Proton-Potassium Pump Hypothesis:

Levitt in the year 1974 combined the points in Steward’s and Scarth’s hypothesis and provide a modified version of the mechanism of stomatal movement that was termed as the proton- potassium pump hypothesis.

According to this theory K⁺ ions are transported into the guard cells in the existence of light. The series of events occurring are:

a. Under the influence of light, protons build up by dissociation of malic acid move from cytoplasm to the chloroplasts of guard cells.

b. To counter the exit of protons, K⁺ ions enter the guard cells from the nearby mesophyll cells.

c. K⁺ ions react with the malate ions existed in the guard cells to form potassium malate.

d. Potassium malate causes rise in the osmotic potential of guard cells causing entry of water into the guard cells as an outcome of which the stoma opens.

e. At night the dissociation of potassium malate occurs and K⁺ ions exit out of guard cells causing loss of water from guard cells and therefore the stoma closes.

Fritz and Noggle in 1976 supported this theory and provide a scheme for opening of stomata.

This hypothesis is the widely accepted one as Levitt was capable to explain increase in K⁺ ion level during the day and the formation of organic acids such as malic acid with the unused CO₂ existed in the guard cells.

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