Question

According to modern theory of stomatal opening, proposed by Levitt, pH of guard cell can rise due to –
A. CO2 assimilation by guard cells only
B. Active efflux of H+ ions by guard cells
C. Conversion of starch into PEP
D. Active uptake of K+ in guard cells

Answer 1

Hint: The concentration of CO2 varies according to time of the day. And the difference in its concentration creates osmotic pressure across guard cells.

Step by step answer:pH is defined as a numeric value which indicates the acidity or alkalinity of a given substance. Guard cells are the cells that are present adjacent to stomata and regulate their opening and closing. The proton transport theory given by Levitt suggests that, when K⁺ ions enter into the guard cells from the surrounding medium, the osmotic pressure inside these cells increases and thus water enters into the cells resulting in the opening of the stomata. This process happens during day time when the CO2 concentration is less as it is constantly utilized for the process of photosynthesis. But during night time, the concentration of CO2 increases as plants cannot perform photosynthesis during the night. A specific hormone gets activated due to the presence of CO2 and inhibits the uptake of K⁺ ions, decreasing the osmotic pressure and water leaves the guard cell and due to their shrinkage, stomata closes. pH rises in presence of a higher concentration of H⁺ ions. The change in the concentration of H⁺ ions depends upon the concentration of CO2.
Hence, the correct answer is option A. CO2 assimilation by guard cells only.
Additional information: The opening and closing of stomatal cells determine the rate of transpiration. In the atmosphere where the humidity and water content in the atmosphere is less, the stomatal opening is inhibited to avoid loss of water due to transpiration.

Note: The proposed theory deals with movement of H⁺ and K⁺ ions across guard cells. An increase in H⁺ ion increases the pH of guard cells.