Starch-Sugar hypothesis has been proposed to explain the mechanism of stomatal transpiration. Most of the transpiration takes place through stomata. Stomata are usually concerned in transpiration as its guard cells exhibit closing and opening movement.
During stomatal transpiration water from xylem of leaf diffuse osmotically to the intercellular space above stomata through the mesophyll when stomata open water vapors come out. Thus true mechanism of this transpiration. Chiefly involve closing and opening of stomata which explain by sugar hypothesis. It explained as follows:
i. Guard cells are specially built in which inner walls are thicker and stronger where as outer wall is much thinner and elastic. These cells are of high chlorophyll content than the surrounding epidermal cells.
ii. During day time in presence of light guard cells synthesize soluble sugar which accumulated in form of insoluble starch in night time.
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iii. This lower the osmotic pressure of the guard cell and their DPD is also decreased with result that water released from the guard cell into the surrounding mesophyll tissue.
iv. Due to this guard cells remain flaccid. As their inner wall is thicker, come close to each other so as to the stomata remain closed to cease transpiration.
Conversion of sugar into starch favored by low temperature and lesser hydrogen ion concentration. Low pH is due to accumulation of CO2 in night.
v. During day time in pressure of light the accumulated insoluble starch in the guradcells converted into soluble sugar in presence of enzyme phosphorylase;
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Soluble sugar increase the O.P of guard cell, resulting an increased D.P.D. Due to this guard cells draw water from the surrounding mesophyll cells by osmotic diffusion and become turgid.
vi. As a result of turgidity the outer wall of guradcells expands outwardly while inner thinker wall is curved inwardly thus the stomata remain opened.
Steward (1964) proposed that conversion of starch and inorganic phosphate into glucose -1 – phosphate does not cause appreciable change in the osmotic pressure as both compounds are equally active in relation to osmosis. Steward further suggested that:
i. Glucose -1- phosphate converted into glucose and inorganic phosphate for the opening of stomata.
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ii. Metabolic energy in form of ATP would be required for closing of stomata which met by respiration.