Answer: Is a comparatively weak force, most active at night when transpiration is low.
- A Pushes water downward from shoot to root, not upward according to standard texts
- B Is generated by evaporation of water from leaf surfaces in daytime under typical physiological conditions
- C Operates in plants whose xylem vessels are non-functional in general clinical practice
- D Is a comparatively weak force, most active at night when transpiration is low
Correct answer: D. Is a comparatively weak force, most active at night when transpiration is low
Explanation: Root pressure is generated by active accumulation of ions in root xylem, but it is a relatively weak force and most evident at night/early morning (causing guttation); it cannot explain water rise in tall trees during the day.
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.