Answer: Soil nitrogen level.
- A Light intensity
- B Air humidity
- C Wind speed
- D Soil nitrogen level
Correct answer: D. Soil nitrogen level
Explanation: Transpiration rate depends on light, humidity, temperature and wind; the soil's nitrogen level does not directly control it.
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.