Question

Explain the mechanism of opening and closing of stomata.

Answer 1

When the guard cells surrounding the stomatal pore are in a turgid state, stomata open and when the guard cells become flaccid stomatal opening gets closed.
Mechanism of Opening and Closing of Stomata:

• Opening and closing of stomata may be called stomatal movement.
• Several hypotheses have been proposed to explain the stomatal movement.

1. Starch:Sugar hypothesis:
This hypothesis was proposed by J.D. Sayre (1923).

• Prior to Sayre, Lloyd (1908) showed the presence of starch in the guard cells.
• According to starch-sugar hypothesis during day time, $$CO_2$$ is used in guards cells in photosynthesis and pH of guard cells increases (7.0).
• At this pH, the phosphorylase enzyme becomes active in guard cells and starch is converted into glucose-1-phosphate.
• On account of this, DPD of guard cells increases and they become turgid due to endosmosis and stomata open.
• During the night, photosynthesis stops and $$CO_2$$ accumulates in the guard cells, due to which the pH of guard cells decreases (5.0).
• At this pH, glucose-1-phosphate converts into starch.
• DPD of guard cells decreases and they become flaccid due to exosmosis, causing the closing of stomata.

Starch + iP → Glucose - 1 - phosphate
2. Steward's hypothesis:

• According to this hypothesis, an increase in pH of guard cells results in the conversion of starch in glucose-1-phosphate, and then into glucose-6-phosphate which finally converts into glucose.
• Glucose and glucose-6-phosphate are more soluble in water as compared to glucose-1-phosphate.
• This results in an increase, in the osmotic concentration of guard cells leading to the opening of stomata.
• Biochemical reaction during day time:

Starch + iP → Glucose-1-phosphate
Glucose-1-phosphate → Glucose-6-phosphate
Glucose-6-phosphate → Glucose + iP

• Biochemical reactions during night time:

Glucose + ATP → Glucose-1-Phosphate
Glucose-1-Phosphate → Starch + iP
3. Active Potassium Ion Transport theory:

• This theory was proposed by Japanese scientists. Imamura and Fujino (1959) and was modified by Levitt (1974).
• Presently this is the most accepted theory for explaining the stomatal movement.
• According to this theory, during the presence of light malic acid is formed in guard cells which breaks into malate and in hydrogen ion ($$H^+$$).
• These $$H^+$$ move out of guard cells and pass to subsidiary cells (influx) or epidermal cells.
• $$K^+$$ from subsidiary cells or epidermal cells move into guard cells where they react with the malate and form potassium malate.
• This increases the osmotic concentration of the guard cells.
• Increase in osmotic concentration of guard cells results into endosmosis in these cells and they become turgid causing opening of stomata.
• During night efflux of $$K^+$$ from guard cells to adjacent epidermal cells takes place.
• $$H^+$$ flow from epidermal cells into guard cells where they react with malate to form malic acid.
• This principle is also called as $$K^+$$ ion exchange or anion exchange or photoactive proton transport theory.
• At present this theory is the most accepted theory for explaining the opening and closing of stomata.