Diurnal trunk shrinkage from transpiration pull

Steps:

• Observe graph: Trunk diameter decreases during day, increases at night.
• Link to transpiration: Daytime evaporation from leaves creates tension in xylem.
• Apply cohesion-tension theory: Water columns pulled upward, reducing trunk water content and diameter.
• Confirm recovery: Nighttime low transpiration allows rehydration via root pressure.

Why A is correct:

• Cohesion-tension theory states transpiration increases cohesive forces in xylem water columns, pulling water up and shrinking trunk diameter during day.

Why the others are wrong:

• B: Water columns rarely break daily; cavitation is stress-induced, not routine diurnal change.
• C: Sucrose mass flow in phloem affects sugars, not direct water-based diameter fluctuations.
• D: Root pressure drives nighttime upward flow for rehydration, not daytime shrinkage.

Final answer: A