Sayre's theory in terms of Turgidity and Flaccidity of Guard Cells - QS Study
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Sayre’s theory:

This theory was proposed by Sayre (1926). First of all Lloyd stated that amount of sugar in GC increases during the day time & starch in night. According to him, the theory stated above is correct but, the inter change between sugar and starch takes place due to the fluctuation of the PH of the cell sap.

Photosynthesis remains stopped in the night time but the respiration goes on continuously. As a result, CO2 produced during the respiration forms carbonic acid (H2CO3) which reduces the PH of guard cells (PH7 to PH5). Thus insoluble starch is produced from glucose-1 phosphate (sugar). Sugar changes into starch during night at low pH in guard cells (Supported by Scarth).

During the day time CO2 is utilized for photosynthesis and thus PH rises (alkaline). Sayre 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.

During the darkness, photosynthesis is stopped and the concentration of CO2 rises as a result of respiration and thus PH decreases (alkaline).