Answer: Potassium.
- A Calcium
- B Iron
- C Potassium
- D Magnesium
Correct answer: C. Potassium
Explanation: Potassium (K+) ions actively accumulate in guard cells in the light, increasing osmotic pressure. Water enters by osmosis, making guard cells turgid and opening stomata.
Atmospheric N₂ is fixed into ammonia by nitrogen-fixing bacteria (protected from oxygen by leghaemoglobin in root nodules), converted to nitrate via nitrification for plant uptake, and eventually returned to the atmosphere as N₂ via denitrification, completing the cycle.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.