Answer: a water shortage.
- A a water shortage
- B excess soil water
- C bright sunlight
- D cool weather
Correct answer: A. a water shortage
Explanation: Under water stress, guard cells lose turgor and the stomata close to save water.
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.