Nitrogen Fixation – Definition, Types, Examples

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Nitrogen Fixation – Definition, Types, Examples

Inspiring act of nature is self-regulation. As all living organisms act as tools for biogeochemical cycles, nitrogen cycle is highly regulated. Life on earth depends on nitrogen cycle. Nitrogen occurs in atmosphere in the form of N2 (N≡N), two nitrogen atoms joined together by strong triple covalent bonds. The process of converting atmospheric nitrogen (N2) into ammonia is termed as nitrogen fixation. Nitrogen fixation can occur by two methods:

  1. Biological
  2. Non-Biological (Figure 12.5).

Nitrogen Fixation img 1

Non – Biological Nitrogen Fixation

  • Nitrogen fixation by chemical process in industry.
  • Natural electrical discharge during lightening fixes atmospheric nitrogen.

Biological Nitrogen Fixation

Symbiotic bacterium like Rhizobium fixes atmospheric nitrogen. Cyanobacteria found in Lichens, Anthoceros, Azolla and coralloid roots of Cycas also fix nitrogen. Non-symbiotic (free living bacteria) like Clostridium also fix nitrogen.

a. Symbiotic Nitrogen Fixation

(i) Nitrogen Fixation with Nodulation

Rhizobium bacterium is found in leguminous plants and fix atmospheric nitrogen. This kind of symbiotic association is beneficial for both the bacterium and plant. Root nodules are formed due to bacterial infection. Rhizobium enters into the host cell and proliferates, it remains separated from the host cytoplasm by a membrane (Figure 12.6).
Nitrogen Fixation img 2

Stages of Root Nodule Formation:

  1. Legume plants secretes phenolics which attracts Rhizobium.
  2. Rhizobium reaches the rhizosphere and enters into the root hair, infects the root hair and leads to curling of root hairs.
  3. Infection thread grows inwards and separates the infected tissue from normal tissue.
  4. A membrane bound bacterium is formed inside the nodule and is called bacteroid.
  5. Cytokinin from bacteria and auxin from host plant promotes cell division and leads to nodule formation

Alnus and Casuarina contain the bacterium Frankia. Psychotria contains the bacterium Klebsiella.

(ii) Nitrogen Fixation Without Nodulation

The following plants and prokaryotes are involved in nitrogen fixation.

  • Lichens – Anabaena and Nostoc
  • Anthoceros – Nostoc
  • Azolla – Anabaena azollae
  • Cycas – Anabaena and Nostoc

b. Non-Symbiotic Nitrogen Fixation

Free living bacteria and fungi also fix atmospheric nitrogen.
Nitrogen Fixation img 3

Difference Between Hydroponics and Aeroponics

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Difference Between Hydroponics and Aeroponics

1. Hydroponics or Soilless culture:

Von Sachs developed a method of growing plants in nutrient solution. The commonly used nutrient solutions are Knop solution (1865) and Arnon and Hoagland Solution (1940). Later the term Hydroponics was coined by Goerick (1940) and he also introduced commercial techniques for hydroponics. In hydroponics roots are immersed in the solution containing nutrients and air is supplied with help of tube (Figure 12.3).
Hydroponics and Aeroponics img 1


This technique was developed by Soifer Hillel and David Durger. It is a system where roots are suspended in air and nutrients are sprayed over the roots by a motor driven rotor (Figure 12.4).
Hydroponics and Aeroponics img 2

Hydroponics and aeroponics are both methods of growing plants. The latter, aeroponics, is a method used to grow plants in the air – without the use of soil. Hydroponics is also a method that does not use soil, but instead, uses only a nutrient solution in a water solvent.

An advanced form of hydroponics, aeroponics is the process of growing plants with only water and nutrients. This innovative method results in faster growth, healthier plants, and bigger yields, all while using fewer resources. Plants grow in a soilless medium called rockwool.

In hydroponics we provide a solution in which plants can feed the amount they need when they want to. Because of this, we have more control over the speed and growth of our plants. Although this is a relatively new method of growing cannabis, it is cheaper than aeroponics.

Hydroponics offers the advantage of no energy wasted searching for nutrients. Aeroponic systems are a specialized version of hydroponics where the roots of the plant extend only in air and the roots are directly sprayed with a nutrient water mix (the recipe).

There are two main types of aeroponic systems: high pressure aeroponics and low pressure aeroponics. The main difference being the droplet size of the mist used in each case. Low-pressure aeroponics uses low-pressure, high-flow pumps, whereas high-pressure aeroponics uses high-pressure, low-flow pumps.

There are six main types of hydroponic systems to consider for your garden: wicking, deep water culture (DWC), nutrient film technique (NFT), ebb and flow, aeroponics, and drip systems.

The major disadvantage of aeroponics is the cost. Aeroponic systems are more expensive than most hydroponic systems and are completely dependent on a power source to run the air and nutrient pumps and the timer. Even a short interruption in power can result in the roots drying out and killing your plants.

One of the best advantages of Aeroponics is that plants grow quickly in such systems. They thrive and are able to produce great harvests. The plants grown are also much stronger and healthier due to this oxygen richness too.

In Aeroponic farming, the plantations can be vertical in the structure which helps the farmers to save a lot of space which in turn produces more food. There would be a great reduction in the disease and the infestation of pests. The Aeroponic farming can also be automated which reduces the cost of labour.

Fruits and Vegetables can also be grown comfortably in Aeroponics systems. Lot of vegetables and fruits can be grown like Beets, Broccoli, Cabbage, Carrots, Cauliflower, Corn, Cucumber, Eggplant, Grapes, Melons, Onions, Peas, Peppers, Potatoes, Radish, Raspberry, Strawberry, Sweet Potato, Tomatoes, and Watermelon.

Both hydroponics and aquaponics have clear benefits over soil-based gardening: lessened, adverse environmental impacts, reduced consumption of resources, faster plant growth, and higher yields. Many believe that aquaponics is a better option over hydroponics when choosing a soilless growing system.

Is hydroponics really good for the environment? Yes, hydroponics is good not just for the environment, but for several other reasons such as higher yield, water conservation and the removal of pesticides and herbicides.

You will have to mix up advanced nutrients for aeroponics of the proper strength consisting all of the required nutrients in the proper proportions (depending upon what your plant needs for its growth). You should not miss the Growing Exotic Hydroponic Plants.

Plants grown through hydroponics and aeroponics have the advantage natural and unrestricted growth. This system has enabled the cultivation of numerous plants that were previously considered difficult or impossible to grow from cuttings, as it becomes possible to propagate them from a single stem cutting.

There are vast numbers of people who have heard of hydroponics, and the majority of those know that systems can be set up indoors. In hydroponics, we can now provide all the light we need to plants to help them grow, so in this case, no they don’t need sunlight.

Critical Concentration and Toxicity of Minerals

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Critical Concentration and Toxicity of Minerals

Critical Concentration

To increase the productivity and also to avoid mineral toxicity knowledge of critical concentration is essential. Mineral nutrients lesser than critical concentration cause deficiency symptoms. Increase of mineral nutrients more than the normal concentration causes toxicity. A concentration, at which 10% of the dry weight of tissue is reduced, is considered as toxic. Figure 12.2 explains about Critical Concentration.
Critical Concentration and Toxicity of Minerals img 1

Mineral Toxicity

a. Manganese Toxicity

Increased Concentration of Manganese will prevent the uptake of Fe and Mg, prevent translocation of Ca to the shoot apex and cause their deficiency. The symptoms of manganese toxicity are appearance of brown spots surrounded by chlorotic veins.

b. Aluminium Toxicity

Aluminium toxicity causes precipitation of nucleic acid, inhibition of ATPase, inhibition of cell division and binding of plasma membrane with Calmodulin. For theories regarding, translocation of minerals please refer Chapter – 11.

Critical concentration. (Science: chemistry) The minimum concentration of units needed before a biological polymer will form. Examples of biopolymers are microtubules from tubulin units, polypeptides from amino acid units, polysaccharides from simple Sugar units, etc.

The term mineral toxicity refers to a condition during which the concentration within the body of anybody of the minerals necessary for all times is abnormally high, and which has an adverse effect on health.

Critical level or concentration is a term that is common in both soil and plant analysis. It is usually defined in plant analysis as the level that results in 90% of maximum yield or growth, which is also a reasonable division of the zones of adequacy and deficiency in the figure below.

These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel. Toxic Elements Any mineral ion concentration in tissues, that reduces the dry weight of tissues by about 10% is considered toxic. For example, Mn inhibit the absorption of other elements.

As a group, minerals are one of the four groups of essential nutrients, the others of which are vitamins, essential fatty acids, and essential amino acids. The five major minerals in the human body are calcium, phosphorus, potassium, sodium, and magnesium.

Critical nutrient range is defined as: that range of nutrient concentration above which we are reasonably confident the crop is amply supplied and below which we are reasonably confident the crop is deficient.

Soil pH affects nutrient availability by changing the form of the nutrient in the soil. Adjusting soil pH to a recommended value can increase the availability of important nutrients. Low pH reduces the availability of the macro- and secondary nutrients, while high pH reduces the availability of most micronutrients.

These symptoms include cardiac arrhythmias, headache, nausea and vomiting, and in severe cases, seizures. Calcium and phosphate: Calcium and phosphate are closely related nutrients.

Critical Concentration is the term which is used to define the concentration of essential elements below which the growth of plant is Retarded or Reduced. Also, if the concentration of essential elements rise above the critical concentrations it leads to toxicity.

Calcium is required by meristematic and differentiating tissues. During cell division it is used in the synthesis of cell wall, particularly as calcium pectate in the middle lamella. It is also needed during the formation of mitotic spindle. It accumulates in older leaves. The criteria of essentiality were stated by Arnon and Stout.

The three criteria of essentiality of an element are:

  1. Deficiency of the given element must cause some specific deficiency symptom so that the vegetative and reproductive stages of the life cycle of plant remain imcomplete.
  2. Such 8 deficiency symptom can be prevented or corrected only by supplying this element.

The element must be critical for the growth and development of the plant. The plant can not complete its life cycle or produce seeds in the absence of the element. The requirement for the element must be specific and not replaceable by another element.

The beneficial elements are not deemed essential for all crops but may be vital for particular plant taxa. The distinction between beneficial and essential is often difficult in the case of some trace elements. These elements are not critical for all plants but may improve plant growth and yield.

Deficiency Diseases and Symptoms

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Deficiency Diseases and Symptoms

The following table (Table 12.2) gives you an idea about Minerals and their Deficiency symptoms:
Deficiency Diseases and Symptoms img 2

Signs and symptoms of vitamin deficiency anemia include:

  • Fatigue
  • Shortness of Breath
  • Dizziness
  • Pale or yellowish Skin
  • Irregular Heartbeats
  • Weight Loss
  • Numbness or tingling in your hands and feet
  • Muscle Weakness

7 Nutrient Deficiencies:

That Are Incredibly Common

  • Iron deficiency. Iron is an essential mineral
  • Iodine Deficiency
  • Vitamin D Deficiency
  • Vitamin B12 Deficiency
  • Calcium Deficiency
  • Vitamin A Deficiency
  • Magnesium Deficiency

Any currently treated or untreated nutrient deficiency or disease. These include, but are not limited to, Protein Energy Malnutrition, Scurvy, Rickets, Beriberi, Hypocalcemia, Osteomalacia, Vitamin K Deficiency, Pellagra, Xerophthalmia, and Iron Deficiency.

Nutritional Deficiencies can lead to conditions such as anemia, scurvy, rickets.

  • Calcium
  • Magnesium
  • Omega-3 fatty acid
  • Folate
  • Potassium
  • Vitamin A
  • Vitamin E
  • Copper

Copper deficiency is more common among people with untreated celiac disease than the general population. Stopping behaviors that contribute to the deficiency, such as unhealthy eating, smoking, and heavy alcohol use, can help prevent vitamin deficiency anemia. Eating a healthy diet can lower your risk of developing the condition. Some people take a daily vitamin supplement to help prevent the condition.

These deficiencies can result in many disorders including anemia and goitre. Examples of mineral deficiency include, zinc deficiency, iron deficiency, and magnesium deficiency.

A deficiency disease can be defined as a disease which is caused by the lack of essential nutrients or dietary elements such as vitamins and minerals in the human body. Deficiency disease examples: Vitamin B1 deficiency causes beriberi, lack of iron in the body can lead to anaemia.

There are four main types of disease: infectious diseases, deficiency diseases, hereditary diseases (including both genetic diseases and non-genetic hereditary diseases), and physiological diseases. Diseases can also be classified in other ways, such as communicable versus non-communicable diseases.

Vitamin and nutrition blood tests can detect gluten, mineral, iron, calcium and other deficiencies, telling you which vitamins you lack and which you are getting enough of through natural sources.

What are the causes of zinc deficiency? A poor diet can cause zinc deficiency. So it is more common in malnourished children and adults and in people who are unable to eat a normal diet due to circumstances or illness. Lots of zinc intake is from meat and seafood, so vegetarians may be more prone to deficiency.

There is a very simple and efficient test for zinc deficiency. For an adult, mix fifty mg of zinc sulphate in a half a glass of water. If it tastes sweet, pleasant or like water, then your body needs it. If it has a strong metallic or unpleasant taste, you are not zinc deficient.

Vitamin E deficiency may cause impaired reflexes and coordination, difficulty walking, and weak muscles. Premature infants with the deficiency may develop a serious form of anemia. The diagnosis is based on symptoms and results of a physical examination. Taking vitamin E supplements corrects the deficiency. Deficiency Diseases and Symptoms img 1

Functions – Mode of Absorption and Deficiency Symptoms of Micronutrients

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Functions – Mode of Absorption and Deficiency Symptoms of Macronutrients

Micronutrients even though required in trace amounts are essential for the metabolism of plants. They play key roles in many plants. Example: Boron is essential for translocation of sugars, molybdenum is involved in nitrogen metabolism and zinc is needed for biosynthesis of auxin. Here, we will study about the role of micro nutrients, their functions, their mode of absorption, deficiency symptoms and deficiency diseases.

1. Iron (Fe):

Iron is required lesser than macronutrient and larger than micronutrients, hence, it can be placed in any one of the groups. Iron is an essential element for the synthesis of chlorophyll and carotenoids. It is the component of cytochrome, ferredoxin, flavoprotein, formation of chlorophyll, porphyrin, activation of catalase, peroxidase enzymes.

It is absorbed as ferrous (Fe2+) and ferric (Fe3+) ions. Absorbtion of Fe2+ ions are comparitively more than Fe3+ ions. Mostly fruit trees are sensitive to iron.

Interveinal Chlorosis, formation of short and slender stalk and inhibition of chlorophyll formation.

2. Manganese (Mn):

Activator of carboxylases, oxidases, dehydrogenases and kinases, involved in splitting of water to liberate oxygen (photolysis). It is absorbed as manganous (Mn2+) ions.

Interveinal chlorosis, grey spot on oats leaves and poor root system.

3. Copper (Cu):

Constituent of plastocyanin, component of phenolases, tyrosinase, enzymes involved in redox reactions, synthesis of ascorbic acid, maintains carbohydrate and nitrogen balance, part of oxidase and cytochrome oxidase. It is absorbed as cupric (Cu2+) ions.

Die back of citrus, Reclamation disease of cereals and legumes, chlorosis, necrosis and Exanthema in Citrus.

4. Zinc (Zn):

Essential for the synthesis of Indole acetic acid (Auxin), activator of carboxylases, alcohol dehydrogenase, lactic dehydrogenase, glutamic acid dehydrogenase, carboxy peptidases and tryptophan synthetase. It is absorbed as Zn2+ ions.

Little leaf and mottle leaf due to deficiency of auxin, Inter veinal chlorosis, stunted growth, necrosis and Khaira disease of rice.

5. Boron (B):

Translocation of carbohydrates, uptake and utilisation of Ca++, pollen germination, nitrogen metabolism, fat metabolism, cell elongation and differentiation. It is absorbed as (borate) BO3- ions.

Death of root and shoot tips, premature fall of flowers and fruits, brown heart of beet root, internal cork of apple and fruit cracks.

6. Molybdenum (Mo):

Component of nitrogenase, nitrate reductase, involved in nitrogen metabolism, and nitrogen fixation. It is absorbed as molybdate (Mo2+) ions.

Chlorosis, necrosis, delayed flowering, retarded growth and whip tail disease of cauliflower.

7. Chlorine (Cl):

It is involved in Anion – Cation balance, cell division, photolysis of water. It is absorbed as Cl ions.

Wilting of leaf tips.

8. Nickel (Ni):

Cofactor for enzyme urease and hydrogenase.

Necrosis of leaf tips.

Functions – Mode of Absorption and Deficiency Symptoms of Macronutrients

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Functions – Mode of Absorption and Deficiency Symptoms of Macronutrients

Macronutrients, their functions, their mode of absorption, deficiency symptoms and deficiency diseases are discussed here:

1. Nitrogen (N):
It is required by the plants in greatest amount. It is an essential component of proteins, nucleic acids, amino acids, vitamins, hormones, alkaloids, chlorophyll and cytochrome. It is absorbed by the plants as nitrates (NO3).

Deficiency Symptoms:
Chlorosis, stunted growth, anthocyanin formation.

2. Phosphorus (P):
Constituent of cell membrane, proteins, nucleic acids, ATP, NADP, phytin and sugar phosphate. It is absorbed as H2PO4+ and HPO4 ions.

Deficiency Symptoms:
Stunted growth, anthocyanin formation, necrosis, inhibition of cambial activity, affect root growth and fruit ripening.

3. Potassium (K):
Maintains turgidity and osmotic potential of the cell, opening and closure of stomata, phloem translocation, stimulate activity of enzymes, anion and cation balance by ion-exchange. It is absorbed as K+ ions.

Deficiency Symptoms:
Marginal chlorosis, necrosis, low cambial activity, loss of apical dominance, lodging in cereals and curled leaf margin.

4. Calcium (Ca):
It is involved in synthesis of calcium pectate in middle lamella, mitotic spindle formation, mitotic cell division, permeability of cell membrane, lipid metabolism, activation of phospholipase, ATPase, amylase and activator of adenyl kinase. It is absorbed as Ca2+ exchangeable ions.

Deficiency Symptoms:
Chlorosis, necrosis, stunted growth, premature fall of leaves and flowers, inhibit seed formation, Black heart of Celery, Hooked leaf tip in Sugar beet, Musa and Tomato.

5. Magnesium (Mg):
It is a constituent of chlorophyll, activator of enzymes of carbohydrate metabolism (RUBP Carboxylase and PEP Carboxylase) and involved in the synthesis of DNA and RNA. It is essential for binding of ribosomal sub units. It is absorbed as Mg2+ ions.

Deficiency Symptoms:
Inter veinal chlorosis, necrosis, anthocyanin (purple) formation and Sand drown of tobacco.

6. Sulphur (S):
Essential component of amino acids like cystine, cysteine and methionine, constituent of coenzyme A, Vitamins like biotin and thiamine, constituent of proteins and ferredoxin. plants utilise sulphur as sulphate (SO4) ions.

Deficiency Symptoms:
Chlorosis, anthocyanin formation, stunted growth, rolling of leaf tip and reduced nodulation in legumes.

Classification of Minerals

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Classification of Minerals

Classification of Minerals Based on Their Quantity Requirements

Essential elements are classified as Macro-nutrients, Micronutrients and Unclassified minerals based on their requirements. Essential minerals which are required in higher concentration are called Macronutrients. Essential minerals which are required in less concentration called are as Micronutrients.

Minerals like Sodium, Silicon, Cobalt and Selenium are not included in the list of essential nutrients but are required by some plants, these minerals are placed in the list of unclassified minerals. These minerals play specific roles for example, Silicon is essential for pest resistance, prevent water lodging and aids cell wall formation in Equisetaceae (Equisetum), Cyperaceae and Gramineae (Table 12. 1).

Macro Nutrients

Micro Nutrients

Unclassified Minerals

Excess than 10 mmole Kg-1 in tissue concentration or 0.1 to 10 mg per gram of dry weight Less than 10 mmole Kg-1 in tissue concentration or equal or less than 0.1 mg per gram of dry weight Required for some plants in trace amounts and have some specific functions
Example: C, H, O, N, P, K, Ca, Mg and S Example: Fe, Mn, Cu, Mo, Zn, B, Cl and N Example: Sodium, Cobalt, Silicon and Selenium

Classification of Minerals Based on Mobility

If you observe where the deficiency symptoms appear first, you can notice differences in old and younger leaves. It is mainly due to mobility of minerals. Based on this, they are classified into:-

  • Actively mobile minerals and
  • Relatively immobile minerals (Figure 12.1).

Macro Nutrients

Micro Nutrients

Unclassified Minerals

Excess than 10 mmole Kg-1 in tissue concentration or 0.1 to 10 mg per gram of dry weight Less than 10 mmole Kg-1 in tissue concentration or equal or less than 0.1 mg per gram of dry weight Required for some plants in trace amounts and have some specific functions
Example: C, H, O, N, P, K, Ca, Mg and S Example: Fe, Mn, Cu, Mo, Zn, B, Cl and N Example: Sodium, Cobalt, Silicon and Selenium

Actively Mobile Minerals

Nitrogen, Phosphorus, Potassium, Magnesium, Chlorine, Sodium, Zinc and Molybdenum. Deficiency symptoms first appear on old and senescent leaves due to active movement of minerals to younger leaves.

Relatively Immobile Minerals

Calcium, Sulphur, Iron, Boron and Copper shows deficiency symptoms first that appear on young leaves due to the immobile nature of minerals.
Classification of Minerals img 1

Classification of Minerals Based on their Functions

Structural Component Minerals:
Minerals like Carbon, Hydrogen, Oxygen and Nitrogen

Enzyme Function:
Molybdenum (Mo) is essential for nitrogenase enzyme during reduction of atmospheric nitrogen into ammonia. Zinc (Zn) is an important activator for alcohol dehydrogenase and carbonic anhydrase. Magnesium (Mg) is the activator for RUBP carboxylaseoxygenase and PEP carboxylase. Nickel (Ni) is a constituent of urease and hydrogenase.

Osmotic Potential:
Potassium (K) plays a key role in maintaining osmotic potential of the cell. The absorption of water, movement of stomata and turgidity are due to osmotic potential.

Energy Components:
Magnesium (Mg) in chlorophyll and phosphorous (P) in ATP.

Mineral Absorption and its Various Types

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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).
Mineral Absorption img 1

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).
Mineral Absorption img 2

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).
Mineral Absorption img 3

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).
Mineral Absorption img 4

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.
Mineral Absorption img 5

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.

Translocation of Organic Solutes

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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).
Translocation of Organic Solutes img 1

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).
Translocation of Organic Solutes img 2

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.
Translocation of Organic Solutes img 3

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.


  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.


  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.

Transpiration – Factors and its Various Types

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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


Transpiration Per Day

Corn Plant 2 Litres
Sunflower 5 Litres
Maple Tree 200 Litres
Date Palm 450 Litres

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.
Transpiration img 1

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.


  • 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.
Transpiration img 2

(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).
Transpiration img 3

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).

Transpiration img 4

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).

Transpiration img 5

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.


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


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).
Transpiration img 6

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.
Transpiration img 7

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.

Ascent of Sap and its Theory

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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).
Ascent of Sap img 1

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.
Ascent of Sap img 2

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.
Ascent of Sap img 3

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.