Diurnal fluctuations in tree trunk diameter from xylem water dynamics

Steps:

• Recognize that tree trunks expand at night when transpiration stops and contract during the day due to water loss.
• Identify K as the daytime phase where diameter reduces from active transpiration.
• Link reduction to tension in xylem vessels pulling water upward.
• Confirm C matches this mechanism, ruling out phloem or sink roles.

Why C is correct:

• Transpiration creates negative pressure (tension) in xylem per the cohesion-tension theory, drawing water from trunk storage tissues and reducing diameter.

Why the others are wrong:

• A: Transpiration causes shrinkage, not expansion; water tension depletes xylem storage.
• B: Phloem translocation transports sugars, not water loss affecting trunk diameter.
• D: Phloem acts as a source/sink for sugars, but nighttime expansion results from xylem relaxation, not phloem.

Final answer: C