Phloem unloading via water potential gradient

Steps:

• Phloem sieve tubes transport sugars under pressure from source to sink.
• Unloading at sink requires sugar movement out of sieve tube into sink cells.
• Water potential in sieve tube must be higher (less negative) than sink to drive unloading.
• Direct symplastic connections are absent in apoplastic unloading pathways.

Why A is correct:

• Sink cells maintain more negative water potential than sieve tube, creating a gradient for solute unloading per pressure-flow hypothesis.

Why the others are wrong:

• B: Symplastic connections are absent, not present, in this unloading scenario.
• C: Sieve tube potential is less negative, not more, to facilitate flow to sink.
• D: Both potential gradient and connections are mismatched for unloading.

Final answer: A