Question

Stomata of a plant open due to
A. Influx of potassium ions
B. Efflux of potassium ions
C. Influx of hydrogen ions
D. Influx of calcium ions

Answer 1

Hint: We know that stomata are an essential part of plants which helps in photosynthesis and gaseous exchange.
- The mechanism of opening and closing of stomata can be described by the proton-potassium pump hypothesis.

Complete Answer:
- Stomata are the openings present in the epidermis of leaves, which play an important role in photosynthesis and gaseous exchange. The stomata consist of a stomatal pore at centre which is surrounded by a pair of bean shaped guard cells.
- The guard cell contains chloroplast and the turgor pressure of the guard cell helps in opening and closing of stomata. The guard cell is surrounded by a subsidiary cell which is covered by the outer epidermal cell.
- The opening of stomata at day time and closing of stomata at night is governed by various mechanisms. These are: -
(I) Starch-sugar interconversion theory
(II) Proton-potassium pump hypothesis
(III) Organic solute synthesis

- Proton-potassium pump hypothesis was given by Levit in 1974. According to the hypothesis, potassium ions are transported by the guard cell in presence of light source, which causes the opening of the stomata during day time.
The mechanism is given by the following ways:
- Under the influence of light, malic acid dissociates to form malate ions and protons which move from cytoplasm into the chloroplasts of guard cells.
- To counter the exit of protons, potassium ions enter the guard cells.
- Now, $K^+$ ions react with the malate ions present in the guard cells to form potassium malate.
- Potassium malate helps in increase of the osmotic potential of guard cells which causes the intake of water into the guard cells as a result of which the stoma opens.

Hence, the correct answer is (A) influx of potassium ions.

Note:
- The opening and closing of the stomata are controlled by the turgor pressure of the guard cell of the stomata.
- The influx of potassium ions helps in the formation of potassium malate which increases the osmotic potential of the guard cell so that the stomata opens in day time.
- During night, potassium malate dissociates and efflux of potassium ions take place which decreases the osmotic potential of the guard cell. So, water is removed from the guard cell and the stomata closes.