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Transpiration, Root Pressure BIOLOGY TRANSPORT IN PLANTS Transpiration, Root Pressure Contents Ascent of Sap ............................................................................................................................................. 3 Transpiration ............................................................................................................................................... 7 Go to Top www.topperlearning.com 2 BIOLOGY TRANSPORT IN PLANTS Ascent of Sap The upward conduction of water in the form of a dilute solution of mineral ions from the roots through the stem to the aerial parts of plants is called the ascent of sap. Several theories were put forward to explain the mechanism of the ascent of sap. These theories were placed under the following categories: i. Vital force theories ii. Root pressure theory iii. Physical force theories Vital Force Theories According to vital force theories, living cells are responsible for the ascent of sap. Some vital force theories: i. Westermaier Theory ii. Godlewski’s Relay Pump Theory iii. Bose’s Pulsation Theory Westermaier Theory Westermaier suggested that the living component of the xylem, the xylem parenchyma, is responsible for the conduction of water, while the tracheids and vessels act as a reservoir of water. Godlewski’s Relay Pump Theory Godlewski’s relay pump theory is also known as the clambering theory. According to this theory, conduction of water takes place because of the activity of xylem parenchyma and medullary rays. When the osmotic pressure of these cells is high, water is absorbed from the surrounding vessels. This results in increased turgor pressure. An increase in turgor pressure pumps the water to the next level of xylem vessels in a staircase-like manner. Bose’s Pulsation Theory Sir J. C. Bose proposed that the pulsation movement which occurs in the cortical cells present just outside the endodermis is responsible for the ascent of sap. A scientist named Molish supported the pulsation theory of J. C. Bose and introduced a detailed description of the pulsation theory. According to him, the pulsatory activity increased to 14 seconds by the application of narcotic drugs to plants. Go to Top www.topperlearning.com 3 BIOLOGY TRANSPORT IN PLANTS Root Pressure Theory When a potted plant is cut below the first leaf and a manometer filled with water is attached on the cut stem, the level of water in the manometer rises. This demonstrates that the water is being pushed by the roots along the stem after it is absorbed. If a stem is cut near its base, xylem sap is seen to flow out. This phenomenon is called exudation or bleeding. Priestly proposed that the upward flow of water in bleeding is due to the heavy pressure exerted by the roots. This pressure is termed root pressure. Root pressure is the hydrostatic pressure developed in the roots because of the continued inward movement of water through cell-to-cell osmosis which helps in the ascent of cell sap upwards through the stem. Root pressure develops because of active absorption which depends on the active accumulation of solute in xylem sap. Root pressure usually develops during the night when absorption is maximum and transpiration is minimum. Root pressure can be inhibited by using cyanide, lack of O2 and low temperature. Drawbacks of Root Pressure Theory Root pressure can generate a pressure of about 2 atm which is insufficient to raise the water up in tall trees. In plants such as conifers, root pressure has never been observed. In temperate regions, root pressure is generally low during summer when the rate of transpiration is high as compared to the rate of absorption. The ascent of sap continues even in the absence of root pressure. Go to Top www.topperlearning.com 4 BIOLOGY TRANSPORT IN PLANTS Root pressure helps to re-establish the continuous chain of water molecules in the xylem which often breaks under the enormous tension created by transpiration. Physical Force Theories According to physical force theories, living cells do not take part in the translocation of water. The ascent of sap takes place through the vessels and tracheids because of some physical forces developing in the dead cells. Some physical force theories: i. Capillarity Theory ii. Imbibition Force Theory iii. Cohesion–Tension Theory Capillarity Theory The capillarity theory was proposed by Boehm. According to this theory, the vessels and tracheids which are present in the xylem function as capillaries and water rises by surface tension in their capillaries and partly because of atmospheric pressure. Imbibition Force Theory The imbibition force theory was developed by Von Sachs. According to this theory, water moves upwards because of the force of imbibitions between the cell wall of the xylem and not through the lumen of the xylem vessels. Cohesion–Tension Theory The cohesion–tension theory is also known as the water column theory or the cohesive force theory. This theory was proposed by Dixon and Jolly. The cohesion–tension theory is based on the following facts: i. Transpiration Pull ii. Cohesion Force iii. Adhesive Force Transpiration Pull Because of the continuous loss of water from the leaves due to transpiration, the diffusion pressure deficit always remains higher in the mesophyll cells. As a result, the mesophyll cells absorb water from the adjacent internal mesophyll cells and compensate for the loss of water. This loss causes a water deficit in the xylem. Rapid transpiration develops a pull or tension in the xylem called the transpiration pull. It is about 20 atm. Transpiration pull is called negative pressure because it develops from the aerial parts and progresses to the underground parts of plants. Cohesion Force of Water A force of mutual attraction present between the water molecules is called cohesive force. Water molecules are held together continuously by a cohesive force to form a continuous water column. This cohesive force is up to 45–270 atm. Go to Top www.topperlearning.com 5 BIOLOGY TRANSPORT IN PLANTS Adhesive Force of Water Water molecules are also attached with the walls of the vessels and tracheids through a force called adhesive force. Cohesive and adhesive forces work together to maintain the continuity in between water and the cell wall. The forces are responsible for maintaining unbroken continuity of the water column from the roots to the leaves. This water column is pulled upwards continuously without breaking, from the roots to the leaves by transpiration. Go to Top www.topperlearning.com 6 BIOLOGY TRANSPORT IN PLANTS Transpiration The loss of water in the form of vapour from the aerial parts of the plant is called transpiration. Of all the water absorbed by plants, approximately 95% is lost by transpiration, while 5% or less is used by the plant. Transpiration is useful for plants for two reasons: i. It creates a suction force in the stem which enables roots to absorb water and minerals. ii. It helps in cooling the plant in hot weather. The loss of water through transpiration is so much that it reduces the water level and can lead to the death of the plant. However, transpiration is essential for water and mineral absorption, ascent of sap and lowering the temperature. Hence, transpiration is called a necessary evil (Curtis) or an unavoidable evil (Steward). Types of Transpiration Stomatal Transpiration •Water vapour escapes through the stomata of the leaf. Cuticular Transpiration •Transpiration occurs directly from the surface of the leaves and stems. Lenticular Transpiration •Transpiration occurs through lenticels present on old stems. Go to Top www.topperlearning.com 7 BIOLOGY TRANSPORT IN PLANTS Bark transpiration occurs through the bark of a woody stem. It contributes to about 1% of the total transpiration. Stomatal Transpiration Stomata are tiny apertures found on the epidermis of leaves and young green stems. Each stoma is surrounded by two epidermal bean-shaped guard cells. The inner wall of the guard cell is thick and elastic, whereas the outer wall is thin. Opening of stomata is aided because of the orientation of the microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radially rather than longitudinally which makes it easier for the stoma to open. Guard cells are bordered by one or more modified epidermal cells called subsidiary or accessory cells. The number of stomata may range from 1,000 to 10,000 per cm2. Go to Top www.topperlearning.com 8 BIOLOGY TRANSPORT IN PLANTS Mechanism of Opening and Closing of Stomata The surfaces of spongy mesophyll cells in the leaf are exposed to intercellular spaces. These cells give out a thin film of water. Water from this film evaporates. Water vapour formed saturates the air in the intercellular spaces, diffuses into the connecting intercellular spaces and reaches the sub-stomatal space. Finally, it escapes in the atmosphere through the air. Water absorbed by the roots rises through the stem and reaches the tissues of leaves. The cell sap in each cell exerts a turgor pressure outward on the cell wall. This pressure forces some water out of the cell wall into the air space between the cells, i.e. intercellular space. Here, the water evaporates and the water vapour from the intercellular spaces diffuses into the sub- stomatal space from where it finally goes out by diffusion through the stomata. There are more stomata on the undersurface of a dicot leaf; therefore, more transpiration occurs from the undersurface. Go to Top www.topperlearning.com 9 BIOLOGY TRANSPORT IN PLANTS Stomatal Regulation of Transpiration The opening and closing of stomata is regulated by the guard cells. During photosynthesis, the osmotic pressure of the contents of the guard cells increases. They absorb more water and become turgid. This makes guard cells more arched outwards, opening the stomata. Stomata allow transpiration, and at the same time, the exchange of gases required for photosynthesis. At night, when there is less water in leaves, the guard cells turn flaccid and the stomata close, and thus, transpiration stops.
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