Question

Describe the Proton - potassium pump hypothesis.

Answer 1

A mechanism of stomatal movement that was known as the proton-potassium pump hypothesis, given by Levitt in the year $1974$ by modifying the points in Steward’s and Scarth’s hypothesis. According to this theory $K^{+}$ ions are transported into the guard cells in the presence of light. The series of events occurring are given as follows-

$i)$ In the presence of light, protons formed by dissociation of malic acid move from cytoplasm to the chloroplasts of guard cells.

$ii)$ To balance the exit of protons, $K^{+}$ ions enter the guard cells from the nearby mesophyll cells.

$iii)$ $K^{+}$ ions react with the malate ions present in the guard cells and form potassium malate.

$iv)$ Potassium malate causes rise in the osmotic potential of guard cells causing endosmosis of water into the guard cells as an outcome of which the stoma opens.

$v)$ At night the dissociation of potassium malate occurs and $K^{+}$ ions exit out of guard cells causing exosmosis of water from guard cells and therefore the stoma closes.