Phloem unloading at sink via pressure-flow

Steps:

• In pressure-flow, sieve tubes at sinks unload sugars, raising their water potential above the sink's low ψ to drive water exit by osmosis.
• Sink tissues maintain low ψ to import solutes and water for growth.
• Sieve tube elements have thin, non-lignified walls with sieve plates for efficient transport.
• Lignification is absent in phloem but present in xylem for support.

Why A is correct:

• Water potential gradient (ψ_sieve > ψ_sink) allows osmosis out, per water potential principle (water moves from high to low ψ); phloem sieve tubes lack lignified walls by definition.

Why the others are wrong:

• B: Lignified walls are absent in phloem sieve tubes.
• C: ψ_sieve must exceed ψ_sink for unloading; lower ψ prevents water exit.
• D: Both ψ_sieve lower than sink (blocks flow) and lignified walls (xylem feature) are incorrect.

Final answer: A