Question

Select the correct events leading to the opening of the stomata.
(A) Decline in guard cell solutes
(B) Lowering of osmotic potential of guard cells
(C) Rise in potassium levels in guard cells
(D) Movement of water from neighbouring cells into guard cells
(E) Guard cells becoming flaccid

• A. A and E only
• B. B , C and D only
• C. A , C and D only
• D. B , D and E only

Answer 1

Answer: (B)

$pH$ of the guard cell rises due to active $H^{+}$ uptake by guard cell chloroplast or mitochondria. A rise in pH causes hydrolysis of starch to form malic acid. Malic acid dissociates into $H^{+}$ and malate. $H^{+}$ ions pass out of the guard cells in exchange for $K^{+}$ ions. This lowers the osmotic potential of guard cells. As a result movement of water begins from neighbouring cells into guard cells which leads to opening of stomata.