Mechanism of opening and closing of stomata
Stomatal transpiration occurs when stomata are fully open. The opening and closing of stomata is a direct response to increase or decrease in the osmotic concentration of the guard cells. When the osmotic concentration of the guard cells increases, they absorb water from the surrounding cells and become turgid.
Increase in the turgidity causes the outer thin elastic wall of the guard cells to stretch towards outside. The inner thick and inelastic wall also gets pulled along with it and assumes a concave shape. The stoma or pore between the two guard cells is open in this condition. Loss in the turgidity of guard cells results in stomatal closing.
Osmotic concentration of the guard cells regulates the turgidity of guard cells. Thus the turgidity of guard cells, is the main cause of stomatal opening and closing.
Various hypothesis are given to explain the stomatal movement from time to time. Some of them are given below:
Lloyd’s hypothesis or starch-sugar hypothesis:
Opening and closing of stomata is mainly due to the turgidity of guard cell. It again depends on the concentration of soluble sugar present in it. According to Lloyd (1908) chloroplast present inside the guard cell synthesizes the soluble sugar or carbohydrate during the daytime and at night these sugars convert into starch.
As the concentration of soluble sugar increases, the osmotic pressure also increases and thus, diffusion pressure deficit increases. When diffusion pressure deficit of guard cell increases, endosmosis occurs and the guard cells become turgid. Under turgid condition, the thin walls of guard cells stretch and the pore between remain wide open. Water vapour diffuses out from stomata to the outer atmosphere through these pores.
At night, the soluble sugar accumulates in the form of insoluble starch. As a result the osmotic concentration of the guard cell decreases. The guard cell, as a result of which, becomes flaccid and the stomata is closed.
Starch+inorganic phosphate ⇌ glucose1-phosphate
The reasons for criticism of the starch sugar hypothesis is as follows:
- Conversion of starch into glucose was not always observed during the opening of stomata. In many plants, starch is converted into organic acids.
- In monocotyledons, the guard cells do not synthesize starch. They contain other polysaccharides.
- Stomatal closure at mid-day often takes place without any change in starch contents.
Sayre ‘s (1926) Theory
Sayre modified the view of Lloyd. According to him, pH determines the interconversion of starch into sugar . During daytime carbon dioxide liberated due to respiration is used in photosynthesis by the mesophyll cells. As a result pH increases to 7-.75. In this alkaline state, starch converts into glucose-1-PO4.
In the dark, carbon dioxide accumulates in the intercellular spaces because it is not used in photosynthesis. It lowers the pH of the guard cells. In this acidic state, glucose1-PO4 converts into starch and the stomata closes.
Starch+inorganic phosphate ⇌ glucose1-phosphate
Yin Tung’s (1948) Theory
Yin Tung showed the presence of the enzyme, phosphorylase in chloroplast. Phosphorylase favours hydrolysis of starch into glucose1-PO4 in the presence of inorganic phosphate at high pH. At low pH, enzyme favours formation of starch from glucose1-PO4.
Starch is osmotically inactive while glucose1-PO4 is osmotically active. Hence, there are changes in the osmotic concentration of the guard cells.
Starch+inorganic phosphate ⇌ glucose-1-phosphate
Steward’s hypothesis (1964):
Steward did not accept the above hypothesis due to the following reasons:
- Inorganic phosphate which is used in the formation of glucose1-PO4 is osmotically as active as glucose1-PO4. Hence, there would be no appreciable change in the osmotic concentration of the guard cells as proposed in Sayre’s and Yin Tung’s hypothesis.
- The stomatal closure requires ATP which is not mentioned in any of the above theories.
Steward proposed the following to explain stomatal movement.
Opening of stomata in light involves the following steps:
- The pH of guard cells increases to 7 due to the use of the carbon dioxide in photosynthesis in the presence of light.
- Due to high pH starch converts into glucose1-PO4 in the presence of phosphorylase.
- Glucose1-PO4 converts to glucose-6-PO4 in the presence of enzyme phosphoglucomutase.
- Glucose6-PO4 now converts to glucose and phosphate in the presence of enzyme, phosphatase.
- Glucose being osmotically active increases osmotic concentration of the guard cells. Hence, water from the surrounding cells enters them by endosmosis.
- The turgor pressure of guard cells increases . Therefore the stoma opens.
Closing of the stomata in dark involves the following steps:
- The pH of guard cells decreases due to accumulation of carbon dioxide in the intercellular spaces as photosynthesis does not occur in dark.
- Low pH favours conversion of glucose into glucose-1-PO4 in the presence of enzyme hexokinase and ATP .
- Glucose-1-PO4 converts into starch in the presence of enzyme, phosphorylase.
Starch being osmotically inactive, osmotic concentration of the guard cells decreases. Water comes out from the guard cells due to exosmosis. This reduces the turgor pressure of the guard cells and they become flaccid. Therefore the stoma closes.
Comparison of opening and closing of stomata in light and dark
|Guard cells in light||Guard cells in dark|
|1. Carbon dioxide accumulated in the intercellular spaces and that released in the process of respiration is utilized in photosynthesis.||1. Carbon dioxide accumulates in the intercellular spaces and guard cells because it is not utilized in photosynthesis.|
|2. pH of the guard cells increases to 7.5 and becomes alkaline.||2. pH of the guard cells decreases to 5 and becomes acidic.|
|3. In the alkaline medium of guard cells, hydrolysis of starch into the sugar takes place in the presence of enzyme phosphorylase.||3. In the acidic medium of guard cells, conversion of sugar into starch occurs in the presence of enzyme phosphorylase.|
|4. The osmotic pressure of the guard cells, increases due to increase in the concentration of sugar in their cell sap.||4. The osmotic pressure of the guard cells, decreases due to conversion of sugar into starch.|
|5. Water from adjoining cells move into the guard cells due to increased concentration of their cell sap.||5. Water from the guard cells passes into the adjoining epidermal cells due to decrease in the osmotic concentration of the guard cells.|
|6. Due to endosmosis, the turgidity of guard cells increases resulting in the opening of stomata.||6. Due to exosmosis, the turgidity of guard cells decreases resulting in the closing of stomata.|