Transpiration: Opening and Closing of Stomata

Introduction

In the form of moisture or water vapour plants release the water inside through the process of transpiration. Some of the water is consumed by the roots in the soil, while the rest evaporates into the atmosphere. Water is evaporated into the atmosphere by plant parts such as stems, tiny pores on leaves, and flowers. In other terms, it is the process by which water from plant leaves and other parts evaporates into the atmosphere.

Types of Transpiration

There are three kinds of transpiration depending on the organ that performs transpiration: stomatal transpiration, cuticular transpiration and lenticular transpiration.

Respiration through the stomata: It is the process of water evaporation through the stomata. In the leaves, stomata are specialized pores. They are responsible for about 80% to 90% of the total water loss from the plants.

Respiration through the Cuticle : It is an impermeable layer found on the leaves and stems that allows water to pass through. They are also responsible for 20% of plant transpiration. Cuticular transpiration is lower in xerophytes due to very thick cuticle and wax coating on the leaves and stem

Respiration through the lenticels: Loss of water that takes place through lenticels which are small minute pores present on the stem of many woody plants is called lenticular transpiration

Stomata

The microscopic apertures seen on the leaf epidermis are known as stomata. Stomata can be seen beneath the light microscope. Some plants have stomata on their stems and other sections.

Stomatal Structure

Each stoma has two guard cells surrounding it. Chloroplasts are found in the guards, which are kidney-shaped. In each stoma, there is a breathing chamber.

Plants acquire the necessary carbon dioxide during photosynthesis and release the oxygen created into the atmosphere. This gas exchange takes place through openings in the epidermal of plant leaves. A stoma is a single orifice derived from the Greek word for “mouth.” Stomata respond to a range of environmental stimuli by opening and closing.

Two specialized guard cells border each stoma, opening when these cells suck in water. The amount of water in the guard cell is controlled by ion transport.

Stomatal Expansion

Stomata are the pores present in abundance at the lower epidermis of the leaf. They are also present in the stem. The gaseous exchange and transpiration occur through stomata. The stomata are surrounded by two guard cells which regulate its opening and closing. Stomata open during the day time for gaseous exchange and also release water vapour through transpiration. The opening and closing of stomata is due to the change in turgor pressure of the guard cell.

During the day the roots absorb water due to greater transpiration pull and it is transported to different parts of the plant through xylem. The guard cell on receiving this water swells and becomes turgid. As a result of which the stomatal pore is open.

At night, the roots absorb less water, thus the guard cell becomes flaccid and shrinks. As a result of which stomatal pores closes.

The guard cells that surround the stomata have thickened radial cell walls. The guard cell’s cell wall is entirely thickened on the side facing the stoma (ventral side). Alternating bands of thick and thin cell walls run along the outside (dorsal side). What causes the stomata to open when exposed to light? A proton pump exports protons (H+) when phototropin detects blue light. The pump is powered by ATP, produced by photosynthesis’ light processes. The cytosol is often more negative than the extracellular solution, and this distinction is important.

Open or Close the Gate – The Role of ABA, Ion Channels, and Diurnal Cycle in Stomatal Movements Regulation

The Regulatory Role of Ion Channels localized in the Guard Cell Membrane in the Opening and Closing Stomata The adjustment of guard cell turgor is dynamically according to environmental conditions and hormonal signals to facilitate the proper gas exchange and prevent excessive water loss. Plasmodesmata are not present in mature guard cells. For this reason, most influx and efflux of solutes occurs via ion channels, transporters, and pumps that are localized in the plasma membrane (PM). The action of ion channels, transporters, and pumps essential for the stomatal function is well documented and supported by molecular studies involving mutants in the genes encoding these proteins. During the stomata opening, the H+-ATPase pump mediates the efflux of H+ from the guard cells. 

The Role of ABA in Stomatal Movement Regulation During the Diurnal Cycle. The diurnal stomatal motions are linked to the ABA mode of action. This association is thought to be based on both the molecular links between ABA and circadian-clock pathways and ABA production and light response. Despite the fact that multiple studies have linked the diurnal cycle to ABA signaling, further research is needed to fully understand this connection.

The Stomata are opened when the Guard Cell Turgor Pressure rises. Light intensity and quality, temperature, relative humidity, and other environmental elements in 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.

The Starch-Sugar Interconversion Theory: Steward proposed this theory in 1964. According to him, during the day, the phosphorylase enzyme converts starch to sugar, increasing the osmotic potential of the guard cell and allowing water to enter the cell. The same response occurs in the opposite direction during the night, closing the guard cell, i.e. the stomata are closed. 

The Proton-Ketose Pump Hypothesis: Levit in 1974 combined the points in Scarth’s and Steward’s hypothesis and gave a modified version of the mechanism of stomatal movement which was called the proton – potassium pump hypothesis. According to this theory, the change that takes place in the turgor pressure of the guard cells that open and close the stomata causes the absorption and loss of potassium ions by guard cells. 

Theory of Glycolate Metabolism: Zelitch (1963) proposed that production of glycolic acid in the guard cells is an important factor in stomatal opening. Glycolate is produced under low concentration of CO2. He suggested that glycolate gives rise to carbohydrate, thus raising the osmotic pressure and also that it could participate in the production of ATP. Which might provide energy required for the opening of stomata.

Active K+ Transport or Potassium Pump Theory and Role of Abscisic Acid: The concept of K+ ion transport was given by Fujino. It was supported and elaborated by Levitt & Rashke in 1975 It appears to be an active mechanism which needs ATP. It is based on recent observations and (explains the mechanism as follows.

Conclusion

Plants acquire the necessary carbon dioxide during photosynthesis and release the oxygen created into the atmosphere. This gas exchange takes place through openings in the epidermal of plant leaves. A stoma is a single orifice (hole) derived from the Greek word for “mouth.” Stomata respond to a range of environmental stimuli by opening and closing. The guard cell turgor is dynamically adjusted to environmental conditions and hormonal signals to facilitate the proper gas exchange and prevent excessive water loss. Mature guard cells do not have plasmodesmata and for this reason, most influx and efflux of solutes occurs via ion channels, transporters, and pumps that are localized in the plasma membrane (PM). Some of the water in the soil is consumed by the roots, while the rest evaporates into the atmosphere. Water is evaporated into the atmosphere by plant parts such as stems, tiny pores on leaves, and flowers. In other terms, it is the process by which water from plant leaves and other parts evaporates into the atmosphere. During the day time plant takes carbon dioxide from the atmosphere for photosynthesis and releases oxygen which we breathe. This gaseous exchange occurs through stomatal pore. At night photosynthesis doesn’t take place. Plants take in oxygen for respiration and release carbon dioxide.

Note: The rate of transpiration is regulated by the guard cells. When the guard cells are turgid, stomata open and when it is flaccid, the stomata closes.