Mechanism of Opening and Closing of Stomata

Mechanism of Opening and Closing of Stomata

MECHANISM OF TRANSPIRATION 

Water is absorbed by the roots and is conducted upwards through the xylem vessels.  Water from the stem enters into the leaves through the xylem elements of petiole, veins and veinlets.  Water is then distributed throughout the leaves through veinlets.  The mesophyll cells of the leaves absorb water from the xylem elements of the veins and veinlets and get saturated.  When sunlight falls on the leaves, the water of mesophyll cells evaporate and the intervening airspaces get saturated with water.

The atmosphere is rarely saturated with water vapours.  The dry air of the atmosphere has a higher diffusion pressure deficit.  The intercellular spaces of the transpiring organs is almost saturated with water vapours.  When the stomata are open, the water vapours are drawn from the substomatal cavities to the outside air.  This increases the diffusion pressure deficit of the substomatal air which draws more water vapours from the intercellular spaces.  The intercellular spaces in turn get water vapour from the wet walls of mesophyll cells.  Stomatal transpiration continues till the stomata are open.  Transpiration depends primarily on two factors:

  1. Difference in vapour pressure between the inside and outside atmosphere of leaves.
  2. Degree of stomatal opening.

Normally, there is always a difference in vapour pressure between the inside and outside atmosphere of leaves but stomata are not always open because of various reasons.  Hence, the rate of transpiration depends upon the number of stomata and the degree of stomatal opening.

MECHANISM OF OPENING AND CLOSING OF STOMATA

 Various hypothesis are given to explain the stomatal movement from time to time.  Some of them are given below:

  1. 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.  Lloyd (1908) observed that the chloroplast present inside the guard cell synthesizes the soluble sugar or carbohydrate during the daytime and at night these sugars get converted into starch.  As the concentration of soluble sugar increase, 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 get stretched 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 which decreases osmotic concentration of the guard cell.  The guard cell, as a result of which, becomes flaccid and the stomata is closed.

Starch+inorganic phosphate ⇌ glucose-1-phosphate

The starch sugar theory have been criticised on the basis of the following facts:

  1. Conversion of starch into glucose was not always observed during the opening of stomata. In many plants, starch is converted into organic acids.
  2. In monocotyledons, starch is not synthesized in the guard cells. They contain other polysaccharides.
  3. Stomatal closure at mid-day often takes place without any change in starch contents

 Sayre (1926) modified the view of Lloyd.  According to him, interconversion of starch into sugar is determined by pH.  During daytime carbon dioxide liberated due to respiration is used in photosynthesis by the mesophyll cells.  This results in increase in pH to 7-7.5.  In this alkaline state, starch is converted into glucose-1-PO4.  In the dark, carbon dioxide accumulates in the intercellular spaces as it is not utilized in photosynthesis.  It lowers the pH of the guard cells. In this acidic state, glucose-1-PO4 is converted into starch and the stomata get closed.

Starch+inorganic phosphate ⇌ glucose-1-phosphate

Yin Tung (1948) showed the presence of the enzyme, phosphorylase in chloroplast.  Phosphorylase favours hydrolysis of starch into glucose-1-PO4 in the presence of inorganic phosphate at high pH.  At low pH, enzyme favours formation of starch from glucose-1-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):  The above hypothesis is criticized by Steward.  The major reasons were as follows:

  1. Inorganic phosphate which is used in the formation of glucose-1-PO4 is osmotically as active as glucose-1-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.
  2. 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:

  1. The pH of guard cells increases to 7 due to the use of the carbon dioxide in photosynthesis in the presence of light.
  2. High pH favours conversion of starch into glucose-1-PO4 in the presence of phosphorylase.

Starch+phosphate→glucose-1-phosphate

  1.  Glucose-1-PO4 is further converted to glucose-6-PO4 in the presence of enzyme phosphoglucomutase.

Glucose-1-phosphate→glucose-6-phosphate

  1. Glucose6-PO4 is now converted to glucose and phosphate in the presence of enzyme, phosphatase.

Glucose-6-phosphate →glucose+phosphate

  1. Glucose being osmotically active increases osmotic concentration of the guard cells. Hence, water from the surrounding cells enters them by endosmosis.
  2. The turgor pressure of guard cells increases and the stoma opens. 

Closing of the stomata in dark involves the following steps:

  1. The pH of guard cells decreases due to accumulation of carbon dioxide in the intercellular spaces as photosynthesis does not occur in dark.
  2. Low pH favors conversion of glucose into glucose-1-PO4 in the presence of enzyme hexokinase. This requires ATP which is made available by respiration.

Glucose+ATP→glucose-1-phosphate

  1. Glucose-1-PO4 is now converted into starch in the presence of enzyme, phosphorylase.

Glucose-1-phosphate→starch

  1. 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.