Chapters
- Role of Stomata in Homeostasis in Plants
- Environmental Stimuli that Cause the Opening of Stomata
- Environmental Stimuli that Cause the Closing of Stomata
- What are the Advantages and Disadvantages of Stomatal Opening & Closure?
- Opening and Closing of Stomata
- Structure of Guard Cells
- The Mechanism for Opening Stomata
- The mechanism for Closing Stomata
- Abscisic Acid and Stomatal Closure
In this article, we will discuss how stomata respond to changes in environmental conditions by opening and closing. We will also explain that stomata have daily rhythms of opening and closing. Moreover, we will describe the structure and function of guard cells and discuss the mechanism by which they open and close stomata. In the end, we will describe the role of abscisic acid in the closure of stomata during times of water stress, including the role of calcium ions as a second messenger. So, let us get started.
Role of Stomata in Homeostasis in Plants
Like animals, plants need to maintain a consistent internal environment, therefore the process of homeostasis also takes place in plants. For instance, plants have mesophyll cells in leaves that need a constant supply of carbon dioxide to carry out the process of photosynthesis.
Stomata in plant cells especially the guard cells control the diffusion of gases in and out of the leaves. It implies that the entry of carbon dioxide into the leaves is controlled by the stomata.
Environmental Stimuli that Cause the Opening of Stomata
The environmental stimuli that cause the opening of stomata in plants are:
- Enhanced intensity of light
- Low concentration of carbon dioxide in the air spaces inside the leaf
Environmental Stimuli that Cause the Closing of Stomata
The environmental stimuli that cause the closing of stomata in plants are:
- Absence of light, i.e. darkness
- High concentration of carbon dioxide in the air spaces inside the leaf
- Hugh temperature
- Low humidity
- Water stress, i.e. when there is limited water supply from the roots or when there are high rates of transpiration
Regulating the stomatal aperture balances the requirement for the uptake of carbon dioxide through diffusion along with the requirement to minimize the loss of water through transpiration.
What are the Advantages and Disadvantages of Stomatal Opening & Closure?
The advantage of stomatal opening during the day is:
- The gain of carbon dioxide by leaves to carry out the process of photosynthesis
The disadvantage of stomatal opening during the day is:
- Loss of large amount of water by leaves through transpiration
The advantage of stomatal closing during the day is:
- The retention of water within the leaf is critical in times of water stress
The disadvantage of stomatal closing during the day is:
- The low supply of carbon dioxide which in turn reduces the rate of photosynthesis
Opening and Closing of Stomata
Every day stomata open and close in a rhythmic form. This daily rhythm of stomata continues in the presence or absence of light. The opening of the stomata during the day maintains the inward diffusion of carbon dioxide and the outward diffusion of oxygen. It also enables the outward diffusion of water vapour in transpiration.
On the other hand, the closing of stomata at night minimizes the rate of photosynthesis and helps to conserve water.
Structure of Guard Cells
Two guard cells surround each stoma. The guard cells have the following characteristics:
- Thick cell walls face the air outside the leaf and the stoma
- Thin cell walls face adjacent epidermal cells
- Around the cell, cellulose microfibrils are arranged in bands
- There are no plasmodesmata in cell walls
- Cell surface membrane is usually folded and has several channels and carrier proteins
- The cytoplasm contains a high density of chloroplasts and mitochondria
- Chloroplasts contain thylakoids, however, they have few grana as compared to mesophyll cell chloroplasts
- Mitochondria contain several cristae
- Instead of a single large vacuole, many small vacuoles are present
The Mechanism for Opening Stomata
- When guard cells gain water by osmosis and become turgid, they open
- The reduced water potential in the guard cells is needed for water to enter the cells through osmosis
- While responding to light, ATP-powered proton pumps in the guard cell surface membranes move hydrogen ions out of the guard cell actively
- Due to this, the inside of the guard cells is negatively charged, unlike the outside
- This causes the opening of channel protein in the cell surface membranes of the guard cell, which enables potassium ions to move down the electrical gradient and enter the guard cells. These potassium ions also diffuse into the guard cells down the concentration gradient. Remember that the combination of the concentration gradient and electrical gradient is referred to as the electrochemical gradient
- The solute concentration within the guard cells is enhanced due to the influx of potassium ions, which in turn lowers the water potential within the cells
- Now, osmosis allows the water to enter the guard cells through aquaporins in the cell surface membranes of the guard cells
- The size of the vacuoles in increased because the majority of the water enters the vacuoles
- It also boosts up the turgor pressure of the guard cells which causes the opening of stomata
- The bands of cellulose microfibrils only increase the length of the guard cells, not their diameter
- The outer walls of the guard cells are thin, so they bend more easily as compared to thick inner walls
- It helps the guard cell to become curved, and the stomata open
The mechanism for Closing Stomata
- When specific environmental stimuli are identified that result in the closing of stomata, the proton pump in the cell surface membranes of the guard cells stops moving hydrogen ions out of the guard cell actively
- The potassium ions exit the guard cells
- Now, there is a reverse water potential gradient and water leaves the guard cells through osmosis
- As a result, the guard cells become flaccid and stomata close
Abscisic Acid and Stomatal Closure
- In times of water stress, plants produce a hormone known as abscisic acid (ABA) which helps them to stimulate the closing of their stomata. Many environmental conditions can be attributed to water stress, like extremely high temperatures and low water supplies
- ABA receptors are present on the cell surface membranes of the guard cells
- The hormone abscisic acid binds with these receptors to inhibit the proton pumps and thus preventing the active movement of hydrogen ions out of the guard cells
- Calcium ions also move into the cytoplasm of the guard cells due to ABA via cell surface membranes
- The calcium ions play the role of second messengers
- The calcium ions cause the opening of channel proteins that enable negatively charged ions to exit the guard cells
- This triggers the further opening of channel proteins that enable potassium ions to exit the guard cells
- The calcium ions also trigger the closing of channel proteins that enable potassium ions to enter the guard cells
- The loss of ions results in the increase in water potential of the guard cells
- Water exits the guard cells through osmosis which causes the guard cells to become flaccid and lead to the closing of stomata
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