Stomatal Movement: Opening and Closing of Stomata, Mechanism, Antitranspirants, and Guttation
Opening and Closing of Stomata
The distinctive shape of the stomata makes them distinct from the nearby epidermal cells. The epidermal cells that surround the stomata nearby can resemble other epidermal cells or they can be unique and specialised. They are referred to as Subsidiary cells. The guard cells differ from ordinary epidermal cells in that they have chloroplasts and a unique thickening on either their surface or the surface of an adjacent cell (in closed stomata). Following an increase in the guard cells’ osmotic pressure (OP) and diffusion pressure deficit (DPD) (caused by an accumulation of osmotically active chemicals), water is osmotically diffused into the guard cells from the mesophyll and epidermal cells around them. The guard cells’ turgor pressure (TP) rises, as a result, making them turgid. The guard cells enlarge, lengthen, and starch develops pores on their adjacent swollen surfaces, allowing the Stomata to Open.
On the other hand, water is released back into the surrounding epidermal and mesophyll cells by osmotic diffusion and the guard cells become flaccid when OP and DPD of guard cells decrease relative to those of the surrounding epidermal and mesophyll cells (due to depletion of osmotically active substances). The stomatal pore and Stomata are Closed when the thicker guard cell surfaces touch one another.
When guard cells’ water potential falls (i.e. becomes more negative) than that of the surrounding epidermal and mesophyll cells, osmotic diffusion of water into those cells occurs. When the guard cells’ osmotic pressure falls in comparison to the cells around them (water moves from a zone of higher water potential to a region of lower water potential), the guard cells become flaccid.
Stomatal Transpiration Mechanism
Three stages can be taken to study the process of Stomatal transpiration.
i. Water moves osmotically via the mesophyll cells in the leaf from the xylem to the intercellular space above the stomata.
ii. Stomata opening and closing (stomatal movement)
iii. Straightforward stomatal diffusion of water vapour from intercellular gaps to another atmosphere.
The mesophyll cells within the leaf are in contact with the xylem, but they are also in contact with the intercellular space above the stomata. Mesophyll cells become turgid as they draw water from the xylem, and as a result, their osmotic pressure (OP) and diffusion pressure deficit (DPD) both drop. This causes them to release water in the form of vapour in intercellular gaps adjacent to stomata by osmotic diffusion. Now that the O.P. and D.P.D. of the mesophyll cells have increased, they can extract water from the xylem through osmotic diffusion.
The Agents or Mechanisms that regulate stomatal movements could vary.
i) Starch hydrolysis into sugars in guard cells
ii) Synthesis of organic acids or sugars in them
iii) The Guard’s active pumping of K+ ions
1. Guard cells starch hydrolysis into sugars
Theory of interconversion of starch and sugar
This traditional approach is founded on the idea that the enzyme starch phosphorylase, which catalyses the reversible conversion of starch + inorganic phosphate into glucose -1 phosphate, is affected by pH.
Daytime pH is high in the guard cells. This encourages the breakdown of insoluble starch into soluble glucose phosphate, which raises osmotic pressure in the guard cells. As a result, water diffuses into the guard cells from the nearby epidermal and mesophyll cells through osmotic diffusion. Stomata open and guard cells develop a turgid appearance.
In the dark, the opposite happens. In the guard cells, glucose 1-phosphate is changed back into starch, lowering osmotic pressure. The guard cell exudes water, wilts, and closes its stomata.
Steward (91964) claims that because inorganic phosphate and glucose-1-phosphate are both equally active osmotically, the conversion of starch and inorganic phosphate into glucose-1-phosphate does not significantly alter the osmotic pressure. For the opening of stomata, he has proposed in this method that glucose-1-phosphate be further transformed into glucose and inorganic phosphate. The shutting of stomata would require metabolic energy, which most likely originates from breathing in the form of ATP.
2. In Guard cells, sugars or organic acids are synthesized.
Guard cells undergo photosynthesis during the day because they have chloroplasts. The formation of soluble carbohydrates during this process may help guard cells’ osmotic potential rise, leading to stomatal opening. The guard cells have, however, yielded very little quantities of soluble sugars (osmotically active), inadequate to alter water potential. The reduction in CO2 concentration in guard cells brought on by photosynthesis causes an increase in the pH of organic acids, primarily malic acid, during this time. The malic acid formation would result in the production of protons that could function in an ATP-driven proton K+ exchange pump, moving protons into neighbouring epidermal cells and K ions into guard cells. This could help to raise the osmotic pressure in the guard cells, which would then cause the stomata to open. In the dark, a reverse procedure would take place.
3. Guard cells have a proton (H+)-K exchange pump that is powered by ATP.
This process states that during the daylight hours, K+ ions build up in the guard cells. The guard cells “pump out” protons (H+) into the neighbouring epidermal cells in exchange for K+ ions, which is an active mechanism regulated by ATP. The guard cells’ non-cyclic photophosphorylation process during photosynthesis produces ATP. Respiration is one more possible source of the ATP needed for the ion exchange process. During the day, guard cells’ water potential is dramatically reduced due to the accumulation of K ions.
As a result, water from the nearby epidermal and mesophyll cells enters them, raising their turgor pressure and allowing the stomatal pore to open. When it is dark and the stomata are closed, the situation is reversed. In dark conditions, there is no buildup of “K” in g cells.
(iii) The straightforward diffusion of water vapour from the intercellular spaces to the atmosphere through open stomata is the final stage in the mechanism of transpiration. This is because close to stomata, the intercellular spaces are moister than the surrounding outside environment.
Antitranspirants
There are several known chemicals that, when given to plants, slow down their transpiration. Antitranspirants are the name given to such compounds. Colourless polymers, silicone, oils, low viscosity waxes, phenylmercuric acetate, abscisic acid, CO2, and other substances are examples of antitranspirants. One group includes colourless polymers, silicone oils, and low viscosity waxes because they are sprayed on leaves to create a layer that is permeable to oxygen and carbon dioxide but not to water. Phenyl mercuric acetate, a fungicide, had a very little harmful effect on leaves when treated at low concentrations (10-4 m) and caused partial stomatal pore closing for two weeks. The plant hormone ABA also causes stomatal closure. CO2 works well as an antiperspirant. Stomata are partially closed by a slight increase in CO2 concentration from the natural 0.03% to 0.05%. Since its higher concentration cannot be utilised, the stomata completely close, negatively impacting both photosynthesis and respiration.
Guttation
Water droplets flow out from the unharmed margins of the leaves where the main vein stops in some plants, including garden nasturtium, tomato, colocasia, etc. This process, known as guttation, typically occurs in the morning when the rate of absorption and root pressure is high and the amount of transpiration is minimal. The occurrence of particular forms of stomata at the leaf margins, known as water stomata or hydathodes, is linked to the guttation phenomena. Each hydathode has a water pore inside of it that is always open. A tiny hollow and loose tissue known as the epithem are located underneath this. This epithem is closely related to the terminals of the vein’s vascular components. The xylem of the veins supplies water to the epithem when there is strong root pressure. Water is released into the cavity from the epithem. The latter starts to leak out as watery droplets through the water hole after this cavity is filled with watery solution.
Frequently Asked Questions
Question: What causes the opening and closing of stomata?
Ans: The guard cells enlarge, lengthen, and starch develops pores on their adjacent swollen surfaces, allowing the Stomata to Open. water is released back into the surrounding epidermal and mesophyll cells by osmotic diffusion and the guard cells become flaccid when the OP and DPD of guard cells decrease relative to those of the surrounding epidermal and mesophyll cells. The stomatal pore and Stomata are Closed
Question: What is the difference between open stomata and closed stomata?
Ans: Stomata open to let oxygen out and take up carbon dioxide for photosynthesis. To stop water from evaporating, the stomata close.
Question: Why is stomata closed at night?
Ans: To reduce transpiration and conserve water, stomata are closed at night.
Question: Why do stomata open during the daytime?
Ans: Stomata are open during the day To perform photosynthesis.
Question: Are stomata closed at night?
Ans: Yes
Question: Which plants keep their stomata open at night?
Ans: Opuntia, cactus,
Question: Why do stomata open?
Ans: To facilitate the effective gas exchange of carbon dioxide and oxygen for photosynthesis, stomata must open.