Mechanism of Opening and Closing of Stomata

Mechanism of Opening and Closing of Stomata

​

An Increase in Guard Cell Turgor Pressure Opens the Stomata

• Environmental factors such as light intensity and quality, temperature, relative humidity, and intracellular CO₂ concentrations are sensed by guard cells, and these signals are integrated into well-defined stomatal responses

• If leaves kept in the dark are illuminated, the light stimulus is perceived by the guard cells as an opening signal, triggering a series of responses that result in opening of the stomatal pore.

Opening of the stomata

• Stomata open and close due to changes in turgor pressure of the guard cells.

• They open when water is absorbed by the guard cells from the surrounding epidermal cells.

• The guard cells then become fully turgid and they push into the epidermal cell besides them.

Closing of the stomata

• When water is lost and the guard cells become flaccid, the pore closes again.

• Stomata tend to open in daylight and close in the dark (but there are exceptions to this diurnal pattern).

Open Ended

What are the factors signal the opening and closing of the stomata?

Levitt (1974)

• Proposed the proton transport concept to explain the mechanism of the opening and closing of the stomata.

​Dissociation of malic acid to H⁺ ions and malate ions

Multiple Choice

According to the proton transport concept, what ion is being moving in and out of the guard cells?

• Ca²⁺

• Na⁺

• K⁺

• Mg²⁺

Multiple Choice

Which act ion storage cells for the guard cells?

• spongy mesophyll cells

• adjacent epidermal cells

• stomatal cells

• vacuole

Multiple Choice

Starch in the guard cells is converted into what type of organic compound?

• glucose

• glycogen

• malic acid

• maltose

Multiple Choice

The converted organic acid is further converted into ____ ions.

• Na⁺ & glucose

• K⁺ and glycogen

• H⁺ & malate

• Cl^- & maleate

Multiple Choice

When H⁺ ions move out from the guard cells, the K⁺ ions move into the guard cells.

• TRUE

• FALSE

Multiple Choice

Movement of the K⁺ to the guard cells creates a decrease in osmotic potential of the guard cells.

• TRUE

• FALSE

Multiple Choice

Endosmosis makes the guard cells flaccid.

• TRUE

• FALSE

Abscisic acid and water stress in the leaf

• ABA is also involved in the closure of stomata under conditions of  drought .

• Green plants that are experiencing drought produce ABA in their chloroplasts, including those in the guard cells, and this ‘stress hormone’ triggers stomatal closure.

Abscisic acid and water stress in the leaf

• This response involves calcium ions as the ‘second messenger’.

• ABA triggers release of calcium ions from the cell sap in the guard cell vacuoles into the guard cell cytosol, via activated calcium ion channels in the tonoplast.

Abscisic acid and water stress in the leaf

• Consequently, the concentration of calcium ions in the cytosol is abruptly increased, and directly triggers:

• 1.  a sudden efflux of anions such as chloride ions from the cytosol, across the cell surface membrane, by activated anion channels

Abscisic acid and water stress in the leaf

• 2.  a sudden efflux of potassium ions from the cytosol, across the cell surface membrane, by activated potassium ion pumps.

• 3.  inhibition of efflux of potassium ions into the guard cells (a feature of the stomatal opening mechanism).

Abscisic acid and water stress in the leaf

• As a result, the water potential of the guard cells is abruptly raised. It is now higher than the water potential of the surrounding cells, and a net efflux of water occurs from the guard cells. The guard cells become flaccid and the pores close.

Open Ended

Suggest why the stomatal apertures of the plant in the very dry conditions differed in both maximum size and duration of opening from those of the plant with adequate moisture.

Open Ended

How do stress conditions within leaves due to water shortage affect stomatal opening, and what part do plant growth regulators play in this response?