Osmotic diffusion and Mechanism of stomatal opening and closing

 i. Osmotic diffusion

In the first stage, water gets evaporated from the surface of turgid cells and collects in the

intercellular spaces increasing the water vapor pressure and lowering its DPD.

Fig.5. V.S. of dicot leaf. The arrows show the

movement of water during transpiration

Inside the leaf, the mesophyll cells are in contact with the xylem, and on the other hand with

intercellular spaces above the stomata. When the mesophyll cells draw water from the xylem,

they become turgid and their DPD and OP decrease with the result that they release water in the

form of vapor into intercellular spaces close to stomata by osmotic diffusion. In turn, the OP and

DPD of the mesophyll cells becomes higher and hence, they drew water from the xylem by osmotic

diffusion (Fig.5)

ii. Mechanism of stomatal opening and closing (Steward’s scheme of stomatal

movements)

Microscopic examination of open and closed stomata reveals that open stomata have

turgid cells while closed stomata have flaccid guard cells. Therefore, it is concluded that the

opening and closing of stomata depend upon the changes in the turgidity of their guard cells. i.e.,

when guard cells are turgid, pores are open but when flaccid, the pores are closed. when the

turgidity increases, the outer thin walls of guard cells stretch outward causing outward stretching

of their inner wall. The inner wall being inelastic (thick), becomes concave and as a result, the

space surrounding the pore widens and pore opens.

Thus, in the opening and closing of stomata, the turgor mechanism is involved. How this

change in turgidity in guard cells is brought about is explained below :

During the night, starch accumulates in the guard cells. The starch is insoluble and therefore,

the OP of the guard cells is not increased. This results in the flaccid condition of the guard cells

leading to the closure of stomata during night conditions. Inlight, the insoluble starch is converted

into soluble sugars in the presence of enzyme phosphorylase and leads to stomatal opening

as follows :

The soluble sugars increase the OP of the guard cells, which also results in an increase in

their DPD. Due to all these changes, the guard cells drawn water from the mesophyll cells by

osmotic diffusion and become turgid. The outer thin walls of the guard cells expand while the

thick inner walls are stretched. This results in the opening of the stomatal pore. High temperature

and pH (about 7.0) favor the conversion of starch into sugars. The increased pH is probably due

to the consumption of CO2 in light in photosynthesis.

In dark, the soluble sugars are converted back to starch. This lowers the OP of the

guard cells and their DPD is also decreased with the result that water is released from the guard

cells into the mesophyll cells. Due to this change, the guard cells become flaccid, their thicker inner

walls come very close to each other leading to the closure of the stomatal pore.

Low temperature and pH (about 5.0) favor the conversion of sugars into starch. The lower pH

the level might be due to the accumulation of CO2 in dark.

With the discovery of the presence of phosphorylase enzyme in the guard cells Steward (1964)

proposed a modified scheme for stomatal opening. The scheme proposed by him is given in Fig.6

below :

Fig.6. Steward’s scheme of metabolic reactions

involved in the opening and closing of stomata.

In his scheme, he has suggested that :

i. glucose-1-phosphate should be further converted into glucose and inorganic phosphate for

the opening of the stomata, and

ii. metabolic energy in the form of ATP will be required for the closing of stomata which

probably comes through respiration.

The mechanism proposed for stomatal opening is :

The light → high rate of photosynthesis in mesophyll cells → removal of CO2 from intercellular

spaces→ increase of pH in guard cells → enzymatic conversion of starch into glucose →

increase in Op of cell sap → endodermis → guard cells become turgid → stomata open.

A reverse scheme would follow during the closure of the stomata.