Diurnal Stem Shrinkage from Transpiration Pull

Steps:

• Transpiration rate rises during daylight, evaporating water from leaves.
• This creates negative pressure (tension) in xylem via cohesion-tension mechanism.
• Tension withdraws water from stem tissues, reducing turgor and circumference.
• At night, transpiration halts, allowing water uptake and stem re-expansion.

Why B is correct:

• Cohesion-tension theory states that daytime transpiration generates tensile forces in xylem water columns, pulling water upward and contracting stem diameter.

Why the others are wrong:

• A: Adhesion aids water retention in xylem but does not vary diurnally to cause circumference changes.
• C: Root pressure drives upward flow mainly at night, not daytime shrinkage.
• D: Sucrose flow in phloem affects transport but not direct water-based girth fluctuations.

Final answer: B