POTASSIUM ION PUMP THEORY OR PROTON TRANSPORT HYPOTHESIS
ROLE OF K+ IN STOMATAL MOVEMENT:
- Imamura and M. Fujino (1959) found a direct correlation between stomatal movement and K+ ion concentration of guard cells. Fujino proposed that stomatal opening and closing are a result of an active transport of K+ ion into the guard cells. They showed the accumulation of K+ in the guard cells during stomatal opening. By using the electron probe micro analyzer, it has been found that opening of stomata in light is accompanied by the transport of K+ into the guard cells from the adjacent cells. It has been found that anions which balance the positive charge of K+ are those of organic acid especially malic acid. The influx of K+ into the guard cells is accompanied by the synthesis of malic acid.
Opening of stomata in light: Stomata open in light due to following reactions.
- In light starch in the guard cells is metabolised into Phospho Enol Pyruvate (PEP). It is later converted into organic acids, particularly malic acid. The reaction takes place in the presence of an enzyme phospho enol pyruvic carboxylase.
- Malic acid dissociates into malate and hydrogen ions in the guard cells.
- Hydrogen ions from guard cells are transported to epidermal cells and K+ ions from epidermal cells are absorbed into the guard cells.
- In the guard cells, K+ ions are balanced by malate anions. Besides, small amount of Cl-ions are also absorbed which neutralize a small percentage of K+ ions.
- The process of K+ ions exchange requires ATP and thus, it is an active process.
- Increased K+ ions and anion concentration in the guard cells increases their osmotic concentration. Hence, water enters the guard cells by endosmosis.
- Turgor pressure of the guard cells increases due to endosmosis and the stoma opens.
Closing of stomata in dark: Closure of stomata in darkness is due to the following reactions:
- As carbon dioxide is not utilized in photosynthesis during night, hence its concentration in the sub stomatal cavity increases.
- Abscissic acid (ABA) functions in the presence of carbon dioxide. It is an inhibitor hormone. It inhibits K+ uptake by changing the diffusion and permeability of guard cells.
- The K+ is now transported back to the epidermal or subsidiary cells from the guard cells. The osmotic concentration of the guard cells decreases. This results in the movement of water out of the guard cells (exosmosis). The guard cells now become flaccid. This causes stomatal closure.
POTASSIUM ION PUMP THEORY
The main features of the theory were put forward by Levitt (1974). Levitt explained the influx of K+ in the guard cells and their role in the stomatal movement. He demonstrated a relationship between stomatal opening and accumulation of K+ ions. According to the hypothesis, pH of the guard cells rises due to active uptake of H+ ions by guard cell. The steps involved are as follows.
- The light induced proton transport from the cytoplasm into the chloroplast creates a negative potential. This negative potential may lead to influx of positively charged K+ ion from the surrounding cells so that the negative potential is decreased and the proton transport is not ceased. This results in the development of maximum pH.
- The pH of cytoplasm is raised to 8-9 and that of the chloroplast is lowered to 5. The rise in the pH causes hydrolysis of starch to form organic acid especially phospho enol pyruvate. Phospho enol pyruvate in the presence of enzyme PEP carboxylase combines with CO2 to produce oxalic acid. This acid is then converted to malic acid.
- Malic acid disassociates into H+ ion and malate ion. H+ ions comes out and in exchange K+ ions enter into the guard cells, thereby increasing the concentration of K+ ion and decreasing the H+ ions in guard cells. This is an active ionic exchange and requires ATP and cytokinin.
- The pH of the cell sap in the guard cells increases simultaneously. The pH becomes more than 7 and the medium becomes alkaline.
- There is also an increased uptake of Cl- anions by the guard cells to maintain the electrical and ionic balance inside and outside guard cells.
- The malate anions formed in the guard cells are neutralized by the K+ ions. K+ ions react with malate to form potassium malate.
- Potassium malate enters into the cell sap reducing the water potential and increasing the osmotic concentration and osmotic pressure of the cell sap.
- Endosmosis occurs and the guard cells become turgid and the stoma opens.
STOMATAL CLOSURE:
When the darkness sets in, H+ ion starts diffusing into the cytoplasm. H+ ion reacts with malate ion to form malic acid. Malic acid undergoes decarboxylation and gets converted into pyruvic acid and carbon dioxide. Pyruvic acid is consumed in respiration. Carbon dioxide gets dissolved in water to form carbonic acid. When potassium malate is converted into malic acid, osmotic pressure of the guard cells decreases .Exosmosis starts and the stomata closes.
- Potassium malate→K++malate ion
- Exflux of K+ and influx of H+ ion occurs.
- H++malate ion→ malic acid.
- Malic acid is consumed during respiration.
- Osmotic pressure of guard cells decreases.
- Exosmosis occurs.
- Stoma closes.