enzymatic theory and ionic theory of stomatal opening explain
Answers
Answer:
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.
Starch Sugar Interconversion Part 1
(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).
ADVERTISEMENTS:
(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:
Starch Sugar Interconversion Part 2
(iv) Steward’s scheme:
ADVERTISEMENTS:
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.
Starch Sugar Interconversion Part 3
Based on the above mentioned theory, process of opening and closing of stomata may be summarized as given below.
In Light:
ADVERTISEMENTS:
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
Mechanism of Opening and Closing of Stomata
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.
Explanation:
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Answer....✌
The closing of stomata: As the stomata open the solute concentration is reduced.
This makes the water from the guard cells to move away into neighbouring cells.
Now, guard cells become flaccid with no water.
They collapse against each other and result in the closing of stomata..
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