Phloem Loading in Sieve Tubes Steps:

• Carbohydrates enter the phloem sieve tube, increasing its solute concentration.
• Higher solute concentration lowers the solute potential, making the water potential more negative.
• The more negative water potential draws water into the sieve tube from surrounding cells.
• Influx of water increases hydrostatic pressure in the sieve tube.

Why D is correct:

• In phloem loading, sugar influx makes sieve tube water potential more negative, solute concentration increases, and hydrostatic pressure increases to drive mass flow (per pressure-flow hypothesis).

Why the others are wrong:

• A: Water potential does not become less negative; solute and pressure changes mismatch loading effects.
• B: Water potential does not become less negative; pressure does not decrease.
• C: Solute concentration and pressure both increase, not decrease.

Final answer: D