Fungal growth in xylem blocks water transport via cohesion

Steps:

• Xylem vessel elements transport water upward from roots to leaves.
• Fungi growing inside these vessels physically obstruct the pathway.
• This blockage disrupts the continuous water column needed for ascent.
• Wilting occurs as water cannot reach leaves, directly impairing cohesion-tension mechanism.

Why A is correct:

• Cohesion between water molecules maintains the unbroken column pulled by transpiration; fungal blockage severs this, halting ascent per the cohesion-tension theory.

Why the others are wrong:

• B: Mass flow of assimilates occurs in phloem, not xylem, so unaffected by vessel blockage.
• C: Root pressure develops in roots below xylem vessels, not directly impacted.
• D: Root hair uptake happens at soil interface, prior to xylem entry.

Final answer: A