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Unit I (Botany) Study material
B.Sc. III Year Mineral nutrition
The process including the absorption and utilization of various mineral ions by plants for their growth and development is called mineral nutrition.
80 – 90 % of tissue is composed of water. The part of the tissue left behind is called the dry matter and typically it is about 10 -20 % of the original weight. The dry matter mainly consists of organic compounds. Its 80 % consists of plant cell walls, primarily cellulose and related carbohydrates. This can be eliminated in the form of gases on combustion at 6000 C. The residue now left is ash which vary in different plant tissues from about 1- 0.15 % of the dry weight. A careful analysis of the ash shows that it contains almost all of the chemical elements present in the soil surrounding plant All these elements are not essential for the plant . Only 16 elements have been so far considered to be essential for the growth.
Criteria of essentiality of an element:- Arnon and Stout 1939 suggested certain criteria that an element must fulfill in order to be classified as essential. These criteria are.
1. An element is essential if in its absence, the plant can not complete its life cycle.
2. An element is essential if it forms a part of any molecule or a constituent of plant that in itself is essential for the plant e.g. Nitrogen in protein. Magnesium in chlorophyll. Iron in cytochrome.
3. The element must act directly inside the plant or not enhance or suppress the availability of some other element.
From a practical point of view an element is considered essential if plants show deficiency symptoms when they are raised without that element in the medium, the symptom is recovered only by injecting the same deficient element.
Sixteen elements have so far been found to fulfill the criteria of essentiality suggested by Arnon and Stout in 1939.
Sachs and Knop have divided these sixteen elements into two categories based on the quantity in which they are required by the plant.
1. Macroelements or Major elements or Macronutrients:- These elements are required by the plants in their large quantities i.e. 1 – 10 mg./ gm of dry weight. The elements are. C, H, O, N, S, P, Mg, K, Ca. Total = 09
2. Micro elements or Micronutrients or Trace elements:- These elements are required by the plants in their lesser quantities i.e. 0.1 mg / gm of dry weight. These elements are Fe, Mn, Bo, Cu, Zn, Mo, Cl. Total = 07
Scientists have added a few more elements to the list of sixteen though they have found them essential for certain group of plants e.g. Vanadium(Va), Silicon(Si), and Iodine(I) are essential for certain algae. Almunium(Al) is essential for some ferns. Selenium(Si) is essential for weeds.
Sources of essential elements:- Depending upon the source of an element for the plant the elements have been divided into.
1. Mineral elements:- These are the elements which the plants get from the soil e.g. S, P, Mg, K, Ca, Fe, Mn, Bo, Cu, Zn, Mo, Cl, N.
2. Non mineral elements:- These are the elements which plant gets from the water and air e.g. C, H, O. Carbon in the form of Co2 from air. Hydrogen in the form of water from the soil and Oxygen from the air. Nitrogen is included in both the categories mineral and non mineral element because its source is both atmosphere and soil.
Role and deficiency symptoms of Macro elements:
1.Carbon, Hydrogen, and Oxygen:- They are absorbed in the form of Co2 and Water. Co2 is mostly obtained from air while water is got from the soil. The three elements enter the composition of all types of organic compounds like carbohydrates, organic acids, fats, proteins, amino acids, enzymes, hormones, etc. In short they build up the protoplasm. Deficiency of either Co2 or water cause retardation of growth.
Apart from being a structural element, hydrogen in he form of H+ ions is highly important. (i). the concentration of H+ ion determines the PH. PH value influences the majority of reactions going on in the cells. (ii). In respiration oxidation of organic compounds involves the transfer of hydrogen from them to certain acceptor substances (iii). There is an exchange of H+ ions with cations during salt absorption.
2. Nitrogen:- The available forms of the Nitrogen in the soil are No-3 and NH+4 ions .Although nitrate ion is preferred and is translocated to leaves as such. Where it is first converted into Ammonia and then to amino acids. Even plants can absorb Ammonium ions but are first converted into amino acids in the roots and amino acids are translocated to leaves. NH+4 fertilizers when added to soil are converted to nitrates by the microbes then absorbed by the plants. Dry plant material contains 2 – 4 % of nitrogen . In green plants protein Nitrogen is by far the largest Nitrogen fraction and accounts for 80 – 85 % of the total Nitrogen.
Role:- Nitrogen is the mineral element that plants require in greatest amount. It is a constituent of important organic compounds like. Amino acids, Proteins, Nucleic acids, Chlorophyll, Hormones, Vitamins, ATP, NADP and NADPH coenzymes
Deficiency symptoms:
1. Poor growth of the plant.
2. The young leaves remain small and the older leaves fall off prematurely.
3. Chlorosis of leaves especially in the older leaves near the base of the plant. Young leaves may not show this symptom because nitrogen can be mobilized from older leaves. Under severe conditions they may turn yellow or Tan and fall off from the plant.
4. Under severe conditions necrosis of the leaves occurs.
5. In cereals there is poor tillering, reduction in number of ears and number of grains per ear.
6. Flower formation is either suppressed or a few flowers are formed fruits and seeds formed are small and less viable. Potato produces fewer tubers.
7. When nitrogen deficiency develops slowly plants may be markedly slender and often woody stems. This woodiness may be due to build up of excess carbohydrates that can not be used in the synthesis of amino acids and other nitrogen compounds and can be used in the synthesis of anthocyanins which lead to purple colouration of leaves petioles and stems e.g. in tomato and certain varieties of corn.
8. Branching restricted.
3. Phosphorus:- The major phosphorus containing ions in soil solution are monovalent (H2PO4-), bivalent (HPo42-), Trivalent(Po43-), Monovalent is present in acidic soils. Bivalent is present in neutral soils. Trivalent phosphate is present in alkaline soils. Monovalent is easily available to roots while bivalent and trivalent are present in bound form. Roots are capable of absorbing phosphate from solution low in phosphate content. The phosphate content of roots and xylem sap is about 100 – 1000 fold higher than that of the soil solution.
Role:- Phosphorus is the constituent of nucleoprotein, ATP, NADP, phospholipids .Another important phosphorus containing compound is phytin found in seeds. It is regarded as the phosphate reserve. During seed germination it is mobilized and converted into other phosphate forms that are needed in the metabolism of young plants.
Deficiency symptoms:
1. Stunted growth.
2. In cereals there is poor tillering, reduction in number of ears and number of grains per ear.
3. Older leaves become darkish green in colour, contain small necrotic spots (dead tissue)
4. Stems of annual plants become reddish in colour due to formation of anthocyanin pigments.
5. Vascular tissues are poorly developed.
6. Death of older leaves.
7. Delay in maturation of plant
8. Production of slender (bur not woody) stems.
9. Some species produce excess anthocyanins giving the leaves a slight purple colouration but not associated with chlorosis.
10. Lateral buds show prolonged dormancy but active buds are not affected because they can function as sinks at the expense of the remaining plant
11. Plants are unable to absorb and accumulate salts.
4. Potassium:- It is absorbed as k+ ions at high rates by the plants.
Role:-
1. It is necessary for meristematic growth.
2. It is involved in maintaining the water status of the plants by maintaining cell turgor.
3. It plays important role in opening and closing of stomata.
4. It is involved in the translocation of photosynthetates.
5. It is the activator of enzymes like,Diastase, Catalase, Invertase.
Deficiency symptoms:
1. Reduced growth rate.
2. First chlorosis and then necrosis on the margins and tips of older leaves.
3. Decrease in turgor.
4. Leaf wilting and abscission.
5. Roots become susceptible to rotting fungi in corn and the plant easily bends to ground (Lodging).
6. Stem becomes slender and weak with abnormally short internodes.
7. Apical buds may die with the result loss of apical dominance is found.
8. In absence of K+ other ions present exert toxic effects.
5. Sulphur:- It is present in the soil in inorganic and organic form. The inorganic forms of sulphur in soil consists mainly of So42-. The organic sulphur of the soil is made available to plants by microbial activity and So42- is produced. Plants mainly absorb sulphur in the form of So42. Plants can utilize So2 also as a sulphur source.
Role:
1. It is the constituent of most important compounds like amino acids (Cysteine and Methionine) Vitamins (Lipoic acid , Thiamin , Biotin , Coenzyme A) electron carrier Ferredoxin.
2. Sulphur forms disulphide bridges in proteins.
3. It forms an alkaloid sinigrin (Diallyl disulphide) which gives pungent odour to Onion and Garlic.
Deficiency Symptoms:
1. Rate of plant growth is reduced. Shoot growth is more effected than root growth
2. Chlorotic symptoms occur first in the younger then most recently formed leaves rather than in old leaves because sulphur is not easily mobilized to the younger leaves in most species..
3. Premature leaf fall.
4. Reduction in nodule formation in legumes.
5. Anthocyanin accumulation.
6. Calcium:- It is absorbed from the soil in the form of Ca 2+ ions.
Role:
1. It is required for cell division because it is used in mitotic spindle during cell division.
2. It is essential for the stabilization of newly synthesized membranes.
3. It is involved in the normal functioning of cell membranes.
4. It is used in the synthesis of new cell walls particularly the middle lamella that separates newly divided cells.
5. It is activator of enzymes like ATPase, Kinase, Phospholipase, α-amylase and succinate dehydrogenase.
6. It functions as secondary messenger for hormonal and environmental signals for this it combines with calmodulin protein present in cytosol and plays role in many metabolic processes.
7. Development of thick cuticle in potomogeton occurs when calcium is supplied in abundance.
8. It plays role in binding proteins and nucleic acids in chromosomes.
9. It controls metabolism of carbohydrates.
10. Calcium counteracts toxities of other elements eg. Oxalo-aceticacid is converted into calcium oxalate which is non toxic.
Deficiency symptoms:
1. Reduced meristematic activity because of necrosis of young meristematic regions.
2. Young leaves become deformed (rolling and curling).
3. The surface of apples is pitted with small brown necrotic spots called bitter pit disease. In tomato this disease is called blossom end rot.
4. Premature drop of flower and fruit by breaking of their stalks
5. Chlorosis starts from margins to middle or in the leaf area between the veins.
7. Magnesium:- Magnesium is absorbed in the form of divalent Mg 2+.
Role:
1. In plant cells magnesium ions have a specific role in the activation of enzymes involved in respiration, photosynthesis, (rubisco) and the synthesis of DNA and RNA.
2. It is the part of ring structure of chlorophyll molecule.
3. Association of two sub units of ribosomes occurs in presence of Mg.
4. It is essential for the formation of carotenoids.
5. It is involved in the synthesis of magnesium pectate of middle lamella.
Deficiency symptoms:
1. Chlorois between the leaf veins occurring first in the older leaves because of mobility of this element. This pattern of chlorosis results because the chlorophyll in the vascular bundles remain unaffected for longer periods than the chlorophyll in the cells between the bundle does. If the deficiency is extensive the leaves may become yellow or white.
2. Premature leaf fall.
3. Chlorosis is followed by necrosis. Defoliation may also occur.
4. Phloem and pith becomes reduced or remain under developed.
5. There is reduced vegetative and reproductive growth.
6. In deficiency of Mg tomato fruits develop pale orange colour, reduced pulp and wooly flesh.
Role and deficiency symptoms of Micronutrients.
1. Iron:- Plants obtain iron in the form of ferric ions Fe3+
Role:
1. Iron has an important role as a component of enzymes involved in the transfer of electrons (redox reactions) such as cytochrome. In this role it is reversibly oxidized from Fe2+ to Fe3+ during electron transfer.
2. It is essential for the development of chloroplasts and maintenance of chlorophyll though no iron dependent enzyme is involved in chlorophyll synthesis.
3. It is essential for the formation of or activity of nucleic acids and synthesis of proteins.
Deficiency symptoms:
1. Chlorosis between the leaf veins occurring first in the younger leaves because iron can not be readily mobilized from older leaves.