BIOLOGY TRANSPORT IN PLANTS
Transpiration, Root Pressure
Contents Ascent of Sap ...... 3 Transpiration ...... 7
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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.
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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.
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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.
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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.
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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.
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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.
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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.
The increase and decrease in the osmotic concentration of guard cells is explained by the theories listed below: i. Photosynthetic Theory ii. Starch Sugar Hypothesis iii. Active K+ Transport/Potassium Pump Theory
Studies of guard cell physiology reveal that red light induces accumulation of sucrose which causes osmoregulation and blue light regulates stomatal movement by changing the osmoregulation of guard cells.
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Factors Affecting the Rate of Transpiration
External Factors Intensity of sunlight On a bright sunny day, stomata open fully, so transpiration is increased. On a cloudy day, stomata open partially, so transpiration is reduced. At night, stomata close; hence, transpiration is greatly reduced or negligible. Temperature Increase in the temperature of air increases the rate of transpiration. Velocity of wind Transpiration increases with rapid or active air movement. Humidity If the air is humid, the rate of transpiration is reduced.
Concentration of Increase in the CO2 level in the atmosphere over normal 0.03% causes carbon dioxide stomatal closure. Hence, it decreases the rate of transpiration. Atmospheric With decrease in atmospheric pressure, the rate of transpiration increases. pressure Available soil water If the available water in the soil is not sufficient, the rate of transpiration is decreased. A high concentration of salts in the soil water also reduces the rate of transpiration because of less absorption of water. Internal Factors Root–shoot ratio If the root–shoot ratio decreases, the rate of transpiration also decreases. Leaf structure Many leaf characteristics such as thick cuticle, waxy coating, thick-walled hypodermis, compact mesophyll cells, reduction in the number of stomata and sunken stomata reduce the rate of transpiration. Age of plants Rate of transpiration is slow at the seedling stage, maximum at maturity and gradually decreases when near senescence.
In succulents such as Opuntia and Bryophyllum, stomata open during the night and close during the day. This is called scotoactive opening. These plants show formation of malic acid during the night time.
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Significance of Transpiration
Absorption of water and ascent of sap to various parts of the plant body are due to transpiration.
Transpiration pull brings about mass movement of water and solutes in the upward direction.
Transpiration helps in dissipating excess solar energy by evaporating water from the leaf surface and thus helps in cooling the plant body.
It results in the development of mechanical tissues, growth of the root system, increase in ash and sugar content of fruits and development of resistance.
Many anti-transpirant chemicals such as phenyl mercuric acetate and abscisic acid have been found to reduce the rate of transpiration without
affecting the rate of CO2 uptake.
Role of Transpiration in Plants Helps in circulating water in the plant body Controls the rate of absorption of water by the roots Regulates the temperature of plant body by evaporative cooling Supplies water for metabolic processes such as photosynthesis and respiration Maintains the shape and structure of plant parts by maintaining the turgidity of cells
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Transpiration and Photosynthesis – A Compromise There exists an intimate relationship between transpiration and photosynthesis. Most of the water absorbed by plants is lost in the atmosphere through transpiration. The humidity of rainforests is mostly because of this process where water is recycled from root to leaf to the atmosphere and back to the soil. This kind of water cycle helps to maintain a continuous supply of water to the leaves which is required for photosynthesis. If the supply of water is reduced because of the slow rate of transpiration, the rate of photosynthesis also becomes limited.
The evolution of the C4 pathway is one of the strategies of maximising the availability of CO2 while minimising the loss of water.
C4 plants are twice as efficient as C3 plants with respect to carbon fixation but lose only half as much
water as a C3 plant for the same amount of CO2 fixed.
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Guttation Sometimes, because of high root pressure, water is forced out through pores at the end of leaf veins which are called hydathodes. The water appears as tiny drops along the margins or the tip of leaves. This loss of excessive water is called guttation.
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