- The following four points will highlight the four important theories of stomatal movement.
- The four important theories of stomatal movement are: (1) Theory of Photosynthesis in Guard Cells (2) Starch Sugar Inter – Conversion Theory (3) Theory of Glycolate Metabolism and (4) Active K + Transport or Potassium Pump Theory and Role of Abscisic Acid or Active Potassium Pump Theory.
- Many theories have been proposed regarding opening and closing of stomata. The important theories of stomatal movement are as follows:
(1) Theory of photosynthesis in guard cell.
(2) Starch Sugar inter – conversion theory.
(3) Theory of glycolate metabolism
(4) Active potassium transport ion concept
Theory 1. Theory of Photosynthesis in Guard Cells:
- Von MohI (1856) observe that stomata open in light and close in the night. He then proposed that chloroplasts present in the guard cells photosynthesize in the presence oflight resulting in the production of carbohydrate due to which osmotic pressure of guard cells increases,
- Its explanation is based on the following sequence:
- Formation of sugar Increase ofosmotic pressure Light – Photosynthesisin guard cells sapEndosmosis takes place from subsidiary. cell to guard cell Increase of TP in district guard cells – Stomata open.
- Increasing the CO². concentration around the leaves should lead to wide opening or stomata but here occurs their purtial closure.
- Chloroplast of guard cells are poorly developed and incapable of performing active photosynthesis.
Theory 2. Starch Sugar Inter – Conversion Theory:
- A ccording to Lloyd (1908) torgidity of guard cell depends on inter – conversion of starch and sugar. It was supported by Loft – field (1921). He found out that guard cells contain sugar during day time when they are open and starch during night when they are closed.
- Sayre (1926) observed that stomala open in neutral oralkalincph, which pretails duringday time due to constant removal of carbon – dioxide by photosynthesis, stomata remain closes during night when there is no photosynthesis and due to accumulation of carbon – dioxide, carbonic acid is formmed that causes thephto be acidic. Thus, stomatal movementisregulated by pH due to inter – conversion of starch and sugar. Sayre concept was supported by Scarth (1932) and Smallet. al. 1942),
- Yin and Tung (1948) isolated for the first time phosphorylase enzyme from the guard cellsi according to them starchi is converted into glucose – l, phosphate in the presence of this cnayme. During the process, inorganic phosphate is also used and light and dark phases as follows: (changing. Co. Concentration) control the changes in pH. The reaction may be represented as follows.
- Steward’s scheme: Steward (1964) proposed another modified scheme of inter – conversion ofstarch and sugar for stomatalmovement. He believes that conversion of starch toGlucose – Johosphdre is not sufficient. It should be converted toglucose in order to increase sufficient osmotic pressure. For this, ATP is also required which means that the process should be through respiration in presence of oxygen. Guard cell carries enzymes like Phosphorylase, closing of the stomata. Phosphoglucomutase, Phosphatase and Phosphorylase. These enzymeshelp in opening and summarized as given below.
- Based on the above mentioned theory. process of opening and closing of stomata may below.
- Photosynthesis – Decreased co, Concentration in leaf cells (2) – Increase in phof guard celbo) Hydrolysis of starch to sugar by enzymes (4) Increase ofOP. of guard cells (s) Endosmosis of water in guard cells (6) → Increase in TRofguard cells (7) -> Aperture opens.
Demerits of the starch – sugar inter – conversion theory:
- Many scientists do not agree with the theory of starch – sugar ioter – conversion due to the following reasons.
- In the presence of light when starch disappears from guard cells, malic acid appears and not the sugars.
- Starch has not been reported in the goard cells of many monocots such as fris, Amatyllis, Allium.
- According to this theory O. P. of guard cells increases due to the formation of glucose – 1 phosphate in guard cells but it is found that the presence ofphosphate ions causes the development of the same OPas does the presence of glucose – phosphate.
- Enzyme phosphorylase helps in conversion of starch toglucose – 1 – phosphate but not in the formation of starch from glucose – 1 – phosphate. This reaction is controlled by some other enzyme about which we do not know as yel.
- The theory could not explain the extra effectiveness of blue light at the time of stomatal opening.
Theory 3. Theory of Glycolate Metabolism:
- Zelitch (1963) proposed that production of glycolic acid in the guard cells is an important factor in stomital opening. Glycolate is produced under low concentration ofco2. He suggested that glycolate gives rise to carbohydrate. Thus raising the osmotic pressure and also that it could participate in the production of ATP which might provide energy required for the opening of stomata.
(I) It fails to explain the opening of slomata iir dark (e. G. – in succulent plants).
(ii) In some plants slomata have been found to remain closed even during daytime.
(iii) In fails to explain the effect of blue light on stomatal opening
Theory 4. Active K+ Transport or Potassium Pump Theory and Role of Abscisic Acid: Or Active Potassium Pump Theory:
(Active K+ ion Transport (K+ ion Pump) Mechanism)
- The concept of Kion transport (K “ion pump) mechanism is the concept of Kion transport was given by Fujino. It was supported and elaborated by Levit &. Rashke in 1975 tappears to be an active mechanism which needs ATP. It is based on recent observations and (explains the mechanism as follows.
- Opening of stomata during daytime (in presence of light):
Opening of stomata depends upon following conditions:
- Presence of light.
- Decrease in starch contents of guard cells.
- Increased concentration of malic acid in guard cells.
- Influx of K+ ions in guard cells,
- Efflux of H+ ions from guard cells.
- Intake of CL ions by guard cells.
- Low CO², concentration in an around guard cells.
- High pH (more than 7) in guard cells (hence. alkaline medium of the cell sap in guard cells)
- High T. P. in guard cells due to endosmosis, (turgidity of cells),
- TP more towards thin wall of guard cell & stomala open.
Explanation of Levitt Concept:
- This is explained as follows:
- In the guard cells, starch is converted into malic acid in presence of light (during day time).
- Protons (Hythus formed are used by the guard cells for the uptake ofK – ions (in exchange for the protonsi). Thisisan active ionic exchange and requires ATPenergy and cytokinin (aplant hormone). In this way, the concentration ofKrions increases in guard cells. At the same time, the concentration of Hrions decreases in guard cells. The pH of the cell sap in guard cells also increases simultaneously (pH becomes more than Tand the medium becomesalkaline),
- There is also an increased uptake of CL (anions) by the guard cellstomaintain the electrical and ionic balance inside and outside the guard cells. The malate anions formed in the guard cells are neutralized by the Kions. This results in the formation ofpotassium malate.
- Malate anions + K+ Potassium malate:
- Potassium malate enters the cellsap of the guard cells thereby reducing the water potential while increasing the osmotic concentration (and the O. P) of the cell ap. Hence. endosmosis occurs. guard cells become turgid and kidney – shaped and the stomata opens.
- It is also observed that the co, concentration is low in and around guard ceils during day time. This issue to high photosynthetic utilization of co. It helps in opening of stomata,
- Closing of stomata in Absence of Light (Darkness / Night Time): Closing of stomata depends on the following conditions:
- Absence of light.
- Decreased concentration of malic acid in guard cells.
- Efflux of K+ ions from guard cells,
- Influx of H+ ions in guard cells.
- Acidic medium of the cell sap in guard cells.
- Loss ofcr ions from guard cells,
- Increases CO², concentration in and around guard cell due to release of CO, in respiration combined with the absence ofphotosyntheticactivity in dark.
- Presence of plant growth inhibiting hormone abscissic acid (ABA),
- Loss of turgidity and loss of kidney – shape by guard cells. All these conditions represent the reversal of the daytime events. Under these conditions, the guard cells lose water by exosmosis and become flaccid. This causes closing of the stomata.
Role of Plant Hormones in Stomatal Movements:
- Presence of Cytokinin (Plant growth regulator) is needed for the active uptake of K+ions.
- Presence of ABA (abscissic acid, a plant growth inhibiting hormone) favors closing of stomata by blocking uptakeofK byguard cells in the dark. It also prevents efflux of Hions from guard cells. ABA and Co, cone, together help in lowering the pH in guard cells and making the medium acidic. This helps in closing of stomata. ABA act as stress hormone during drought condition.