Which of the following statements correctly describes the role of transpiration in the cohesion-tension theory of water transport in plants?

• A. Transpiration causes water to move from the phloem to the xylem by osmosis.
• B. Transpiration directly pumps water from the roots to the leaves using active transport.
• C. Transpiration only occurs at night when stomata are closed.
• D. Transpiration creates a negative pressure (tension) in the leaf, pulling water upward through the xylem.

Answer: (D)

Verified step by step guidance

1. Step 1: Begin by understanding the cohesion-tension theory. This theory explains how water moves upward through the xylem in plants due to the combined effects of cohesion (water molecules sticking together) and tension (negative pressure created by transpiration).
2. Step 2: Define transpiration. Transpiration is the process by which water evaporates from the stomata (tiny openings) in the leaves. This evaporation creates a negative pressure or tension in the leaf's air spaces.
3. Step 3: Explain how negative pressure drives water movement. The negative pressure generated by transpiration pulls water upward from the roots through the xylem vessels. This movement is facilitated by the cohesive properties of water molecules, which allow them to form a continuous column.
4. Step 4: Address the incorrect statements. Transpiration does not cause water to move from the phloem to the xylem by osmosis, nor does it directly pump water using active transport. Additionally, transpiration primarily occurs during the day when stomata are open, not at night.
5. Step 5: Conclude with the correct statement. Transpiration creates a negative pressure (tension) in the leaf, which is the driving force for pulling water upward through the xylem, aligning with the cohesion-tension theory.