Phloem Loading Mechanism in Sieve Tubes

Steps:

• Active loading of sugars into sieve tube elements from source cells decreases solute potential.
• Lower solute potential creates a water potential gradient, drawing water into sieve tubes via osmosis.
• This establishes turgor pressure for mass flow toward sinks.
• Sieve tube walls remain unlignified to allow flexibility and prevent blockage during transport.

Why C is correct:

• Lower water potential in sieve tubes than source drives osmotic influx of water, per the pressure-flow hypothesis; unlignified walls maintain sieve plate porosity for translocation.

Why the others are wrong:

• A: Higher water potential prevents sugar and water entry into sieve tubes.
• B: Higher water potential blocks loading; lignification would rigidify and obstruct flow.
• D: Lignification is absent in sieve tubes, as it would impede living transport functions.

Final answer: C