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Question

How does guard cells help in opening of stomata?

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Solution

The most accepted theory of regulation of stomata by guard cells is ‎K+ ion pump theory which was proposed by Levitt in 1974. According to this theory, there is accumulation of ‎K+ ions in the guard cell during the day time.
During the day time, a rise in pH causes hydrolysis of starch and an organic acid phosphoenol pyruvate is formed which combines with available CO2 to produce malic acid. The malic acids dissociate to produce H+ and malate. The H ions are pumped out of guard cells and in exchange ‎K+ ions are pumped into the guard cells from the adjacent subsidiary cells. This is followed by entry of Cl ions into the guard cells. This increases the osmotic concentration of guard cells and water is absorbed by the guard cells from the adjacent epidermal or subsidiary cells. As a result, guard cells swell up and this leads to opening of stomata.

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