Mechanism of Stomatal Movement­

Mechanism of Stomatal Movement­

Stomatal movements are regulated by the change of turgor pressure in guard cells. When water enters the guard cell, it swells and its unevenly thickened walls stretch up resulting in the opening of stomata. This is due to concave non-elastic nature of inner wall pulled away from each other and stretching of the convex elastic natured outer wall of guard cell.

Different theories have been proposed regarding opening and closing of stomata. The important theories of stomatal movement are as follows,

1.     Theory of Photosynthesis in guard cells

2.     Starch – Sugar interconversion theory

3.     Active potassium transport ion concept

1.     Theory of Photosynthesis in guard cells

Von Mohl (1856) observed that stomata open in light and close in the night. According to him, chloroplasts present in the guard cells photosynthesize in the presence of light resulting in the production of carbohydrate (Sugar) which increases osmotic pressure in guard cells. It leads to the entry of water from other cell and stomatal aperture opens. The above process vice versa in night leads to closure of stomata.

Demerits

1.           Chloroplast of guard cells is poorly developed and incapable of performing photosynthesis.

2.           The guard cells already possess much amount of stored sugars. 

2.     Starch – Sugar Interconversion theory

i.  According to Lloyd (1908), turgidity of guard cell depends on interconversion, of starch and sugar. It was supported by Loftfield (1921) as he found guard cells containing sugar during the daytime when they are open and starch during the night when they are closed.

ii.  Sayre (1920) observed that the opening and closing of stomata depends upon change in pH of guard cells. According to him stomata open at high pH during day time and become closed at low pH at night. Utilization of CO₂ by photosynthesis during light period causes an increase in pH resulting in the conversion of starch to sugar. Sugar increase in cell favours endosmosis and increases the turgor pressure which leads to opening of stomata. Likewise, accumulation of CO₂ in cells during night decrease the pH level resulting in the conversion of sugar to starch. Starch decreases the turgor pressure of guard cell and stomata close.

iii.  The discovery of enzyme phosphorylase in guard cells by Hanes (1940) greatly supports the starch-sugar interconversion theory. The enzyme phosphorylase hydrolyses starch into sugar and high pH followed by endosmosis and the opening of stomata during light. The vice versa takes place during the night.

iv.  Steward (1964) proposed a slightly modified scheme of starch-sugar interconversion theory. According to him, Glucose-1-phosphate is osmotically inactive. Removal of phosphate from Glucose- 1-phosphate converts to Glucose which is osmotically active and increases the concentration of guard cell leading to opening of stomata (Figure 11.15).

Objections to Starch-sugar interconversion theory

i.  In monocots, guard cell does not have starch.

ii.  There is no evidence to show the presence of sugar at a time when starch disappears and stomata open.

iii.  It fails to explain the drastic change in pH from 5 to 7 by change of CO₂.

3. Theory of K+ transport

This theory was proposed by Levit (1974) and elaborated by Raschke (1975). According to this theory, the following steps are involved in the stomatal opening:

In light

i. In guard cell, starch is converted into organic acid (malic acid).

ii.           Malic acid in guard cell dissociates to malate anion and proton (H+).

iii.        Protons are transported through the membrane into nearby subsidiary cells with the exchange of K+ (Potassium ions) from subsidiary cells to guard cells. This process involves an electrical gradient and is called ion exchange.

iv.        This ion exchange is an active process and consumes ATP for energy.

v.           Increased K+ ions in the guard cell are balanced by Cl– ions. Increase in solute concentration decreases the water potential in the guard cell.

vi.        Guard cell becomes hypertonic and favours the entry of water from surrounding cells.

vii.      Increased turgor pressure due to the entry of water opens the stomatal pore (Figure 11.16).

In Dark

i. In dark photosynthesis stops and respiration continues with accumulation of CO₂ in the sub-stomatal cavity.

ii. Accumulation of CO₂ in cell lowers the pH level.

i.             Low pH and a shortage of water in the guard cell activate the stress hormone Abscisic acid (ABA).

iv. ABA stops further entry of K+ ions and also induce K+ ions to leak out to subsidiary cells from guard cell.

v. Loss of water from guard cell reduces turgor pressure and causes closure of stomata (Figure 11.17).