what are the two theories about the opening and closing of stomata?
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1. Theory of photosynthesis in guard cell
2. Starch Sugar inter-conversion theory
2. Starch Sugar inter-conversion theory
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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).
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).
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