OPENING AND CLOSING OF STOMATA:

The guard cells function as multisensory hydraulic valves.

Environmental Factors:

Environmental factors such as:

(i) Light intensity and quality.

(ii) Temperature.

(iii) Relative humidity and

(iv) Intracellular CO2 concentration are sensed by guard cells and these signals are integrated (combine) into well-defined stomatal responses.

These are two hypothesis which may explain the opening and closing of stomata.

(1) Starch Sugar Hypothesis:

The German Botanist H. Van Mohl proposed that the guard cells are the only photosynthesising cells of epidermis of leaf.

Sugar are produced in the guard cells during day time when light is available.

(i) During Day Time:

When sugar level rises i.e. solute concentration increases or water potential decreases and the guard cells become turgid, and they separate from one another and stoma or pore opens.

(ii) During Night:

During night there is no photosynthesis the sugars are either converted into in soluble starch or are used in respiration, this decreases free sugars in cell.

So the osmotic pressure of guard cells is lowered, and water leaves the guard cells. They become flaccid and stoma or pore between them closes.

But these process are not fast enough to account for the rapid rise in turgor, of guard cells.

(2) Influx of Kᐩ Ions:

Potassium concentration in guard cells increases several folds,

Depending upon plant species.

(i) Stomata Open:

Stomata open due to active transport of potassium ions (Kᐩ) into the guard cells from the surrounding epidermis.

The accumulation of Kᐩ decreases the osmotic potential of guard cells.

Water enters the guard cells by osmosis, which become more turgid and stretched and stomata are opened.

(ii) Stoma Closes:

The stoma closes by reverse process; involving passive diffusion of K⁤ᐩ from guard cells followed by water moving out by osmosis.

What Controls the Movement of Kᐩ into and out of Guard Cells ?

• Level of carbon dioxide in the spaces inside the leaf end.
• Light, control this environment.

(i) Level of Carbon Dioxide:

A low level of carbon dioxide favors opening of the stomata, thus allowing an increased carbon dioxide level and increased rate of photosynthesis.

(ii) Light Quality and Intensity:

Exposure to blue light, which is also effective in photosynthesis has been shown to acidify the environment of the guard cells (i.e. pumps out protons) which enable the guard cells to take up Kᐩ followed by water uptakes resulting in increased turgidity of guard cells.

So in general stoma are open during day and closed at night.

This prevents needless loss of water by the plant when it is too dark for photosynthesis.

(iii) Actively Pumping Potassium:

The plant open their stomata by actively pumping potassium in guard cells causing water to follow by osmosis.

Guard cells become turgid and stoma or pore opens.

When Potassium leaves the guard cells (during night) water leaves the guard cells by exosmosis and guard cells become flaccid and stoma or pore between guard cells closes.