Question

how does opening and closing of stomata take place ? why does water enter the guard cell?

Answer 1

Stomata are the small pores present on leaves through which gaseous exchange and transpirational loss of water occurs. To control these inputs and outputs, mechanisms to control stomatal opening are developed. There are several theories given to explain stomatal movement, but the most accepted one among these is potassium ion theory. This theory was given by Levitt. According to this theory, during daytime:

1. In the presence of light, starch in guard cells is converted to malic acid.
2. Malic acid dissociates into hydrogen ions and malate.
3. Hydrogen ions are exported to epidermal cells and potassium ions enter the guard cells in a process called ion exchange which is an active, energy requiring process.
4. Charges on potassium ions are balanced by organic malate ions and some chloride ions are also taken into guard cells to balance the charge.
5. Increased solute concentration increases solute potential of guard cell cytoplasm.
6. As a result, water potential of the guard cells decrease.
7. Water has a tendency to move from solution of higher water potential to the solution of lower potential. As a result, water from surrounding cells enter the guard cells and they become turgid and stomata open.

Potassium ions move out of the guard cells at night,and the guard cells become flaccid and stomata close.