Active K transport theory:

Experimental evidences show that stomata also open even in C0o free air and in absence of light by increasing the K+ concentration in the guard cells. Imamura and Fujiro (1959) observed the accumulation of K” in guard cells during stomata opening this theory is also called potassium pump theory It is an active process.

The important steps leading to the opening of stomata in light conditions are given below:

1. In light condition starch of the guard cells is converted to phosphoenol pyruvate (PEP) which is later oenverted to malic acid by an enzyme phosphoenolpyruvickinase.


2. Malic acid dissociates into

3. Due to increase in H + concentration in the guard cells, FT ions diffuse into the epidermal and accessory cells in exchange of K~ through the agency of hydrogen potassium ion exchange pump of the plasmamembrance.

4. During this inflow of KT into guard cells some amount of CI” ions are also transported to neutralize excess of KT absorbed.

This exchange involves use of ATP.


Increase and accumulation of Kr in the guard cells increase the osmotic concentration of guard cell so water from neighbouring cells enters into the guard cells by osmotic diffusion.

The guard cells become turgid and stomata open.