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Steward put forward the starch-sugar interconversion theory; according to this theory during the day time the enzyme phosphorylase convert the starch to sugar. This creates a decrease in the osmotic potential in the guard cell resulting in the entry of water; the guard cells become turgid so that the stomata open. The inter conversion takes place in the night which results in the closing of stomata. The glucose is converted back to the starch which increases the osmotic potential so that the water is released; the guard cell becomes flaccid and the stomata get closed. 

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