Biology

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Mineral Absorption and its Various Types

Minerals in soil exist in two forms, either dissolved in soil solution or adsorbed by colloidal clay particle. Previously, it was mistakenly assumed that absorption of mineral salts from soil took place along with absorption of water. But absorption of minerals and ascent of sap are identified as two independent processes. Minerals are absorbed not only by root hairs but also by the cells of epiblema.

Plasma membrane of root cells are not permeable to all ions and also all ions of same salt are not absorbed in equal rate. Penetration and accumulation of ions into living cells or tissues from surrounding medium by crossing membrane is called mineral absorption. Movement of ions into and out of cells or tissues is termed as transport or flux.

Entry of the ion into cell is called influx and exit is called efflux. Various theories have been put forward to explain this mechanism. They are categorized under passive mechanisms (without the involvement of metabolic energy) and active mechanisms (involvement of metabolic energy).

Passive Absorption

1. Ion-Exchange:

Ions of external soil solution were exchanged with same charged (anion for anion or cation for cation) ions of the root cells. There are two theories explaining this process of ion exchange namely:

• Contact Exchange and
• Carbonic acid Exchange

Contact Exchange Theory:

According to this theory, the ions adsorbed on the surface of root cells and clay particles (or clay micelles) are not held tightly but oscillate within a small volume of space called oscillation volume. Due to small space, both ions overlap each other’s oscillation volume and exchange takes place (Figure 11.23).

Carbonic Acid Exchange Theory:

According to this theory, soil solution plays an important role by acting as a medium for ion exchange. The CO₂ released during respiration of root cells combines with water to form carbonic acid (H₂CO₃). Carbonic acid dissociates into H⁺ and HCO₃^– in the soil solution.

These H⁺ ions exchange with cations adsorbed on clay particles and the cations from micelles get released into soil solution and gets adsorbed on root cells (Figure 11.24).

Active Absorption

Absorption of ions against the concentration gradient with the expenditure of metabolic energy is called active absorption. In plants, the vacuolar sap shows accumulation of anions and cations against the concentration gradient which cannot be explained by theories of passive absorption. Mechanism of active absorption of salts can be explained through carrier concept.

Carrier Concept:

This concept was proposed by Van den Honert in 1937. The cell membrane is largely impermeable to free ions. However, the presence of carrier molecules in the membrane acts as a vehicle to pick up or bind with ions to form carrier-ion-complex, which moves across the membrane. On the inner surface of the membrane, this complex breaks apart releasing ions into cell while carrier goes back to the outer surface to pick up fresh ions (Figure 11.25).

The concept can be explained using two theories:

1. Lundegardh’s Cytochrome Pump Theory:

Lundegardh and Burstrom (1933) observed a correlation between respiration and anion absorption. When a plant is transferred from water to a salt solution the rate of respiration increases which is called as anion respiration or salt respiration. Based on this observation Lundegardh (1950 and 1954) proposed cytochrome pump theory which is based on the following assumptions:

• The mechanism of anion and cation absorption are different.
• Anions are absorbed through cytochrome chain by an active process, cations are absorbed passively.
• An oxygen gradient responsible for oxidation at the outer surface of the membrane and reduction at the inner surface.

According to this theory, the enzyme dehydrogenase on inner surface is responsible for the formation of protons (H⁺) and electrons (e^–). As electrons pass outward through electron transport chain there is a corresponding inward passage of anions.

Anions are picked up by oxidized cytochrome oxidase and are transferred to other members of chain as they transfer the electron to the next component (Figure 11.26).

The theory assumes that cations (C⁺) move passively along the electrical gradient created by the accumulation of anions (A^–) at the inner surface of the membrane. Main defects of the above theory are:

• Cations also induce respiration.
• Fails to explain the selective uptake of ions.
• It explains absorption of anions only.

2. Bennet-Clark’s Protein-Lecithin Theory:

In 1956, Bennet-Clark proposed that the carrier could be a protein associated with phosphatide called as lecithin. The carrier is amphoteric (the ability to act either as an acid or a base) and hence both cations and anions combine with it to form Lecithinion complex in the membrane. Inside the membrane, Lecithin-ion complex is broken down into phosphatidic acid and choline along with the liberation of ions.

Lecithin again gets regenerated from phosphatidic acid and choline in the presence of the enzyme choline acetylase and choline esterase (Figure 11.27). ATP is required for regeneration of lecithin.

Donnan Equilibrium

Within the cell, some of the ions never diffuse out through the membrane. They are trapped within the cell and are called fixed ions. But they must be balanced by the ions of opposite charge. Assuming that a concentration of fixed anions is present inside the membrane, more cations would be absorbed in addition to the normal exchange to maintain the equilibrium.

Therefore, the cation concentration would be greater in the internal than in the external solution. This electrical balance or equilibrium controlled by electrical as well as diffusion phenomenon is known as the Donnan equilibrium.

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Translocation of Organic Solutes

Leaves synthesize food material through photosynthesis and store in the form of starch grains. When required the starch is converted into simple sugars. They must be transported to various parts of the plant system for further utilization. However, the site of food production (leaves) and site of utilization are separated far apart. Hence, the organic food has to be transported to these areas.

The phenomenon of food transportation from the site of synthesis to the site of utilization is known as translocation of organic solutes. The term solute denotes food material that moves in a solution.

Path of Translocation

It has now been well established that phloem is the path of translocation of solutes. Ringing or girdling experiment will clearly demonstrate the translocation of solute by phloem.

Ringing or Girdling Experiment

The experiment involves the removal of all the tissue outside to vascular cambium (bark, cortex, and phloem) in woody stems except xylem. Xylem is the only remaining tissue in the girdled area which connects upper and lower part of the plant.

This setup is placed in a beaker of water. After some time, it is observed that a swelling on the upper part of the ring appears as a result of the accumulation of food material (Figure 11.20).

If the experiment continues within days, the roots die first. It is because, the supply of food material to the root is cut down by the removal of phloem. The roots cannot synthesize their food and so they die first. As the roots gradually die the upper part (stem), which depends on root for the ascent of sap, will ultimately die.

Direction of Translocation

Phloem translocates the products of photosynthesis from leaves to the area of growth and storage, in the following directions.

Downward Direction:
From leaves to stem and roots.

Upward Direction:
From leaves to developing buds, flowers, fruits for consumption and storage. Germination of seeds is also a good example of upward translocation.

Radial Direction:
From cells of pith to cortex and epidermis, the food materials are radially translocated.

Source and Sink

Source is defined as any organ in plants which are capable of exporting food materials to the areas of metabolism or to the areas of storage. Examples: Mature leaves, germinating seeds.

Sink is defined as any organ in plants which receives food from source. Example: Roots, tubers, developing fruits and immature leaves (Figure 11.21).

Phloem Loading

The movement of photosynthates (products of photosynthesis) from mesophyll cells to phloem sieve elements of mature leaves is known as phloem loading. It consists of three steps.

1. Sieve tube conducts sucrose only. But the photosynthate in chloroplast mostly in the form of starch or triose-phosphate which has to be transported to the cytoplasm where it will be converted into sucrose for further translocation.
2. Sucrose moves from mesophyll to nearby sieve elements by short distance transport.
3. From sieve tube to sink by long-distance transport.

Phloem Unloading

From sieve elements sucrose is translocated into sink organs such as roots, tubers, flowers and fruits and this process is termed as phloem unloading. It consists of three steps:

1. Sieve Element Unloading:
Sucrose leave from sieve elements.

2. Short Distance Transport:
Movement of sucrose to sink cells.

3. Storage and Metabolism:
The final step when sugars are stored or metabolized in sink cells.

Mechanism of Translocation

Several hypotheses have been proposed to explain the mechanism of translocation. Some of them are given below:

1. Diffusion Hypothesis

As in diffusion process, this theory states the translocation of food from higher concentration (from the place of synthesis) to lower concentration (to the place of utilization) by the simple physical process. However, the theory was rejected because the speed of translocation is much higher than simple diffusion and translocation is a biological process which any poison can halt.

2. Activated Diffusion Theory

This theory was first proposed by Mason and Maskell (1936). According to this theory, the diffusion in sieve tube is accelerated either by activating the diffusing molecules or by reducing the protoplasmic resistance to their diffusion.

3. Electro-Osmotic Theory

The theory of electro osmosis was proposed by Fenson (1957) and Spanner (1958). According to this, an electric-potential across the sieve plate causes the movement of water along with solutes. This theory fails to explain several problems concerning translocation.

4. Munch Mass Flow Hypothesis

Mass flow theory was first proposed by Munch (1930) and elaborated by Craft (1938). According to this hypothesis, organic substances or solutes move from the region of high osmotic pressure (from mesophyll) to the region of low osmotic pressure along the turgor pressure gradient. The principle involved in this hypothesis can be explained by a simple physical system as shown in figure 11.22.

Two chambers “A” and “B” made up of semipermeable membranes are connected by tube “T” immersed in a reservoir of water. Chamber “A” contains highly concentrated sugar solution while chamber “B” contains dilute sugar solution. The following changes were observed in the system,

(i) The high concentration sugar solution of chamber “A” is in a hypertonic state which draws water from the reservoir by endosmosis.

(ii) Due to the continuous entry of water into chamber “A”, turgor pressure is increased.

(iii) Increase in turgor pressure in chamber “A” force, the mass flow of sugar solution to chamber “B” through the tube “T” along turgor pressure gradient.

(iv) The movement of solute will continue till the solution in both the chambers attains the state of isotonic condition and the system becomes inactive.

(v) However, if new sugar solution is added in chamber “A”, the system will start to run again. A similar analogous system as given in the experiment exists in plants:

Chamber “A” is analogous to mesophyll cells of the leaves which contain a higher concentration of food material in soluble form. In short “A” is the production point called “source”. Chamber “B” is analogous to cells of stem and roots where the food material is utilized.

In short “B” is consumption end called “sink”. Tube “T” is analogous to the sieve tube of phloem.

Mesophyll cells draw water from the xylem (reservoir of the experiment) of the leaf by endosmosis leading to increase in the turgor pressure of mesophyll cell.

The turgor pressure in the cells of stem and the roots are comparatively low and hence, the soluble organic solutes begin to flowen masse from mesophyll through the phloem to the cells of stem and roots along the gradient turgor pressure.

In the cells of stem and roots, the organic solutes are either consumed or converted into insoluble form and the excess water is released into xylem (by turgor pressure gradient) through cambium.

Merits:

1. When a woody or herbaceous plant is girdled, the sap contains high sugar containing exudates from cut end.
2. Positive concentration gradient disappears when plants are defoliated.

Objections:

1. This hypothesis explains the unidirectional movement of solute only. However, bidirectional movement of solute is commonly observed in plants.
2. Osmotic pressure of mesophyll cells and that of root hair do not confirm the requirements.
3. This theory gives passive role to sieve tube and protoplasm, while some workers demonstrated the involvement of ATP.

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Transpiration – Factors and its Various Types

Water absorbed by roots ultimately reaches the leaf and gets released into the atmosphere in the form of vapour. Only a small fraction of water (less than 5%) is utilized in plant development and metabolic process. The loss of excess of water in the form of vapour from various aerial parts of the plant is called transpiration. Transpiration is a kind of evaporation but differs by the involvement of biological system. The amount of water transpired is astounding (Table 11.4). The water may move through the xylem at a rate as fast as 75cm/min.

Rate of Transpiration in Some Plants

Types of Transpiration
Transpiration is of following three types:

1. Stomatal Transpiration

Stomata are microscopic structures present in high number on the lower epidermis of leaves. This is the most dominant form of transpiration and being responsible for most of the water loss (90 – 95%) in plants.

2. Lenticular Transpiration

In stems of woody plants and trees, the epidermis is replaced by periderm because of secondary growth. In order to provide gaseous exchange between the living cells and outer atmosphere, some pores which looks like lens-shaped raised spots are present on the surface of the stem called Lenticels. The loss of water from lenticels is very insignificant as it amounts to only 0.1% of the total.

3. Cuticular Transpiration

The cuticle is a waxy or resinous layer of cutin, a fatty substance covering the epidermis of leaves and other plant parts. Loss of water through cuticle is relatively small and it is only about 5 to 10% of the total transpiration. The thickness of cuticle increases in xerophytes and transpiration is very much reduced or totally absent.

Structure of Stomata

The epidermis of leaves and green stems possess many small pores called stomata. The length and breadth of stomata is about 10-40µ and 3-10µ respectively. Mature leaves contain between 50 and 500 stomata per square mm.

Stomata are made up of two guard cells, special semi-lunar or kidneyshaped living epidermal cells in the epidermis. Guard cells are attached to surrounding epidermal cells known as subsidiary cells or accessory cells.

The guard cells are joined together at each end but they are free to separate to form a pore between them. The inner wall of the guard cell is thicker than the outer wall (Figure 11.14). The stoma opens to the interior into a cavity called sub-stomatal cavity which remains connected with the intercellular spaces.

Mechanism of Stomatal Movement

Stomatal movements are regulated by the change of turgor pressure in guard cells. When water enters the guard cell, it swells and its unevenly thickened walls stretch up resulting in the opening of stomata. This is due to concave non-elastic nature of inner wall pulled away from each other and stretching of the convex elastic natured outer wall of guard cell.

Different theories have been proposed regarding opening and closing of stomata. The important theories of stomatal movement are as follows,

1. Theory of Photosynthesis in guard cells
2. Starch – Sugar interconversion theory
3. Active potassium transport ion concept

1. Theory of Photosynthesis in Guard Cells Von Mohl

(1856) observed that stomata open in light and close in the night. According to him, chloroplasts present in the guard cells photosynthesize in the presence of light resulting in the production of carbohydrate (Sugar) which increases osmotic pressure in guard cells. It leads to the entry of water from other cell and stomatal aperture opens. The above process vice versa in night leads to closure of stomata.

Demerits

• Chloroplast of guard cells is poorly developed and incapable of performing photosynthesis.
• The guard cells already possess much amount of stored sugars.

2. Starch – Sugar Interconversion Theory

(i) According to Lloyd (1908), turgidity of guard cell depends on interconversion, of starch and sugar. It was supported by Loftild (1921) as he found guard cells containing sugar during the daytime when they are open and starch during the night when they are closed.

(ii) Sayre (1920) observed that the opening and closing of stomata depends upon change in pH of guard cells. According to him stomata open at high pH during day time and become closed at low pH at night. Utilization of CO₂ by photosynthesis during light period causes an increase in pH resulting in the conversion of starch to sugar.

Sugar increase in cell favours endosmosis and increases the turgor pressure which leads to opening of stomata. Likewise, accumulation of CO₂ in cells during night decrease the pH level resulting in the conversion of sugar to starch. Starch decreases the turgor pressure of guard cell and stomata close.

(iii) The discovery of enzyme phosphorylase in guard cells by Hanes (1940) greatly supports the starch-sugar interconversion theory. The enzyme phosphorylase hydrolyses starch into sugar and high pH followed by endosmosis and the opening of stomata during light. The vice versa takes place during the night.

(iv) Steward (1964) proposed a slightly modified scheme of starch-sugar interconversion theory. According to him, Glucose-1-phosphate is osmotically inactive. Removal of phosphate from Glucose-1-phosphate converts to Glucose which is osmotically active and increases the concentration of guard cell leading to opening of stomata (Figure 11.15).

Objections to Starch-sugar Interconversion Theory

• In monocots, guard cell does not have starch.
• There is no evidence to show the presence of sugar at a time when starch disappears and stomata open.
• It fails to explain the drastic change in pH from 5 to 7 by change of CO₂.

3. Theory of K⁺ Transport

This theory was proposed by Levit (1974) and elaborated by Raschke (1975). According to this theory, the following steps are involved in the stomatal opening:

In Light

• In guard cell, starch is converted into organic acid (malic acid).
• Malic acid in guard cell dissociates to malate anion and proton (H⁺).
• Protons are transported through the membrane into nearby subsidiary cells with the exchange of K⁺ (Potassium ions) from subsidiary cells to guard cells.
• This process involves an electrical gradient and is called ion exchange.
• This ion exchange is an active process and consumes ATP for energy.
• Increased K⁺ ions in the guard cell are balanced by Cl^– ions. Increase in solute concentration decreases the water potential in the guard cell.
• Guard cell becomes hypertonic and favours the entry of water from surrounding cells.
• Increased turgor pressure due to the entry of water opens the stomatal pore (Figure 11.16).

In Dark

• In dark, photosynthesis stops and respiration continues with accumulation of CO₂ in the sub-stomatal cavity.
• Accumulation of CO₂ in cell lowers the pH level.
• Low pH and a shortage of water in the guard cell activate the stress hormone Abscisic acid (ABA).
• ABA stops further entry of K⁺ ions and also induce K⁺ ions to leak out to subsidiary cells from guard cell.
• Loss of water from guard cell reduces turgor pressure and causes closure of stomata (Figure 11.17).

Factors Affecting Rate of Transpiration

The factors affecting the rate of transpiration can be categorized into two groups. They are:-

1. External or Environmental factors and
2. Internal or plant factors

1. External or Environmental Factors

(i) Atmospheric Humidity:
The rate of transpiration is greatly reduced when the atmosphere is very humid. As the air becomes dry, the rate of transpiration is also increased proportionately.

(ii) Temperature:
With the increase in atmospheric temperature, the rate of transpiration also increases. However, at very high-temperatures stomata closes because of flaccidity and transpiration stop.

(iii) Light:
Light intensity increases the temperature. As in temperature, transpiration is increased in high light intensity and is decreased in low light intensity. Light also increases the permeability of the cell membrane, making it easy for water molecules to move out of the cell.

(iv) Wind Velocity:
In still air, the surface above the stomata get saturated with water vapours and there is no need for more water vapour to come out. If the wind is breezy, water vapour gets carried away near leaf surface and DPD is created to draw more vapour from the leaf cells enhancing transpiration. However, high wind velocity creates an extreme increase in water loss and leads to a reduced rate of transpiration and stomata remain closed.

(v) Atmospheric Pressure:
In low atmospheric pressure, the rate of transpiration increases. Hills favour high transpiration rate due to low atmospheric pressure. However, it is neutralized by low temperature prevailing in the hills.

(vi) Water:
Adequate amount of water in the soil is a pre-requisite for optimum plant growth. Excessive loss of water through transpiration leads to wilting. In general, there are three types of wilting as follows,

a. Incipient Wilting:
Water content of plant cell decreases but the symptoms are not visible.

b. Temporary Wilting:
On hot summer days, the freshness of herbaceous plants reduces turgor pressure at the day time and regains it at night.

c. Permanent Wilting:
The absorption of water virtually ceases because the plant cell does not get water from any source and the plant cell passes into a state of permanent wilting.

2. Internal Factors

(i) Leaf Area:
If the leaf area is more, transpiration is faster and so xerophytes reduce their leaf size.

(ii) Leaf Structure:
Some anatomical features of leaves like sunken stomata, the presence of hairs, cuticle, the presence of hydrophilic substances like gum, mucilage help to reduce the rate of transpiration. In xerophytes the structural modifications are remarkable. To avoid transpiration, as in Opuntia the stem is flattened to look like leaves called Phylloclade.

Cladode or cladophyll in Asparagus is a modified stem capable of limited growth looking like leaves. In some plants, the petioles are flattened and widened, to become phyllodes example Acacia melanoxylon.

Plant Antitranspirants

The term antitranspirant is used to designate any material applied to plants for the purpose of retarding transpiration. An ideal antitranspirant checks the transpiration process without disturbing the process of gaseous exchange. Plant antitranspirants are two types:

1. To act as a physical barrier above the stomata Colourless plastics, Silicone oil and low viscosity waxes are sprayed on leaves forming a thin film to act as a physical barrier (for transpiration) for water but permeable to CO₂ and O₂. The success rate of a physical barrier is limited.

2. Induction of Stomata Closure

Carbon-di-oxide induces stomatal closure and acts as a natural antitranspirant. Further, the advantage of using CO₂ as an antitranspirant is its inhibition of photorespiration. Phenyl Mercuric Acetate (PMA), when applied as a foliar spray to plants, induces partial stomatal closure for two weeks or more without any toxic effect. Use of abscisic acid highly induces the closing of stomata. Dodecenyl succinic acid also effects on stomatal closure.

Uses:

• Antitranspirants reduce the enormous loss of water by transpiration in crop plants.
• Useful for seedling transplantations in nurseries.

Guttation

During high humidity in the atmosphere, the rate of transpiration is much reduced. When plants absorb water in such a condition root pressure is developed due to excess water within the plant. Thus excess water exudates as liquid from the edges of the leaves and is called guttation. Example: Grasses, tomato, potato, brinjal and Alocasia.

Guttation occurs through stomata like pores called hydathodes generally present in plants that grow in moist and shady places. Pores are present over a mass of loosely arranged cells with large intercellular spaces called epithem (Figure 11.18).

This mass of tissue lies near vein endings (xylem and Phloem). The liquid coming out of hydathode is not pure water but a solution containing a number of dissolved substances.

Measurement of Transpiration

1. Ganongs Potometer

Ganongs potometer is used to measure the rate of transpiration indirectly. In this, the amount of water absorbed is measured and assumed that this amount is equal to the amount of water transpired.

Apparatus consists of a horizontal graduated tube which is bent in opposite directions at the ends. One bent end is wide and the other is narrow. A reservoir is fixed to the horizontal tube near the wider end. The reservoir has a stopcock to regulate water flow. The apparatus is filled with water from reservoir. A twig or a small plant is fixed to the wider arm through a split cock.

The other bent end of the horizontal tube is dipped into a beaker containing coloured water. An air bubble is introduced into the graduated tube at the narrow end (Figure 11.19). keep this apparatus in bright sunlight and observe.

As transpiration takes place, the air bubble will move towards the twig. The loss is compensated by water absorption through the xylem portion of the twig. Thus, the rate of water absorption is equal to the rate of transpiration.

2. Cobalt Chloride (CoCl₂) Paper Method

Select a healthy dorsiventral leaf and clean its upper and lower surface with dry cotton. Now place a dry Cobalt chloride (CoCl₂) strips on both surface and immediately cover the paper with glass slides and immobilize them.

It will be observed after some time that the CoCl₂ strip of lower epidermis turns pink. This indicates that CoCl₂ becomes hydrated (CoCl₂.2H₂O or CoCl₂.4H₂O) due to water vapours coming out through stomata. The rate of transpiration is more on the lower surface than in the upper surface of the dorsiventral leaf.

Significance of Transpiration

Transpiration leads to loss of water, as stated earlier in this lesson 95% of absorbed water is lost in transpiration. It seems to be an evil process to plants. However, number of process like absorption of water, ascent of sap and mineral absorption directly rely on the transpiration. Moreover plants withstand against scorching sunlight due to transpiration. Hence the transpiration is a “necessary evil” as stated by Curtis.

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Ascent of Sap and its Theory

In the last chapter, we studied about water absorption from roots to xylem in a lateral direction and here we will learn about the mechanism of distribution of water inside the plant. Like tributaries join together to form a river, millions of root hairs conduct a small amount of water and confluence in xylem, the superhighway of water conduction.

Xylem handles a large amount of water to conduct to many parts in an upward direction. The water within the xylem along with dissolved minerals from roots is called sap and its upward transport is called ascent of sap.

The Path of Ascent of Sap

There is no doubt; water travels up along the vascular tissue. But vascular tissue has two components namely Xylem and Phloem. Of these two, which is responsible for the ascent of sap? The following experiment will prove that xylem is the only element through which water moves up.

Cut a branch of balsam plant and place it in a beaker containing eosin (red colour dye) water. After some time, a red streak appears on the stem indicating the ascent of water. Remove the plant from water and cut a transverse section of the stem and observe it under the microscope. Only xylem element is coloured red, which indicates the path of water is xylem. Phloem is not colored indicating that it has no role in the ascent of sap (Figure 11.12).

Mechanism of Ascent of Sap

In ascent of sap, the biggest challenge is the force required to lift the water to the top of the tallest trees. A number of theories have been put forward to explain the mechanism of the ascent of sap. They are:-

A. Vital force theories
B. Root pressure theory, and
C. Physical force theory.

Vital Force Theories

According to vital force theories, living cells are mandatory for the ascent of sap. Based on this the following two theories derived:

1. Relay Pump Theory of Godlewski (1884)

Periodic changes in osmotic pressure of living cells of the xylem parenchyma and medullary ray act as a pump for the movement of water.

2. Pulsation Theory of J.C.Bose (1923)

Bose invented an instrument called Crescograph, which consists of an electric probe connected to a galvanometer (Figure 11.13). When a probe is inserted into the inner cortex of the stem, the galvanometer showed high electrical activity.

Bose believed a rhythmic pulsating movement of inner cortex like a pump (similar to the beating of the heart) is responsible for the ascent of sap. He concluded that cells associated with xylem exhibit pumping action and pumps the sap laterally into xylem cells.

Objections to Vital Force Theories

(i) Strasburger (1889) and Overton (1911) experimentally proved that living cells are not mandatory for the ascent of sap. For this, the selected an old oak tree trunk which when immersed in picric acid and subjected to excessive heat killed all the living cells of the trunk. The trunk when dipped in water, the ascent of sap took place.

(ii) Pumping action of living cells should be in between two xylem elements (vertically) and not on lateral sides.

Root Pressure Theory

If a plant which is watered well is cut a few inches above the ground level, sap exudes out with some force. This is called sap exudation or bleeding. Stephen Hales, father of plant physiology observed this phenomenon and coined the term ‘Root Pressure’.

Stoking (1956) defined root pressure as “a pressure developing in the tracheary elements of the xylem as a result of metabolic activities of the root”. But the following objections have been raised against root pressure theory:

1. Root pressure is totally absent in gymnosperms, which includes some of the tallest plants.
2. There is no relationship between the ascent of sap and root pressure.
3. For example, in summer, the rate of the ascent of sap is more due to transpiration in spite of the fact that root pressure is very low.
4. On the other hand, in winter when the rate of ascent of sap is low, a high root pressure is found.
5. Ascent of sap continues even in the absence of roots.
6. The magnitude of root pressure is about 2atm, which can raise the water level up to few feet only, whereas the tallest trees are more than 100m high.

Physical Force Theory

Physical force theories suggest that ascent of sap takes place through the dead xylem vessel and the mechanism is entirely physical and living cells are not involved.

1. Capillary Theory

Boehm (1809) suggested that the xylem vessels work like a capillary tube. This capillarity of the vessels under normal atmospheric pressure is responsible for the ascent of sap. This theory was rejected because the magnitude of capillary force can raise water level only up to a certain height. Further, the xylem vessels are broader than the tracheid which actually conducts more water and against the capillary theory.

2. Imbibition Theory

This theory was first proposed by Unger (1876) and supported by Sachs (1878). This theory illustrates, that water is imbibed through the cell wall materials and not by the lumen. This theory was rejected based on the ringing experiment, which proved that water moves through the lumen of the cell and not by a cell wall.

3. Cohesion-tension or Cohesion and Transpiration Pull Theory

Cohesion-tension theory was originally proposed by Dixon and Jolly (1894) and again put forward by Dixon (1914, 1924). This theory is based on the following features:

(i) Strong cohesive force or tensile strength of water

Water molecules have the strong mutual force of attraction called cohesive force due to which they cannot be easily separated from one another. Further, the attraction between a water molecule and the wall of the xylem element is called adhesion.

These cohesive and adhesive force works together to form an unbroken continuous water column in the xylem. The magnitude of the cohesive force is much high (350 atm) and is more than enough to ascent sap in the tallest trees.

(ii) Continuity of the water column in the plant

An important factor which can break the water column is the introduction of air bubbles in the xylem. Gas bubbles expanding and displacing water within the xylem element is called cavitation or embolism. However, the overall continuity of the water column remains undisturbed since water diffuses into the adjacent xylem elements for continuing ascent of sap.

(iii) Transpiration Pull or Tension in the Unbroken Water Column

The unbroken water column from leaf to root is just like a rope. If the rope is pulled from the top, the entire rope will move upward. In plants, such a pull is generated by the process of transpiration which is known as transpiration pull. Water vapour evaporates from mesophyll cells to the intercellular spaces near stomata as a result of active transpiration. The water vapours are then transpired through the stomatal pores.

Loss of water from mesophyll cells causes a decrease in water potential. So, water moves as a pull from cell to cell along the water potential gradient. This tension, generated at the top (leaf) of the unbroken water column, is transmitted downwards from petiole, stem and finally reaches the roots. The cohesion theory is the most accepted among the plant physiologists today.

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Absorption of Water

Terrestrial plants have to absorb water from the soil to maintain turgidity, metabolic activities and growth. Absorption of water from soil takes place in two steps:

1. From soil to root hairs – either actively or passively.
2. From root hairs further transport in the lateral direction to reach xylem, the superhighway of water transport.

Water Absorbing Organs

Usually, absorption of water occurs in plants through young roots. The zone of rapid water absorption is root hairs. They are delicate structures which get continuously replaced by new ones. Root hairs are unicellular extensions of epidermal cells without cuticle. Root hairs are extremely thin and numerous and they provide a large surface area for absorption (Figure 11.10).

Path of Water Across Root Cells

Water is first absorbed by root hair and other epidermal cells through imbibition from soil and moves radially and centripetally across the cortex, endodermis, pericycle and finally reaches xylem elements osmotically. There are three possible routes of water (Figure 11.11).
They are:-

1. Apoplast
2. Symplast
3. Transmembrane route.

1. Apoplast

The apoplast (Greek: apo = away; plast = cell) consists of everything external to the plasma membrane of the living cell. The apoplast includes cell walls, extra cellular spaces and the interior of dead cells such as vessel elements and tracheids. In the apoplast pathway, water moves exclusively through the cell wall or the non living part of the plant without crossing any membrane. The apoplast is a continuous system.

2. Symplast

The symplast (Greek: sym = within; plast = cell) consists of the entire mass of cytosol of all the living cells in a plant, as well as the plasmodesmata, the cytoplasmic channel that interconnects them.

In the symplastic route, water has to cross plasma membrane to enter the cytoplasm of outer root cell; then it will move within adjoining cytoplasm through plasmodesmata around the vacuoles without the necessity to cross more membrane, till it reaches xylem.

3. Transmembrane Route

In transmembrane pathway water sequentially enters a cell on one side and exits from the cell on the other side. In this pathway, water crosses at least two membranes for each cell. Transport across the tonoplast is also involved.

Mechanism of Water Absorption

Kramer (1949) recognized two distinct mechanisms which independently operate in the absorption of water in plants. They are:-

1. Active Absorption
2. Passive Absorption

1. Active Absorption

The mechanism of water absorption due to forces generated in the root itself is called active absorption. Active absorption may be osmotic or non-osmotic.

(i) Osmotic Active Absorption

The theory of osmotic active absorption was postulated by Atkins (1916) and Preistley (1923). According to this theory, the first step in the absorption is soil water imbibed by cell wall of the root hair followed by osmosis. The soil water is hypotonic and cell sap is hypertonic. Therefore, soil water diffses into root hair along the concentration gradient (endosmosis).

When the root hair becomes fully turgid, it becomes hypotonic and water moves osmotically to the outer most cortical cell. In the same way, water enters into inner cortex, endodermis, pericycle and finally reaches protoxylem. As the sap reaches the protoxylem a pressure is developed known as root pressure. This theory involves the symplastic movement of water.

Objections to Osmotic Theory:

• The cell sap concentration in xylem is not always high.
• Root pressure is not universal in all plants especially in trees.

(ii) Non-Osmotic Active Absorption

Bennet-Clark (1936),Thmann (1951) and Kramer (1959) observed absorption of water even if the concentration of cell sap in the root hair is lower than that of the soil water. Such a movement requires an expenditure of energy released by respiration (ATP). This, there is a link between water absorption and respiration.

It is evident from the fact that when respiratory inhibitors like KCN, Chloroform are applied there is a decrease in the rate of respiration and also the rate of absorption of water.

2. Passive Absorption

In passive absorption, roots do not play any role in the absorption of water and is regulated by transpiration only. Due to transpiration, water is lost from leaf cells along with a drop in turgor pressure. It increases DPD in leaf cells and leads to withdrawal of water from adjacent xylem cells.

In xylem, a tension is developed and is transmitted downward up to root resulting in the absorption of water from the soil. In passive absorption (Table 11.3), the path of water may be symplastic or apoplastic. It accounts for about 98% of the total water uptake by plants.