Water Relations of Vascular Plants - Term Paper Example

Water relations of vascular plants Water potential equation The propensity of water to travel from a higher concentration region to that of a lower one is known as water potential (Ψ). However, in order to move water from one place to another, it is required to apply considerable force which would result into the displacement of water. It has been found that the maximum water potential for any water volume may be identified as 0. However, to achieve this, it has to be assumed that standarised atmospheric pressure needs to be exerted on the given water volume. It has also been found that distilled water can displace other things effectively as it has the maximum potential to progress from one place to another (Taiz & Zeiger 1998). Research has indicated that energy can be found in two type i.e. kinetic energy and potential energy. In case of water, the potential energy within its molecules help them in moving as per the action of where the molecules were originally placed and where are they planning to reach. Two more forces of pressure and gravity are also exerted on water that helps in the movement. Such forces can also be found in the water or hydrological cycle. However, in the water cycle, the water moves from uphill to downhill. But, the water to fall as rain, it also needs to get into the atmosphere to form clouds, which is being done through evaporation. However, such a process also requires energy to complete the cycle (Raven et al 1999). In case of plant cells, it has been observed that they can retain large quantities of water in the vacuole. This helps in maintaining the cells in a proper and turgid manner. Pressure potential is considered to be one of the biggest elements of water potential with regards to plant cells. It has been seen that the pressure potential augments with the entering of water into the plant cell. With the water going across the cell membrane and cell wall, the quantity of water found in the cell also increases. This in turn puts pressure on the plant, which is then restrained by the rigid structure of the cell wall. It has been found that living plants mostly show positive pressure potential. In fact, the pressure potential in plasmolysed cells is found to be about zero. Further, the pressure potential can get into negative figure when the water comes out of open spaces like the xylem vessels of the plant. However, the xylem vessels are mostly able to endure the negative pressure. This process of surviving pressure is known as tension and is one of the significant adjustments of xylem vessels (Holbrook et al 2005). As distilled water is said to have better water potential as compared to the plant cell, it is obvious that the water would get inside the cell if the plant cell is put into a glass of distilled water. However, with the central vacuole of the plant cell being filled with water, it has also been observed that the cell would give out water into its neighboring cells as well. Nonetheless, the pressure exerted on the cell does not burst the cell because of the presence of cell wall (Raven et al 1999). The below diagram presents how cell wall works when a plant cell is placed inside distilled water. Figure 1: Cell wall of a plant cell inside distilled water Research has found that plant cells consist of cell walls that are very strong. When the cell begins to capture water through osmosis, the cell begins to expand however; this expansion does not burst the cell as the cell wall helps in preventing it. Thereafter, the cell pressure increases immensely and at the end the internal pressure becomes too high for the cell to accommodate any more water inside it. The process of turgidity is said to be important for the sustenance of plants as it keeps the green portions of the plants upright and face the sun, which further helps in creating the plant’s food through photosynthesis (Taiz & Zeiger 1998). Plant cells are also found to contain sugars, amino acids, organic acids and ions, along with other components in substantial absorptions in the vacuoles of the cells. These components help the water to enter into the cells and the plant cells are able to create positive internal pressure, which is also known as turgor pressure. Further, it also helps in maintaining the stability and rigidity of the plant tissue. Also, the cells create pressure on the surrounding cells, which further generate additional tissue tension. Plants which constantly take out water become wilted as the turgor pressure inside the cells declines. This results in the non-maintenance of the tissue stability. However, the wilted plants can regain easily if it is watered as the process of watering the plant creates turgor pressure inside the living plant cells (Raven et al 1999). The turgor pressure can also be compared to the presence of air pressure inside a tire. Further, the physical forces may be explained with the help of an equation. The equation being ψ = P + π, wherein the water potential is being described through ψ, which is pronounced as ‘psi’, the pressure potential is defined as P and the osmotic potential through π. Thus, the equation explains that water potential in a cell is equivalent to the sum of pressure potential and osmotic potential of the cell. Further, it has also been indicated that water tends to travel from higher water potential areas to lower water potential regions (Raven et al 1999). Uptake of water by roots Research has indicated that soil water gets into the roots of a plant through the help of epidermis. Thereafter, the water moves into the symplast or the cytoplasm of the root cells. Thus, the water travels along the plasma membrane and then through one cell to the other to reach plasmodesmata. However, in the parts of the root that is not living, which is also called apoplast, the water is not found to pass through such parts (Sperry 2003). Similarly, the cortex’s inside lining or boundary, which is known as the endodermis, is found to be resistant to water. This is due to the presence of casparian strip, a strip of suberized matrix. Therefore, for the water to get into the stele, the water must get into the symplasm and from there move into plasmodesmata and finally into the stele cells. Through this process, water is able to move along without getting into the apoplast (Sperry 2003). However, once the water enters into the stele, it is able to move around freely and travel from one cell to another easily. In case of a young plant, the water gets directly inside the tracheids or the xylem vessels. Once the water enters the xylem, the water which contains rich deposits of minerals continues its upward movement through the tracheids and vessels. Further, at any given level, the water may be able to forsake xylem and go through in a lateral manner to provide water to other tissues. In case of a leaf, the xylem is found in the petiole, which then passes inside the leaf veins. Water therefore, travels to the veins and thereafter gets into the cells of the palisade and spongy layers. In the leaves, the water is mainly used for metabolism. However, it has been seen that most of the water gets lost during transpiration (Sperry 2003). The below diagram explains the pathway of water from the roots to the xylem. Figure 2: Pathway of water It has been found that the xylem tissue, which comprises of vessels and tracheids is the plant’s one of the most important conducting mechanism. Further, there are three mechanisms that are found to be involved in the transporting of water from the roots to the various parts of the plants. These mechanisms are: Osmosis: In this process the water is seen to travel from the soil to the root system of the plant, from where it further gets into the xylem cells. The concentration of water through the soil and root is being sustained through twine methods. One through the constant progress of water from the root to the xylem, and another through the concentration of high quantity of minerals in the stele that is being upheld through the careful course of ions passed by the endodermis. This kind of root pressure, also known as osmotic force, is defined as guttation, which means development of saps in small droplets on the grass leaves and herbs during the early morning time. However, it has been also seen that in case of most of the environmental situations, the root pressure is able to produce only a small amount of force that does not have any major impact on the progression of water in plants (Taiz & Eduardo 1998). Capillary action: In this type of action, the water passes through narrow tubes and helps in the progress of water to reach xylem. The capillary action is the result of the adhesion between the capillary tube and water. Adhesion is known as the attraction of molecules between different substances. The forces get together to put water inside the tubes, which results in the formation of a crescent-shaped plane at the end of the water column towards the top. However, in case of active xylem cells, water creates constant column and does not include menisci. Therefore, the impact of the capillary action is very less and is only restricted to very small cavities found in the cell walls’ cellulose microfibrils (Taiz & Eduardo 1998). Cohesion-tension theory: It has been seen that most of the progression of water to the xylem is due to the cohesion-tension theory. As per this theory, some of the major concepts that govern water movement include (Raven et al 1999): Transpiration: This means the removal of water from the plans, which may cause negative pressure inside the xylem and leaves tissues. This is being explained in detail in the next section (Holbrook et al 2005). Cohesion: The cohesion that is being found between molecules of waters helps in developing a single column of water that is being carried through the leaves from the roots. This process may be explained as the attraction of water molecules between similar substances. Cohesion in water is formed due to the polarity of molecules of water. This further develops hydrogen bonding which is found in adjacent molecules of water (Raven et al 1999). Bulk water flow: Xylem cells also helps in transporting bulk water due to the fact that most water molecules tend to evaporate from the surface of the leaf. Thus, once the water is lost from leaves, the molecules pull the entire water flow that is behind it and this result in the bulk flow of water through the xylem (Raven et al 1999). Transpiration The progression of water inside the plant is known as transpiration stream. The process starts with the root epidermis and thereafter, it progresses apoplastically and symplastically towards the endodermis. Thereafter, the movement continues inside the root’s vascular cylinder. The mineral and water solution that is being concentrated by the endodermal helps in moving proteins to the xylem, root, stem and leaf. Inside the leaf the solution helps in coating the mesophyll cells and evaporating the spaces that is found between the cells. Thereafter, it finds its escape from the leaf with the help of stomata and gets into the atmosphere. Therefore, the water moves inside the plant and gets out through small pores known as stomata and this process is known as transpiration (Langensiepen 2008). There are four major factors that impact transpiration. These include: Root pressure: It has been found that root pressure helps in moving water to the xylem. The movement of water inside the roots is not found to be in uniform, which is being showed in the figure below. The growing suberization in the parts of the root that is old restrains the water to reach endodermis. Thus, the water is being restricted to the zone that is at the back of the root tips’ elongation area (Langensiepen 2008). Figure 3: Root pressure Capillary movement: This type of action has been already discussed in the previous section. The capillary action helps in moving the water to the xylem and is dependent on the adhesion factor of the water, which helps in transporting the water inside the cell wall of the xylem (Langensiepen 2008). Cohesion: In order for the water to move through the xylem, it is important for the water to move from the root xylem to the stem xylem and then to the leaf xylem in a continuous manner without any break. This is especially necessary in case of a tall tree, as the water’s tensile strength needs to be strong to help in keeping the water column intact (Langensiepen 2008). Evaporation: The process of evaporation also helps in pulling water into the xylem. Due to evaporation, the spaces between the cells in a leaf get empty and therefore a pull is generated which helps in getting water from the top of the water column found in the xylem (Langensiepen 2008). Thus, despite the gravitational force, this process helps in generating considerable amount of force that lifts the water column even in tall trees, which is being explained below. Lifting water into tall trees The tree trunk’s wooden core is mostly made up of thick collection of tubes that are narrow which are being called xylem that helps in carrying water to the leaves from the roots. Through the process of transpiration, the water is being transported to the xylem. This process is also helped by evaporation and capillary action as well. Once the water reaches the openings in the xylem, the water often gets evaporated from these open spaces found in the leaves of the trees and gets into the atmosphere. With the evaporation of water, more water through the capillary action gets into the xylem and from there into the leaves. Further, at the same instance, the electrostatic attraction that occurs between the molecules of water which helps in pushing the water into the leaves produces substantial cohesive force upon the total water column that it pulls larger quantity of water from the ground level to even the topmost part of a very tall tree (Langensiepen 2008). Further, the water column has a weight of itself which also helps in putting great amount of pressure on the cohesive forces and gets the water at the top of the tree. The water is given a free reign to move inside the xylem, although the walls found inside the xylem tube does not give any straight support to the water found inside the tube. Instead, the support is given by the water itself. The cohesiveness found inside the water column helps in making the water like a long string with the tension between the water molecules supporting the water’s weight. Research has found that the force or pressure on the water inside the xylem is considerably on the higher side. For instance, in case of a tree that is thirty feet high, the tension or pressure augments by around 15 pounds every square inch. Thus, a 360 feet high xylem tube can feel pressure of up to 180 pounds every square inch at the top (Langensiepen 2008). The xylem sap consists of various minerals and other compositions and is mostly consistent to the leaf from the root. It has been found that the gravitational pull inside the xylem increases by 0.1 MPa in every 10 meters. Vacuum is further required for the pressure potential to bypass the resistance by the perforation plates. The bundle sheath is also made up turgid cells that aids in loading sugar inside the phloem as well as unloading the water from the xylem. However, due to water evaporation from this solution, it is being made very concentrated. This area is also known as non-cellular with the water being passed from the pits of the wall so that the pressure potential can be negative (Holbrook et al 2005). The water also gets into the atmosphere due to this evaporation of water. As the gas has higher level of humidity, the water potential becomes low. But, this is much larger than the atmosphere found in the exterior of the leaf. This is what gives the energy to pull through the process of evaporation. This exterior atmosphere found outside the leave is quite dry, with the solute potential to be much lower than the inside atmosphere of the leaf. Thus, the difference between this water potential would help in providing the force for the water to move from the leaf openings known as stomata into the atmosphere (Holbrook et al 2005). This process is being explained in detail in the next section. Stomata The stomata are responsible for regulating gas exchange and transpiration through the use of guard cells. It has been seen that the stomata pores remains open in case the turgor pressure inside the guard cells becomes too high and closes once the turgor pressure becomes low. This kind of change also takes place with the modification in the intensity of light, concentration of carbon dioxide or concentration of water (Morison 2003). Potassium ions from the neighbouring epidermal cells get into the stomata’s guard cells. This process requires immense energy. Through this process the stomata gets opened as the water potential inside the stomata decreases, which results in the water transporting inside the guard cells and augmenting the turor pressure. Further, with the release of the potassium ions, the water potential changes and the water goes away from the cells. Research has indicated that with water stress, the stomata would close. Further, due to the giving off of potassium ions because of the presence of hormones, the guard cells may close the pores (Morison 2003). It has been seen that most of the plants tends to close their stomata at night time and open them during the day time. Nonetheless, there are some plants that often do the converse by closing their stomata in the day and opening them at night. Due to the heavy loss of water during the day time, the water potential in the leaf decreases and results in the stomata to close. If the stomata do not close with the fall in water level, the leaves may loose excessive water. This would further result in damaging the cell membrane and the photosynthesis process. Further, cavitation, which means breaking of the water column in the xylem, would also occur. This would result in the non-transferring of the water from the roots through the xylem to the top parts of the plant. However, due to the closing of stomata, cavitation is restrained and water is being able to travel through the xylem cell easily (Willmer 1993). However, it should also be kept in mind that the water loss is beneficial for the plants as it allows them to get minerals and nutrients from the soil. Further, the stomata opens during the photosynthesis process, wherein it helps in taking in the carbon dioxide inside the leaf and giving off water vapour. The photosynthesis process gets hampered when the stomata closes in order to prevent too much water loss (Morison 2003). Reference: Holbrook, N. M., Burns, M. J. & Field, C. B. 1999, ‘Negative Xylem Pressures in Plants: A Test of the Balancing Pressure Technique’, Science, vol. 270, 1183–1192. Langensiepen, Matthias 2008, 'Transpiration: Scaling from Leaves and Canopies', Encyclopedia of Water Science, Second Edition, Taylor & Francis. Morison, J. I. L. 2003, ‘Plant Water Use, Stomatal Control’, Encyclopedia of Water Science, Taylor & Francis. Raven, P. H., Ray F. E. & Susan, E. E. 1999, ‘Biology of Plants,’ 6th edition, New York: W. H. Freeman and Company. Sperry, J. S. 2003, 'Evolution of water transport and xylem structure,' Int. J. Plant Science, vol. 164, no. 3, S115-S127. Taiz, L. & Eduardo, Z. 1998, ‘Plant Physiology,’ 2nd edition, Sunderland, MA: Sinauer Associates. Willmer, C. M. 1993, ‘The evolution, structure and functioning of stomata’, Botanical Journal of Scotland, vol. 46, no. 3, 433 – 445. Read More