malate or K+ ion pump hypothesis
This is the most accepted modern theory. It is also known as hormonal regulation theory (due to ABA secretion).
This theory may be represented as follows:
In light
First of all, the starch is converted into organic acids particularly phosphoenol pyruvic acid (PEP). Phosphoenol pyruvic
acid then combines with CO 2 in the presence of PEP carboxylase enzyme to produce oxalo acetic acid (OAA) and then
malic acid.
The organic acids, viz., malic acid, dissociate into malate anion and H+ in the guard cells. H+ are transported to epidermal
cells and K+ are taken into the guard cells in exchange of H+. The process is called ion exchange. K+ are balanced by
organic anions (i.e., malate). Some Cl– ions are also taken in to neutralize a small percentage of K+.
H+– K+ exchange is an active process which requires involvement of energy (ATP) supplied either by respiration or
photophosphorylation. Increased concentration of K+ and malate ions in the vacuole of guard cells causes sufficient
osmotic pressure to absorb water from surrounding cells. Increased turgor of guard cells due to entry of water causes
stomatal pore to open.
In dark
Higher concentration of CO 2 in sub-stomatal cavity prevents proton gradient across protoplasmic membrane in guard
cells (Sharpe and Zeiger, 1981). As a result, active transport of K+ into guard cells ceases.
Cowan et al. (1982) proposed that closure mechanism involves participation of an inhibitor hormone-abscisic acid, which
functions at lower pH. As soon as the pH of guard cells decreases, the abscisic acid inhibits K+ uptake by changing the
diffusion and permeability of the guard cells. Malate ions present in the guard cell cytoplasm combine with H+ to produce