Question

Which of the following elements regulates the opening and closing of stomata?

• A. N
• B. K
• C. S
• D. P

Hint:

Potassium (\ce{K+}) controls stomatal opening/closing by driving water movement in guard cells.

Answer 1

The Correct Option is B

Potassium (K, option 2) is critical for regulating the opening and closing of stomata in plants. Stomata, small pores on plant leaves, control gas exchange and water loss. The movement of potassium ions (\ce{K+}) into and out of guard cells surrounding the stomata regulates their turgor pressure. When \ce{K+} enters the guard cells, water follows via osmosis, causing the cells to swell and the stomata to open, facilitating photosynthesis and transpiration. Conversely, when \ce{K+} exits, water leaves, reducing turgor and closing the stomata to conserve water. Other elements like Nitrogen (N), Sulfur (S), and Phosphorus (P) are essential for plant growth but do not directly regulate stomatal movement. Thus, option (2) is correct.

Answer 2

Elimination approach:
Stomatal opening and closing is controlled by turgor changes in guard cells, which are driven by ion transport rather than nutrient supply alone. Consider each option: Nitrogen (N) is a structural component of proteins and chlorophyll, Sulfur (S) is used in amino acids and coenzymes, and Phosphorus (P) is central to energy transfer (ATP) and nucleic acids, none of these ions move rapidly enough across the guard cell membrane to drive the osmotic swelling/shrinking cycle. Potassium (K), being a small monovalent cation, is actively pumped into guard cells via \(H^+\)/\(K^+\) exchange pumps, lowering water potential and drawing water in by osmosis, which swells the cells and opens the pore; efflux of \(K^+\) reverses this and closes it. Eliminating N, S, and P by their non-osmotic roles leaves Potassium (K) as the only option consistent with the guard-cell mechanism, confirming option (2).