Explain the mechanism of stomatal opening and closing.

Question

Explain the factors affecting stomatal movement.

Explain the proton-potassium pump hypothesis?

Answer 1

The mechanism of stomatal opening and closing depends upon the turgidity of guard cells. When the turgidity increases, the pore opens where as it closes when turgidity decreases.

Entry of water takes place by the osmotic pressure, which cause the elongation of guard cell by the radial orientation of cellulose fibrils.

As the water enters the guard cells, turgidity increases that pulls the cells causes the opening of stomata.

The closing of stomata has the reverse process of opening.

During the day time, water enters the cells due to the less water potential that creates high concentration of solutes.

Blue light of day light, activates proton pumps that is detected by the photoreceptor. By the activation of proton pumps, protons enter in the guard cells from the cytoplasm.

Due to the entry of protons, the proton motive force is created that opens the voltage operated channels in the membrane that passes the positive potassium ions into the cell.

As the concentration of potassium increases in the cell, chloride ions enters via chloride-protons symport mechanism, into the cell to balance the cell’s internal charges.

Blue light also induces the production of malate that increases the cell concentration, supports the increase in turgidity by opening the pores of the stomata.

There are some factors that affects the opening and closing of the stomata:

I. Light:

1. In presence of light stomata opens but in some plants the open during night time also.
2. In succulent plants, stomata closed during day time whereas open during night time.
3. in 1856 Von Mohl observed the stomatal mechanism during day light and night. By the observation he classified three main groups according to the daily movement:
   1. The stomata opens during day time and close during night. E.g.: peas, beans, mustards, etc.
   2. The stomata closes for a short time period during evening. E.g.: allium, cabbage, etc.
   3. The stomata opens for a short time period during day time. E.g.: barley, other cereals, etc.

II. Temperature:

1. Temperature is directly proportional to the size of the stomatal pore.
2. Higher the temperature, wider the pores and lower the temperature, narrow the pores.

III. CO₂ concentration:

1. The concentration of CO₂ is inversely proportional to the stomatal movement.
2. Higher the concentration of CO₂ in the leaf is responsible for the stomatal closing.
3. The concentration of CO₂ of the internal environment affects the opening and closing of the pores.

IV. Availability of water:

1. When the water availability is more, the rate of transpiration goes high. Due to that there is a higher loss of water from the leaves that creates the water stress in the plant.
2. The water stress is responsible for the plant retardation and the whole situational process is responsible for the stomatal closing.