, increasing overall water potential.
• Higher water potential in sieve tube relative to surroundings drives water outflow via osmosis.
• Water loss decreases the volume of solution in the sieve tube element.

Why A is correct:

• Water potential rises (higher) as solute concentration falls, per the formula ψ = ψ_s + ψ_p where ψ_s becomes less negative; this prompts water efflux, decreasing volume.

Why the others are wrong:

• B: Water potential rises, not lowers; volume decreases, not increases.
• C: Volume decreases due to water loss, not increases.
• D: Water potential rises, not lowers.

Final answer: A