Question

According to $H^{+}-K^{+}$ exchange pump mechanism of stomatal movement
I. $K^{+}$ ions accumulate in guard cells during the day.
II. $H^{+}$ ions are pumped out from guard cells and $K^{+}$ ions are pumped into guard cells.
III. Water potential of guard cells decreases with the accumulation of $K^{+}$ ions together with $C{l}^{-}$ and organic acid ions.
IV. Water enters into guard cells from epidermal and mesophyll cells due to decreased water potential of guard cells.

• A. I and II are correct
• B. I, II and IV are correct
• C. II, III and IV are correct
• D. All are correct

Answer 1

Answer: (D)

All are correct

H${}^{+}$−K${}^{+}$ exchange pump mechanism was proposed by Levitt. According to this mechanism, during day, starch is converted to phosphoenol pyruvic acid which combines with CO and form oxaloacteic acid (OAA). OAA is then converted to malate. H${}^{+}$ ions are pumped out from guard cells and K${}^{+}$ ion are pumped in and accumulate inside guard cells along with malate. This is called ion exchange. Water potential of guard cells decreases with the accumulation of K${}^{+}$ ions which is balanced by taking in some Cl${}^{-}$ ions. This ion exchange occurs using ATP as energy. Increased K${}^{+}$ concentration results in decreased water potential of guard cells which results in absorption of water from epidermal and mesophyll cells. This in turn increases turgor pressure of guard cells which leads to the opening of stomatal pores.

Thus, the correct answer is option D.