NEET Biology Notes Mineral Nutrition Essential Mineral Elements
Essential Mineral Elements
Plants absorb a wide variety of mineral elements. Not all the mineral elements that they absorb are required by the plants. Out of the more than 105 elements discovered so far, less than 21 ar’fe essential and beneficial for normal plant growth and development.
The elements, which are absolutely essential for plant growth and metabolism are called essential elements.
Diverse Functions of Essential Elements
- They are components of biomolecules and are the structural elements of cells, e.g. carbon, hydrogen, oxygen and nitrogen.
- They are components of energy related chemical compounds in plants, e.g. magnesium in chlorophyll and phosphorus in ATP.
- They are components that activate or inhibit enzymes, e.g. Mg2+ is an activator of both ribulose bisphosphate carboxylase-oxygenase and phosphoenol pyruvate carboxylase, both are required in carbon fixation.
- Some essential elements play role in alteration of osmotic potential of a cell, e.g. potassium plays an important role in the opening and closing of stomata.
- There are 17 essential elements, which can be divided into macronutrients and micronutrients, based on their quantitative requirements.
Macroelements or macronutrients are required in large amounts for plant tissues (in excess of 10 m mol/kg of dry matter). The macroelements include carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium. Of these carbon, hydrogen and oxygen are mainly obtained from C02 and H20, while the others are absorbed from the soil as mineral nutrition.
Micronutrients or trace elements or microelements are required in very small amounts (less than 10 m mol/kg of dry matter). These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.
Role of Macro and Micronutrients
Essential elements play their role in various metabolic processes, i.e. in the permeability of cell membrane, maintenance of osmotic concentration of cell sap, electron transport systems, buffering action and enzymatic activity. Various forms and functions of essential elements are given below:
It is the mineral element, which is required by plants in greatest amount. It is obtained from the soil in the form of nitrites (N02), nitrates (NO3) and ammonium (NH4) salts. Nitrogen is a constituent of amino acids, proteins, amides, nucleic acids,- enzymes, some coenzymes, chlorophyll and alkaloids. It is, therefore essential for cell division, full vegetative and reproductive growth, metabolic activities of cell, photosynthesis, etc.
Deficiency Symptoms of Nitrogen
- Chlorosis, which starts from older leaves and progresses to younger leaves.
- The leaves show mottled appearance of purple or red anthocyanin pigments over the vein, e.g. tomato and apple.
- Lateral buds remain dormant.
- There is little tillering in cereals.
- Flower formation is either suppressed or only a few flowers are formed, which lead to the formation of small fruits and less viable seeds.
- In potato, smaller and fewer tubers are produced.
It is obtained from the soil in the form of phosphates .
It is a component of nucleic acid, phospholipids, ATP, nucleoprotein, NAD, NADP and a number of coenzymes. It is required in all phosphorylation reactions. Since, the young cells are the seat of maximum metabolic activities, thus, maximum phosphorus is found in the meristems, fruits and seeds.
Deficiency Symptoms of Phosphorus
- Stunted growth.
- Premature leaf fall.
- Chlorosis appears later, which is of mottled type and is shown first by older leaves.
- Necrosis in lamina, petiole and skin of soft fruits.
- Lateral buds shows prolonged dormancy, but active buds are not affected.
- Vascular tissues are poorly developed.
It is absorbed from the soil in the form of sulphate (S024~), though small quantities of sulphur dioxide can be received from the air by foliar absorption.
It is a component of two vitamins (B and biotin) and several coenzymes (coenzyme-A). In onion and garlic, sulphur occurs in the form of glucosides, which provide a characteristic odour to these plants.
It enhances the number of nodules and nitrogen fixing bacteria in legumes and is essential for the formation of chlorophyll.
Deficiency Symptoms of Sulphur
- Leaves show chlorosis.
- Reduction in juice content of citrus fruits.
- Fruit formation, in general is retarded.
- Tea yellow in tea plant.
- Premature germination of lateral buds, killing of young branches.
- Smaller and chlorotic leaves.
- Necrosis of leaf margin and tip.
- Inward rolling of leaf margin and rapid defoliation.
It is absorbed in the form of K+ ions. It is required for the metabolic activities of cells in young growing parts like root tips and young leaves but seeds are an exception.
It is essential for the functioning of about forty enzymes, which take part in glycolysis, Krebs’ cycle, photosynthesis, synthesis of starch, nucleic acids, chlorophyll and the activity of ATP in many reactions. It maintains hydration, permeability and reactive state of protoplasm.
It produces turgor pressure inside cells for their movements. The stomatal opening and closing is linked to its influx and efflux from the guard cells.
Deficiency Symptoms of Potassium
- Leaves show chlorosis.
- Necrosis at the leaf tip, margin or irregular patches.
- Citrus leaves become bronzed and twisted.
- Growth is stunted, due to reduced apical activity.
- Apical buds may die. Loss of apical dominance is a characteristic deficiency symptom of K+ ions and it leads to bushy growth.
- Cereals may show lodging.
It is absorbed as Mg2+ ions. It is a component of chlorophyll and magnesium pectate. It is essential for the formation of carotenoids, and is required by a, large number of enzymes connected with phosphate transfer. Higher concentrations of Mg2 + ions are found in seeds and growing areas of root and stem.
Deficiency Symptoms of Magnesium
- Chlorosis yellow, red and purple tints are often found in the chlorotic areas, especially towards the margins.
- Followed by necrosis.
- Defoliation may occur.
- Reduced vegetative and reproductive growth.
- Phloem and pith become reduced or remain undeveloped.
- Tomato fruits having pale orange colour, reduced pulp and woolly flesh.
It is absorbed as Ca2+ ions. Calcium is a component of calcium pectate, which is found in the middle lamella.
It acts as an activator of enzymes like ATPase, some kinases, phospholipases, a-amylase and succinate dehydrogenase and takes part in lipid metabolism. It is required in the maintenance of cell membrane and is essential for control of carbohydrate metabolism.
It plays a role in binding of nucleic acids with proteins in chromosomes and counteracts toxicities of other metallic ions.
Deficiency Symptoms of Calcium
- Fragility of chromosomes.
- Roots become translucent and apical growth stops.
- Young leaves shows marginal and apical withering.
- Flower and fruit stalks break resulting in premature falling.
It is absorbed from soil in both Fe2+ and Fe3+ forms of ions. It is a constituent of cytochromes, ferredoxin, nitrogenase, catalase peroxidase, hematin, etc. It is essential for the development of chloroplast and chlorophyll. Many of iron containing enzymes take part in electron transfer in both photosynthesis and respiration. The structure of polyribosomes is dependent upon iron containing compounds. It is activator of nitrate reductase and is involved in synthesis of proteins and DNA.
Deficiency Symptom of Iron
- Interveinal chlorosis.
It is absorbed as Mn2+ ions. It acts as an activator of enzymes like oxidases, peroxidases, dehydrogenase, kinases and dicarboxylases, maintenance of lamellar structure of chloroplast. It is involved in and is essential for photolysis of water and evolution of oxygen.
Deficiency Symptoms of Manganese
- Chlorotic patches are in the form of specks or reticulations in dicots and stripes in monocots.
- Necrosis may follow.
- Leaves either show premature fall or do not develop at all.
- Both stem and root experience stunted growth, as apices may die back.
- Flowers are often sterile.
- Grey spot disease in oat develops.
It is obtained as Zn2+ ions. It is constituent of carbonic anhydrase and certain enzymes of redox reactions. It is essential for supply of C02 to the chloroplast and for the evolution of C02, during respiration. It is used in production of LAA (auxin).
Deficiency Symptoms of Zinc
- Interveinal chlorosis.
- Followed by necrosis.
- Terminal bud dies and leads to leaf rosettes.
It is absorbed as borate ions (BOg’or B402~). It favours absorption of calcium, membrane functioning, pollen tube formation translocation of sugars, cell elongation and differentiation and promotes root nodules in legumes.
Deficiency Symptoms of Boron
- Causes disintegration of softer tissues.
- Browning of cauliflower.
- Heart rot of sugarbeet.
- Reduces transpiration due to the defunctioning of stomata.
It is mostly absorbed as Cu2+ ions. It take part in electron transport system in both respiration and photosynthesis, being a constituent of enzyme plastocyanin. It acts as the activation of many enzyme,
i. e. lactose, oxidase, etc., and synthesis of ascorbic acid.
Deficiency Symptoms of Copper
- Appearance of dark green colour in young leaves followed by chlorosis.
- In exanthema, the tree barks show deep slits, from which gum exudes.
- Tips of leaves undergo chlorosis. Hence, also known as the leaf-tip disease or reclamation disease.
- The reclamation disease is named so, because of its widespread the presence in reclaimed lands of Europe.
It is absorbed from soil as molybdate (M022+) ion and is essential for nitrogen fixation, modulation and has a protective role in chloroplasts.
Deficiency Symptoms of Molybdenum
- Yellow spot disease of citrus fruits.
- Whiptail disease in crucifers like cabbage.
- Flowers shows premature fall.
- In cauliflower, the inflorescence loses its compact form.
It is absorbed as Cl” ions. Its precise role is not well known. However, with Na+ and K+, it helps in determining solute concentration and anion-cation balance in the cells. It plays an important role in photosynthesis and takes part in the water splitting reaction, thus releasing oxygen.
Deficiency Symptoms of Chlorine
- Leaves become wilted.
- Chlorosis and then necrosis.
- Leaves develop bronze colour.
- Root growth is stunted.
- Fruit formation is retarded.
Toxicity of Micronutrients
- Any mineral ion concentration in tissues that reduces the dry weight of tissues by about 10% is considered toxic. Such critical concentration, vary widely among different micronutrients.
- The moderate decrease of micronutrients causes deficiency symptoms and a moderate increase causes toxicity. Toxicity levels for any element also vary for different plants. For example, excess of manganese may induce the deficiency of iron, magnesium and calcium. Thus, the magnesium toxicity actually causes the deficiency symptoms of iron, magnesium and calcium.
Soil as Reservoir of Elements
Essential elements become available to the roots due to the weathering of rocks. These processes enrich the soil with dissolved ions and inorganic salts. Soil are not only supplies minerals, but also harbours nitrogen fixing bacteria and other microbes.
It holds water and supplies air to the roots thus, acts as a matrix that stabilises the plant. The deficiency of essential minerals in soil can be restored by applying fertilisers and manures, for the better crop yield.
Translocation of Solutes
Mineral salts are translocated through xylem along with the ascending stream of water, which is pulled up through the plant by transpirational pull. The analysis of xylem sap shows the presence of mineral salts in it.
Mechanism of Absorption of Elements
The initial rapid uptake of ions into the ‘free space’ or ‘outer space’ of cells, i.e. the apoplast is passive. In the second phase of uptake, the ions are taken in slowly into the ‘inner space’, i.e. the symplast of the cells.
The passive movement of ions into the apoplast usually occurs through ion-channels, the trans membrane proteins that function as selective pores.
The entry or exit of ions to and from the symplast requires the expenditure of metabolic energy, which is an active process.
The movement of ions is usually called flux. The inward movement into cells is influx and the outward movement is efflux.
Hydroponics-Method to Study Mineral Requirements
The technique of growing plants in a mineral solution in complete the absence of soil is called hydroponics. It was discovered by Julius von Sachs, a German botanist in 1860.
The hydroponics method requires purified water and mineral nutrient salts. Hydropsies is used successfully in commercial production of vegetables, such as tomato, seedless cucumber and lettuce.
In nature, nitrogen cycle and biological nitrogen fixation are two important processes in the nitrogen metabolism.