OPENIING AND CLOSING OF STOMATA:
· Plants have certain openings on the outer and lower surfaces of leaves that allow water to escape are called stomata. They are usually found on the underside of a leaf to reduce excess water loss and they are surrounded by guard cells that open and close the pores.
· Stomata release water, their main purpose is to exchange of gases. Plants need to breathe CO2 from the atmosphere in order to photosynthesize or change into usable chemical energy. They also need to release oxygen back into the atmosphere as a waste product of cellular processes. This gas exchange occurs through the stomata and while this happens, some water is lost from the plant.
· Stomata are small force present in the epidermal cells of leaves in plants. Stomata are open during the day and close during night.
· Stomata take in C02 required for the photosynthetic activity during the day. They give out excess water released in the process of respiration during night along with CO2.
· Opening and closing of stomata is controlled by concentration of solutes in guard cells.
Mechanisms of opening and closing of stomata
· Opening and closing of stomata takes place due to turgor changes in guard cells.
Opening of Stomata
Solutes from neighbouring epidermal and mesophyll cell enter the guard cell lowering it osmotic potential and water potential. This lowered water potential and osmotic potential will allow movement of water in guard cells from neighbouring cells. Guard cell become turgid and swells in size resulting in the stomata opening due to water accumulation in them.
Closing of stomata
As the stomata open the solute concentration is reduced. This makes the water from the guard cell to move away into the neighbouring cell and making them flaccid with no water. They collapse against each other and results in the closing of stomata.
Factor regulating the Opening and closing of Stomata
1. Light: Blue and red light is effective in both photosynthesis and opening. Blue light is more effective in stomatal opening relative to the red light. At low levels, blue light may cause stomatal opening when red light has no effect at all. Blue light causes movement of k+ ion. Stomata open in the presence of light and closed in darkness. Light intensity is required to open the stomata in very low as compared to the intensity required for photosynthesis. In CAM (Crassulacean Acid Metabolism) Plants, Stomata open during Dark and remained closed during the day even moonlight is sufficient to keep the stomata open is some plant species.
2. Temperature:Rise in temperature induces stomata opening while fall in temperature caused its closure. At 38-40°C, Stomata open even in darkness. In some plant species, stomata remain closed even under continuous light at 0°C.
3. CO2: Low CO2 concentration induced stomata opening and high CO2 concentration induce its closure.
4. O2: It is essential for stomata opening.
5. Water Availability: Water stressed (Less Water Availability to Plant and High transpiration Rate) plats induce stomata closure due to the formation of absiccic acid (ABA) and lowering of water potential in epidermal cells.
6. Potassium: Influx of K+ ion caused opening of stomata while efflux of k+ ion from guard cells caused closure of stomata.
7. Mechanical Shock: It causes closing of stomata.
8. Hormones: ABA brings about closure of Stomata. Cytokinin is required for keeping the stomata open.
9. Mineral elements: Deficiency ofcertain mineral elements like nitrogen, phosphorous and potassium has some effect on the opening and closing of stomata.
Laws of Water in the form of liquid is guttation, it generally occurs from the tips and margin of leaves during night and early morning when there is high atmospheric humidity as during wet season.
1. HYDATHODES OR WATER PORESare special pores on the leaves in the region guttation which are permanently open pores.
2. GUTTED water possesses minute quantities of both organic and in organic substance.
How do Guard cells function?
Guard cells are cells surrounding each stoma. They help to regulate the rate of transpiration by opening and closing the stomata. To understand how they function, study the following figures. As you look at the figures, keep in mind that an increase in solute concentration lowers the water potential of the solution and that water moves from a region of higher water potential to a region of lower water potential.
Notice that in figure A the guard cells are turgid or swollen and the stomatal opening is large. This turgidity is caused by the accumulation of k+ ion in the guard cells. As k+ ion tends to increase in the guard cells, the water potential of the guard cells drops and water enters the guard cells. In figure B, the guard cells have lost water which causes the cells to become flaccid and the stomatal opening to close. This may occur when the plant has lost an excessive amount of water. In addition, it generally occur daily a light levels drop and the use of CO2 in photosynthesis decreases.
What causes the stomata to open and close?
There are guard cells surrounding each stoma that causes them to open or close throughout the life cycle of the plant. This occurs in response to water and less concentration in the pant cell.
How do guard cells regulate the opening and closing of stomata?
A plant’s guard cells regulate the opening and closing of the epidermal stomata by expanding or contracting in response to environmental signals. When a pair of guard cells surrounding stoma receives the signal that the stomatal pore needs to open the guard cells pair fill with water, changing the cell’s shape and opening the pores. An inverse process occurs when the guard cells receive a signal to close the stomata, initiating the loss of water and causing them to shrink and close the pore.
Plants need water to grow and maintaining the turgidity of cell. There are three types of transport occur in plants: Diffusion, facilitated diffusion and active transport.
Water is the most abundant constituent of all physiologically active plant cells. Several physiological processes of plants like osmosis, imbibition, plasmolysis are accomplished by water.
Transpiration explains how water moves up the plant against gravity in tubes via a xylem. Plants pump the water up from the soil to deliver nutrients to their leaves. It is influenced by several factors.
Diffusion is the movement of particles from the region of higher concentration to the region of lower concentration. It is affected by temperature, pressure, density of medium, distance,number of particles per unit volume.
Minerals are move into the roots by using energy in the form of ATP. After the uptake minerals are transported to all parts by transpiration pull. Later they are absorbed by active or passive transport.
Food is synthesized through the process of photosynthesis. It can be transported to different parts of the plant, this function is performed by phloem.
Water transport is important for the uptake of mineral nutrients from the soil. There are three pathways which facilitate the movement of water in plants like apoplast, symplast, trans-membrane pathways.
Mass flow hypothesis states that the mass flow of solute take place from the source(mesophyll cells) to the sink (where photosynthesis needed). This theory was proposed by Ernst Munch.