Question

What is active potassium transfer theory?

Answer 1

Active potassium transfer theory was observed by Fujino (1967) that opening of stomata occurs due to the influx of $K^{+}$ ions into the guard cells. The source of $K^{+}$ ions are the neighbouring subsidiary and epidermal cells, thereby increasing the concentration from 50mM to 300mM in guard cells.
It has been proved that the accumulation of $K^{+}$ ions bring the opening of stomata and loss of $K^{+}$ ions, the closing of stomata.
During day light
$\downarrow$
Accumulation of $K^{+}$ ions by the guard cells
$\downarrow$
Increased solute concentration
$\downarrow$
Decreased water potential
$\downarrow$
Endosmosis of water
$\downarrow$
Increased turgidity
$\downarrow$
Stoma open

During night/dark
$\downarrow$
Loss of $K^{+}$ ions by the guard cells
$\downarrow$
Decreased solute concentration
$\downarrow$
Increased water potential
$\downarrow$
Exosmosis of water
$\downarrow$
Decreased turgidity
$\downarrow$
Stoma close

The increase in K ion concentration increases the osmotic concentration of guard cells thus leading to stomatal opening. ATP helps in entry of Kions into the guard cells.

Role of potassium, chloride and malate ions in stomatal opening (PEPcase = Phosphoenol pyruvate carboxylase)

Levitt (1974) observed that proton \((H^{+})\) uptake by guard cells, chloroplasts takes place with the help of ATP. This leads to increase in the value of pH in guard cells. A rise in pH converts starch into organic acid like malic acid.

$Starch\to Phosphoglycericacid\stackrel{carboxylase}{\to }\stackrel{Phosphoenolpyruvicacid}{\stackrel{\downarrow }{Malicacid\leftarrow Oxaloaceticacid}}+C{O}_2$
The uptake of $K^{+}$ ions is balanced by:
1) Uptake of chloride (Cl) ions
2) Transport of H ions released from organic acid (malic acid)
3) By negative charges of organic acids when they lose H ions + Thus all these factors lead to the opening of stomata. The stomata closure is due to excretion of K ions from guard cells surrounding epidermal and subsidiary cells + The stomatal closure is considered to be brought about by a passive or highly catalysed excretion of $K^{+}$ ions and $C{l}^{-}$ ions from the guard cells to the epidermal tissue in general and the subsidiary cells in particular. It is believed that subsidiary cells have an active reabsorption mechanism of $K^{+}$ ions.