Stomatal Physiology

CHAPTER: 7 (C)

Stomatal Physiology

Stomatal Transpiration (Stomatal Physiology):

(Mechanism of Stomatal Transpiration)

Stomata Transpiration:

•      The loss of water from the stomata of the leaves in the form of water vapor is known as stomatal transpiration.

•      It is the most important type of transpiration.

•      Stomatal transpiration constitutes about 50-97% of the total transpiration.

•      It occurs through the stomata. The stomata are found mostly on the leaves.

•      A few of them occur on the young stems, flowers and fruits.

Stomata and its structure:

•      Stomata are minute pores of elliptical shape surrounded by 2 specialized epidermal cells called guard cells.

•      They are kidney shaped (Dicots) and dumbbell shape (Monocots).

•      The wall of the guard cells surrounding the pore is thick and inelastic but rest of the walls are thin elastic.

•      Each cell has a cytoplasmic lining, a central vacuole containing cell sap.

•      The cytoplasm contains a nucleus and number of chloroplasts.

Mechanism of Stomatal Transpiration:

•      Roots absorb water and it is translocated by xylem of the stem and distributed to leaves and all other aerial parts of the plant.

•      The leaves consist of mesophyll cells.

•      This water is supplied to the mesophyll cells of the leaves through xylem bundles which form the network of veins of the leaves.

•      When leaves absorb radiant energy water is converted into water vapour and collect within intercellular spaces.

•      The intercellular space form a connected system extending up to sub stomatal cavity.

•      When stomata remain open water vapours are diffused out of the leaf because its concentration is high in sub-stomatal cavity than the adjoining atmosphere.

•      The vapours are formed in the first place from the thin film of water on outer surface of mesophyll cells of the leaves.

Ø Mechanism of stomatal transpiration involves following steps:

1. Osmotic diffusion of water from xylem to inter cellular spaces through mesophyll cells
2. Opening and closing of stomata
3. Diffusion of water vapors from intercellular space to outer atmosphere through open stomata.

1. Osmotic diffusion of water from xylem to inter cellular spaces through mesophyll cells:

•      Inside the leaf, mesophyll cells are in contact with xylem and on the other hand with inter-cellular spaces above the stomata.

•      When water saturates the cell wall, protoplasm and vacuoles of mesophyll cells by the water supplied by xylem of leaf, then the cells become turgid.

•      Their diffusion pressure deficit and osmotic pressure decrease with the result  that they release water  into  the inter cellular spaces close to stomata by osmotic diffusion.

•      In turn the O.P. and D.P.D of mesophyll cells become higher and hence they draw water from xylem by osmotic diffusion.

2. Opening and closing of stomata:

•      When the water from mesophyll cells reach the microcellular spaces above stomata in form of vapor then stomatal movement or closing and opening of stomata is necessary for transpiration. The chief mechanism involved in stomatal transpiration is the mechanism of stomatal movement.

Mechanism of Stomatal Opening and Closing (Stomatal Movement):

•      The stomatal movements are brought about by changes in the volume and shape of the guard cell of stomata.

•      The expansion and contraction of the guard cells must be due to the turgidity and flaccidity respectively.

Ø  In Dicots:

•      In dicots when the guard cells absorbs water from the surrounding cells and become turgid, the pressure exerted on their walls causes the thinner and weaker outer side to bulge outward.

•      The outward bulging causes the pulling apart of the inner thick walls resulting in the opening of the stoma.

•      When the guard cells lose the tugidity, the thick walls revert to their original position narrowing the stoma and ultimately closing it.

Ø  In Monocots:

•      In monocots when bulbous ends of the guard cells swell up due to high turgor pressure the two ends are pulled in opposite directions resulting the middle portions of the inner wall getting separated from each other which causes the opening of the stoma.

•      When the guard cells lose tugidity, the condition will revert resulting in closing of stoma.

Factors affecting stomatal movement:

1.         Light:

•      Stomata open in the presence of light and close in darkness. Light intensity required to open the stomata is very low, as compared to the intensity required for photosynthesis. In CAM (Crassulacean Acid Metabolism) plants, stomata open during dark and remain closed during the day. Even moonlight is sufficient to keep the stomata open in some plant species.

2.         Temperature:

•      Rise in temperature induces stomatal opening while fall in temperature causes closure. At 38°-40°C, stomata open even in darkness. In some plant species, stomata remain closed even under continuous light at 0°C.

3.          Carbon dioxide:

•      Low CO₂ concentration induces stomatal opening and vice versa. In closed stomata, external CO₂ concentration has no effect.

4.         Oxygen:

•      Essential for stomatal opening.

5.         Water availability:

•      Water stressed (less water available to plant and high transpiration rate) plants induces stomatal closure, lowering of water potential in epidermal cells.

6.         Potassium:

•      Influx of K⁺ causes opening of stomata while efflux of K⁺ from guard cells causes closure of stomata.

7.         Mechanical shock:

•      Causes stomatal closure.

8.         Hormones:

•      Abscisic acid brings about closure of stomata. Cytokinins are required for keeping the stomata open.

Hypothesis (Theories) of stomatal movement:

Ø  There are several hypothesis which has been proposed to explain stomatal movement. They are as follows:

1. Mohl's hypothesis (Photosynthesis hypothesis):

•      Von Mohal (1856) gave the hypothesis that, the chloroplast present in guard cells manufactures substances which increase the osmotic pressure of guard cells. As a result of which endosmosis takes place and that increases the turgidity in guard cells, consequently cause opening of stomata.

•      In high concretions of Co2 around stomata would cause opening of stomatal pore, but the pore closes.

•      On the other hand guard cells have feeble role in photosythesis in compared to mesophyll.

•      Hence the hypothesis was rejected.

2. Starch-sugar hypothesis :

•      This hypothesis was postulated by Lloyd (1908), Loftfields (1921) and sayre (1926). These workers noted that, starch content of guard cells is high during night and low during day time. Sayre further observed that, stomata closes at a pH lower or higher that pH 4.2-4.4.

This hypothesis postulates that:

1. During day time Co2 which released in respiration is utilized in photosynthesis of mesophyll cells. Therefore concentration of Co2 around guard cells and neighboring cells reduced with rise in pH.
2. High pH favors conversion of starch into osmotically active reducing sugars which get soluble in cell sap.
3. In dark Co2 is accumulated in guard cells as photosynthesis stopped. It cause fall in pH of guard cells. At low pH conversion of sugar into starch takes place. Guard cells become flacid and stomata closed.

Steward's Criticisim to Starch-Sugar Hypothesis :

•      Steward (1964) criticized this above starch sugar hypothesis proposed by Lloyed and other and pointed out that, unless glucose 1 - phosphate is further broken down to glucose and inorganic phosphate, no appreciable change occur in the osmotic pressure.

Steward proposed his own scheme, According to which:

i.          At high pH the opening of stomata is caused by conversion of starch into glucose.

ii. The closing of stomata requires metabolic energy (ATP), O2 and the enzyme hexokinase which help in conversion of sugars into starch.

Ø  Starch-sugar hypothesis is also subjected to criticism in following ground:

1. It fails to explain rise of pH on basis of Co2 concentration.
2. Sugar never noticed in cell sap of guard cells during opening of stomata.
3.  Starch sugar interconversion is very slow which does not effect quick stomatal movement.

3. Maltase Hypothesis or K⁺ ion exchange Hypothesis:

•      According to this, under the influence of light malic acid is produced in the guard cells. Malic acid probably comes from starch via the respiratory pathway. However, it is not understood how light triggers the formation of malic acid in guard cells. According to Levit malic acid is produced in the chloroplast from where it is excreted into the cytoplasm of guard cells.

•      In light malic acid, being a weak acid, dissociated into malate ions and H⁺ ions. H⁺  are excreted from the guard cells and K⁺ enter the guard cell from neighboring cells. In other words, exchange of H⁺ ions and K⁺ ions between guard cells and adjacent epidermal cells takes place. This is an active process and involves expenditure of metabolic energy. Accumulation of  K⁺  ions and malate ions in guard cells cause an increase in the osmotic pressure of guard cells. This in turn would attract flow of water into guard cells. This increases turgor pressure which leads opening of stomata

•      In the Dark, H⁺ ions and K⁺ ions exchange pump again operates. But in this case hydrogen ion enter and potassium ion leave the guard cells. Malate and hydrogen ions would combine to form malic acid which is used in metabolic reactions, particularly in respiration.  This leads to decrease in osmotic pressure of guard cells which leads to loss of turgidity and  closing of stomata ooccur.

Ø  Potassium ion exchange mechanism has been reported to operate in all the plants so far tested and appears to be free from criticism.

3. Diffusion of water vapors from intercellular spaces to outer atmosphere through open stomata:

•      When leaves absorb radiant energy water is converted into water vapour and collect within intercellular spaces.

•      The intercellular space form a connected system extending up to sub stomatal cavity.

•      When stomata remain open water vapours are diffused out of the leaf because its concentration is high in sub-stomatal cavity than the adjoining atmosphere.

•      The vapours are formed in the first place from the thin film of water on outer surface of mesophyll cells of the leaves.