Prasanna

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Special Modes of Nutrition

Nutrition is the process of uptake and utilization of nutrients by living organisms. There are two main types such as autotrophic and heterotrophic nutrition. Autotrophic nutrition is further divided intophotosynthetic and chemosynthetic nutrition. Heterotrophic nutrition is further divided into saprophytic, parasitic, symbiotic and insectivorous type. In this topic you are going to learn about special mode of nutrition.

Saprophytic Mode of Nutrition in Angiosperms

Saprophytes derive nutrients from dead and decaying matter. Bacteria and fungus are main saprophytic organisms. Some angiosperms also follow saprophytic mode of nutrition. Example: Neottia. Roots of Neottia (Bird’s Nest Orchid) associate with mycorrhizae and absorb nutrients as a saprophyte. Monotropa (Indian Pipe) grow on humus rich soil found in thick forests. It absorbs nutrient through mycorrhizal association (Figure 12.9).

Parasitic Mode of Nutrition in Angiosperms

Organisms deriving their nutrient from another organism (host) and causing disease to the host are called parasites.

a. Obligate or Total parasite:
Completely depends on host for their survival and produces haustoria.

(i) Total Stem Parasite:
The leafless stem twine around the host and produce haustoria. Example: Cuscuta (Dodder), a rootless plant growing on Zizyphus, Citrus and so on.

(ii) Total Root Parasite:
They do not have stem axis and grow in the roots of host plants produce haustoria. Example: Rafflesia, Orobanche and Balanophora.

b. Partial Parasite:
Plants of this group contain chlorophyll and synthesize carbohydrates. Water and mineral requirements are dependent on host plant.

(i) Partial Stem Parasite:
Example: Loranthus and Viscum (Mistletoe) Loranthus grows on fig and mango trees and absorb water and minerals from xylem.

(ii) Partial Root Parasite:
Example: Santalum album (Sandal wood tree) in its juvenile stage produces haustoria which grows on roots of many plants (Figure 12.10).

Symbiotic Mode of Nutrition

a. Lichens:
It is a mutual association of Algae and Fungi. Algae prepares food and fungi absorbs water and provides thallus structure.

b. Mycorrhizae:
Fungi associated with roots of higher plants including Gymnosperms. Example: Pinus.

c. Rhizobium and Legumes:
This symbiotic association fixes atmospheric nitrogen.

d. Cyanobacteria and Coralloid Roots:
This association is found in Cycas where Nostoc associates with its coralloid roots. (Figure 12.11).

Insectivorous Mode of Nutrition

Plants which are growing in nitrogen deficient areas develop insectivorous habit to resolve nitrogen deficiency. These plants obtain nitrogen from the insects.

a. Nepenthes (Pitcher plant):
Pitcher is a modified leaf and contains digestive enzymes. Rim of the pitcher is provided with nectar glands and acts as an attractive lid. When insect is trapped, proteolytic enzymes will digest the insect.

b. Drosera (Sundew):
It consists of long club shaped leaves with tentacles that secrete sticky digestive fluid which looks like a sundew and attracts insects.

c. Utricularia (Bladder Wort):
Submerged plant in which leaf is modified into a bladder to collect insect in water.

d. Dionaea (Venus Fly Trap):
Leaf of this plant modified into a colourful trap. Two folds of lamina consist of sensitive trigger hairs and when insects touch the hairs it will close and traps the insects.(Figure 12.12).

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Nitrogen Cycle and Nitrogen Metabolism

Nitrogen Cycle

This cycle consists of following stages:

1. Fixation of Atmospheric Nitrogen

Di-nitrogen molecule from the atmosphere progressively gets reduced by addition of a pair of hydrogen atoms. Triple bond between two nitrogen atoms (N≡N) are cleaved to produce ammonia (Figure 12.7).

Nitrogen fixation process requires Nitrogenase enzyme complex, Minerals (Mo, Fe and S), anaerobic condition, ATP, electron and glucose 6 phosphate as H⁺ donor. Nitrogenase enzyme is active only in anaerobic condition.

To create this anaerobic condition a pigment known as leghaemoglobin is synthesized in the nodules which acts as oxygen scavenger and removes the oxygen. Nitrogen fixing bacteria in root nodules appears pinkish due to the presence of this leghaemoglobin pigment.

Overall Equation:
N₂ + 8e^– + 8H⁺ + 16ATP → 2NH₃⁺ + H₂ + 16ADP + 16Pi

2. Nitrification

Ammonia (NH₃⁺) is converted into Nitrite (NO₂^–) by Nitrosomonas bacterium. Nitrite is then converted into Nitrate (NO₃^–) by Nitrobacter bacterium. Plants are more adapted to absorb nitrate (NO₃^–) than ammonium ions from the soil.

3. Nitrate Assimilation

The process by which nitrate is reduced to ammonia is called nitrate assimilation and occurs during nitrogen cycle.

4. Ammonification

Decomposition of organic nitrogen (proteins and amino acids) from dead plants and animals into ammonia is called ammonification. Organism involved in this process are Bacillus ramosus and Bacillus vulgaris.

5. Denitrification

Nitrates in the soil are converted back into atmospheric nitrogen by a process called denitrification. Bacteria involved in this process are Pseudomonas, Thiobacillus and Bacillus subtilis. The overall process of nitrogen cycle is given in Figure 12.8.

Nitrogen Metabolism Ammonium Assimilation (Fate of Ammonia)

Ammonia is converted into amino acids by the following processes:

1. Reductive Amination

Glutamic acid or glutamate is formed by reaction of ammonia with α-ketoglutaric acid.

2. Transamination

Transfer of amino group (NH₃⁺) from glutamic acid (glutamate) to keto group of keto acid. Glutamic acid is
the main amino acid from which other amino acids are synthesised by transamination. Transamination requires the enzyme transaminase and co enzyme pyridoxal phosphate (derivative of vitamin B6 – pyridoxine)

3. Catalytic Amination: (GS/GOGAT Pathway)

Glutamate amino acid combines with ammonia to form the amide glutamine.

(GOGAT – Glutamine – 2 – Oxoglutarate aminotransferase)

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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).

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).

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

Non-Legume:
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.

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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).

Aeroponics:

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 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.

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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.

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.

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Deficiency Diseases and Symptoms

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

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.

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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.

Deficiency:
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.

Deficiency:
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.

Deficiency:
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.

Deficiency:
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) BO^3- ions.

Deficiency:
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.

Deficiency:
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.

Deficiency:
Wilting of leaf tips.

8. Nickel (Ni):

Cofactor for enzyme urease and hydrogenase.

Deficiency:
Necrosis of leaf tips.

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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.

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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 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.

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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.