Guard cells in leaves are most closely associated with which plant process?

• A. Transpiration.
• B. Photosynthesis.
• C. Evaporation of Water From the Plant.
• D. Respiration.

Answer: (C)

Guard cells regulate the openings (stomata) on the leaf surface. By changing their turgor, they decide when the pore is open or closed. When the stomata are open, water inside the leaf can escape as water vapor to the air. This loss of water is evaporation from the plant, which is the process known as transpiration. Guard cells are directly involved in controlling this water loss, tying them closely to evaporation from the plant. (Photosynthesis needs CO2, but the guard cells’ primary direct role is in regulating water loss through stomatal opening.)