Question

How does guard cells help in opening of stomata?

Answer 1

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.