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 Mohl (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 of light resulting in the production of carbohydrate due to which osmotic pressure of guard cells increases.
Its explanation is based on following sequence:
Light → Photosynthesis in guard cells → Formation of sugar Increase of osmotic pressure of cell sap → Endosmosis takes place from subsidiary cell to guard cell → Increase of TP in guard cells → Stomata open.
Demerits:
1. Increasing the CO2 concentration around the leaves should lead to wide opening of stomata but here occurs their partial closure.
2. Chloroplast of guard cells are poorly developed and incapable of performing active photosynthesis.
Theory # 2. Starch Sugar Inter-conversion Theory:
(i) According to Lloyd (1908) turgidity 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.
(ii) Sayre (1926) observed that stomata open in neutral or alkaline pH, which prevails during day time due to constant removal of carbon-dioxide by photosynthesis. Stomata remain closed during night when there is no photosynthesis and due to accumulation of carbon-dioxide, carbonic acid is formed that causes the pH to be acidic. Thus, stomatal movement is regulated by pH due to inter-conversion of starch and sugar. Sayre concept was supported by Scarth (1932) and Small et. al. (1942).
(iii) Yin and Tung (1948) isolated for the first time phosphorylase enzyme from the guard cells. According to them starch is converted into glucose-1, phosphate in the presence of this enzyme. During the process, inorganic phosphate is also used and light and dark phases (changing CO2 concentration) control the changes in pH. The reaction maybe represented as follows:
(iv) Steward’s scheme:
Steward (1964) proposed another modified scheme of inter-conversion of starch and sugar for stomatal movement. He believes that conversion of starch to Glucose -1 phosphate is not sufficient. It should be converted to glucose 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, Phosphoglucomutase, Phosphatase and Phosphorylase. These enzymes help in opening and closing of the stomata.
Based on the above mentioned theory, process of opening and closing of stomata may be summarized as given below.
In Light:
Photosynthesis (1) →Decreased CO2Concentration in leaf cells (2) →Increase in pH of guard cells(3) → Hydrolysis of starch to sugar by enzymes (4) → Increase of O.P. of guard cells(5) → Endosmosis of water in guard cells (6) →Increase in T.R of guard cells (7) →Aperture opens (Fig. 4.6)
Demerits of the starch-sugar inter-conversion theory:
Many scientists do not agree with the theory of starch-sugar inter-conversion due to the following reasons.
1. In the presence of light when starch disappears from guard cells, malic acid appears and not the sugars.
2. Starch has not been reported in the guard cells of many monocots such as Iris, Amatyllis, Allium.
3. 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 of phosphate ions causes the development of same O.P as does the presence of glucose-phosphate.
4. Enzyme phosphorylase helps in conversion of starch to glucose-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 yet.
5. 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 stomatal opening. Glycolate is produced under low concentration of CO2. 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.
Demerits:
1. It fails to explain the opening of slomata in dark (e.g., – in succulent plants).
2. In some plants slomata have been found to remain closed even during daytime.
3. It 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 K+ ion transport was given by Fujino. It was supported and elaborated by Levitt & Rashke in 1975 It appears to be an active mechanism which needs ATP. It is based on recent observations and (explains the mechanism as follows.
A. Opening of Stomata during Daytime (in presence of light):
Opening of stomata depends upon following conditions:
(a) Presence of light.
(b) Decrease in starch contents of guard cells.
(c) Increased concentration of malic acid in guard cells.
(d) Influx of K+ ions in guard cells.
(e) Efflux of H+ ions from guard cells.
(f) Intake of CI ions by guard cells.
(g) Low CO2 concentration in an around guard cells.
(h) High pH (more than 7) in guard cells (hence, alkaline medium of the cell sap in guard cells).
(i) High T.P. in guard cells due to endosmosis, (turgidity of cells).
(j) TP more towards thin wall of guard cell & stomata 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 (H+) thus formed are used by the guard cells for the uptake of K+ ions (in exchange for the protons H+). This is an active ionic exchange and requires ATP energy and cytokinin (a plant hormone). In this way, the concentration of K+ ions increases in guard cells. At the same time, the concentration of H+ ions decreases in guard cells. The pH of the cell sap in guard cells also increases simultaneously (pH becomes more than 7 and the medium becomes alkaline).
There is also an increased uptake of CI” (anions) by the guard cells to maintain the electrical and ionic balance inside and outside the guard cells. The malate anions formed in the guard cells are neutralized by the K+ ions. This results in the formation of potassium malate.
Malate anions + K+ →Potassium malate:
Potassium malate enters the cell sap of the guard cells thereby reducing the water potential while increasing the osmotic concentration (and the O.P.) of the cell sap. Hence, endosmosis occurs, guard cells become turgid and kidney-shaped and the stomata opens.
It is also observed that the CO2 concentration is low in and around guard ceils during day time. This is due to high photosynthetic utilization of CO2. It helps in opening of stomata.
B. Closing of Stomata in Absence of Light (Darkness/Night Time):
Closing of stomata depends on following conditions:
(a) Absence of light.
(b) Decreased concentration of malic acid in guard cells.
(c) Efflux of K+ ions from guard cells.
(d) Influx of H+ ions in guard cells.
(e) Acidic medium of the cell sap in guard cells.
(f) Loss of Cl– ions from guard cells.
(g) Increases CO2 concentration in and around guard cell due to release of CO2 in respiration combined with the absence of photosynthetic activity in dark.
(h) Presence of plant growth inhibiting hormone abscissic acid (ABA),
(i) 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:
(i) Presence of Cytokinin (Plant growth regulator) is needed for the active uptake of K+ ions
(ii) Presence of ABA (abscissic acid, a plant growth inhibiting hormone) favours closing of stomata by blocking uptake of K+ by guard cells in the dark. It also prevents efflux of H+ ions from guard cells. ABA and CO2 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.