Answer: Root pressure exceeds a low transpiration rate, mostly at night or dawn.
- A Root pressure exceeds a low transpiration rate, mostly at night or dawn
- B Xylem vessels get blocked internally by trapped air bubbles in most observed cases
- C Stomata remain wide open overnight under dry conditions under most conditions studied
- D Transpiration runs highest during the hottest daylight hours in standard reference material
Correct answer: A. Root pressure exceeds a low transpiration rate, mostly at night or dawn
Explanation: Guttation happens when root pressure pushes excess xylem sap out through specialised pores (hydathodes) at leaf margins, most noticeable at night/early morning when transpiration is minimal and humidity is high.
Water can travel through cell walls (apoplast) or cell-to-cell through the cytoplasm (symplast) - but the Casparian strip at the endodermis blocks the apoplast route entirely, forcing everything through a selectively permeable membrane (symplast) before it can enter the xylem.
Concept context
Water and mineral absorption, the ascent of sap, transpiration, and phloem transport via the pressure flow hypothesis.