PLANT NUTRIENTS AND FUNCTIONS
By S.M. ALAM
Feb 07 - 13, 2005
Plants contain small amount of ninety or more nutrient elements, but only sixteen of which are currently known to be essential for plant growth and development. These elements are carbon, hydrogen, oxygen and nitrogen, derived from air, water and soil. Nitrogen, along with other minerals are either supplied from the reserves in the soil or through the application of fertilizers. Nitrogen, P, K, S, Ca and Mg are designated as macronutrients in contrast to those Fe, Mn, Zn, Cu, B, Mo Co, Na and Cl, which are used in small quantities and are referred to as micronutrients. Cobalt is also classified as an essential element. Certain plant species have been shown to benefit from the presence of Na, Si and possibly V in the growth medium. Soil is the source of 13 of the 16 for plant growth most of them are come from parent rock. Nitrogen is not present in the parent rock. The original source of nitrogen is atmospheric air, where it is present nearly to 78%. Plants take up nutrients from the soil mostly in the form of ions. Carbon, O, H and N formed about 95% of the dry weight of plant. The major and minor elements are account for the rest of the total dry weight of the plant. A predictable and timely supply of plant nutrient is essential for successful crop production.
Plants take up nutrients from the soil, mostly in the form of ions (cation and anion) for their growth and development. Shortage of nutrients in the plants cause deficiency symptoms, which reduces the growth productivity of plants. Similarly, leaf chlorosis is due to reduce chlorophyll formation. Sometimes get production of anthogenic pigment, low organic matter and residues with high carbon content are responsible for N deficiency in soils. Due to N deficiency, limited number of tillers, leaves are short and erect. Plant are stunted with small thick cell walls. Seed formation and fruits are reduced. The problem of low N availability is aggravated in salt-affected soils, where organic matter and N content are low and mineralization of N is partially or completely inhibited. Since organic matter is a predominant source of plant available N, retardation of its mineralization is bound to exert a negative effect on the availability of N to plants. Nitrogen and P shortage limit N2 fixation in bean.
PHOSPHORUS: Phosphorus is the second major element essential for plant growth and development, as it is involved in most metabolic processes. Phosphorus is a constituent of nucleic acids, phospholipids, phosphoproteins, phosphate esters, dinucleotides and adenosiue triphosphate. Hence, P is required for the storage and transfer of energy, photosynthesis, electron transport processes, the regulation of some enzymes (for example, in the synthesis of sugars and starchs), and the transport of carbohydrates. As in the case of nitrogen, the need for P is large.
Phosphorus is involved in many of the energy yielding and energy utilizing reactions of plant from photosynthetic phosphorylation to respiratory metabolism. Its role in energy storage and transfer is singly the most important. Phosphate compounds act as 'energy currency' within plants. It is also a constituent of the cell nucleus and is essential for cell division and for the development of meristematic tissue. Plants utilize monovalent phosphate (H2PO4-) much more readily than divalent form (HPO4--) as the pH of the solution decreases from 7 to 5. Phosphorus increases the strength of cereal straw, stimulates root development and shoot growth, promotes flower formation and fruit production. It is considered essential for seed formation. It hastens the maturity of crops growth on soil low in P. Adequate P fertilization may improve quality of certain fruits, forage, vegetable and grain crops and increases their resistance to diseases under adverse conditions.
Most soils of Pakistan are alkaline and calcareous and deficient in P. Farmers frequently supply P fertilizers for correcting P deficiency. Availability of P to plants is affected by soil texture, moisture, temperature, aeration, soil pH, clay content and CaCO3 in soil. These factors also control chemical reactions of applied P resulting in its conversion into forms, which are not available to crops. The availability of P decreases due to adsorption and precipitation in the soil system. Lack of adequate P lead to retarded growth with the result that plants are sickly, stunted, spindly and weak. When P deficiency is severe, visual symptoms appear, such as dark bluish green or dull grayish-green colours on mature leaves of leaf edges, or purpling of stems due to synthesis of anthocyanin pigment, abnormally dark green in some species and rusty brown lesions in potato tubers. As the plants mature, P is translocated into the seeds and fruit. Phosphorus deficiency causes delayed ripening, poor fruiting and retarded root growth, reduced tillering in cereals, reduced fruit quality and storage potential.
POTASSIUM: Potassium is a very slowly soluble mineral constituent, such as orthoclase. Soil contains about 2% P, but only 1/5 is available in the soil. Potassium is necessary for maintaining turgor, stomatal opening and control, pH stabilization and osmoregulation of cells. Adequate P in plant tissue is essential for inorganic osmoticum. It is required for the synthesis of proteins, for the metabolism of carbohydrates, and lipids and is an activator of numerous enzymes, such as pyruvate kinase, acetic thiokinase, acetadelyde dehydrogenase, B-galactosidase, glycerol dehydrase, fructokinase, malic enzyme. Plants take up ionic K from soil solution. Plants may contain around 4% K. Potassium uptake is rapid i.e. selective uptake mechanism. Luxury uptake of K does occur i.e. over and above requirement. Potassium can replace rubidium and sodium.
In deficient plants, protein synthesis, photosynthesis and cell extension are impaired and localized cell death occurs. Potassium deficiency is made worse by high rainfall leaching of soils, and soils with high illite-type clay minerals. Some of the crops, which are sensitive to potassium deficiency are sugarbeet, potatoes, brass and fruit vines. Potassium is very mobile in plant and deficiency is first shown in the older leaves. Margins of the leaves become yellowish and then brown condition being referred to as marginal scorch.
CALCIUM: Most of the Ca in the plant occurs in the cell wall, where it is associated with pectin in the membrane structure. It controls cell wall permeability. Calcium may be present as calcium oxalate. Calcium is also required for membrane integrity and function. It increases the uptake of monovalent cations, but Ca and K are antagonistic at high levels. Calcium associated with some enzymes are phospholipase, arginine kinase, adenosine triphosphatase, adenyl kinase and pyrase. It plays a positive role in mitosis, and chromosome stability in mitosis. It is a dominant exchange cation.
Because Ca is essential for cell division and growth, the growing tips of roots and shoots are particularly vulnerable to Ca deficiency. The Ca which is delivered in the transpiration stream to old leaves cannot be exported to young Ca-deficient tissue since Ca is not transportable in the phloem. Symptoms of Ca deficiency typically cell death, appear first in developing organs such as buds and elongation leaves.
MAGNESIUM: A major function of magnesium is as a co-ordinated metal in chlorophyl. Magnesium is also required for protein synthesis, the regulation of cellular, pH and cation-anion balance. Magnesium is also involved in the ionic activator of many enzymes for carbohydrate metabolism, such enzymes are glucokinase, fructokinase, galactokinase, hexokinase, triosokinase, phosphopentokinase, pyruvic kinase, phosphoglyceric kinase, carboxylase, lactic dehydrogenase. It is also an important binding agent in microsomal particles. Manganese and Co can partially replace the ionic function of Mg. Magnesium deficiency symptoms are first expressed in fully expanded leaves, and when deficiency is severe, they extend into the young foliage.
SULPHUR: Sulphur is essential for the formation of proteins containing amino acids cysteine and methionin. Sulphur is also required for the synthesis of thiamine, co-enzyme A and sulfolipids. Sulphur is poorly phloem mobile. Consequently, symptoms of S deficiency first appear on young leaves, which become uniformly yellow as the chlorophyll content declines.
In S-deficient plants, the interveinal areas of expanding leaves turn pale green. With time, the leaves become uniformly yellow and symptoms spread from expanding to fully expanded leaves may be impaired before symptoms become obvious. Under severe deficiency, the young leaves become pale red, the leaf tips die and shrivel and the terminal bud aborts. Leaves which are uniformly yellow are indistinguishable from those suffering N deficiency. However, N deficiency symptoms first appear in mature leaves and the young leaves develop a red blush. Sulphur is closely associated with the protein metabolism and its uptake is increased with nitrogen. Sulphur deficiency problem is considered as one of the limiting factors in crop production.
MICRONUTRIENTS: Plant need small quantities of certain elements the so-called trace or minor elements for their nutrition and these include Fe, Mn, Zn, Cu, B, Mo, while Co and Cl are beneficial under certain circumstances. These elements are called trace elements, because only very small quantities, ranging from a few grams to a few kgs per hectare are usually needed by the crop. Literature reveals that a full crop of oats only removes about 20 g of Cu, 100 g of Zn and 500 g of Mn compared with 8 kgs of P per ha. A shortage of one or more of these elements, usually affects the visual appearance of the plants, giving the leaves a chlorotic, bronzed, or mottled colour, or altering its habit of growth. Micro-nutrients are components of some key substances in plant metabolism or are essential to the functioning of an enzyme action. Although, their quantities required for normal plant growth may be small compared to major elements, their deficiency may have extremely distributing effect on vital plant processes. Micro-nutrients availability to plants are affected with various changes occurring in the soils. The soil and the climatic conditions in Pakistan are quite conducive to micro-nutrient deficiencies in plants. Most of the soils of Pakistan are generally light to medium textured, high in pH, and low in organic matter, deficient in nitrogen and calcareous in nature, where Fe, Mn, Zn, Cu and B, except Mo may form insoluble compounds and become unavailable to plants. Introduction of high yielding crop varieties and intensive crop production system, need very heavy application of N and P fertilizers, which interact with Zn, Cu, Fe and Mn and thus decrease their uptake by plants. It is quite usual that only 0.5% - 5% of micro-nutrient can be recovered compared with 30-60% N and K and 10-15 %P. The role and function of some micro elements are as:
IRON: Iron is the fourth most abundant element in the earth crust, with an average of 5.1% by weight. Yet in spite of this abundance, Fe is frequently a limiting nutrient in crop production, especially in calcareous soils. Iron is an important micro-nutrient required by plants, since it is involved in energy producing and utilizing processes in the plant and is important in many redox - reactions. In crop plants, the essentially of iron for the development and maintenance of photosynthetically active tissues has been recognized, because iron is involved in the synthesis of chlorophyll and of chloroplastic protein. Iron is also the constituent of aconitase, catalase, cytochromes, ferredoxins, nitrogenase and peroxidases. Its role in photosynthesis, No3 - and SO4 — reduction and N2 assimilation underlines the vital functions, Fe performs in overall plant metabolism.
The first symptom of iron is the chlorosis of the young leaves. This reflects the immobility of Fe in plants and primary need in chlorophyll synthesis. Iron chlorosis is commonly seen in calcareous soils that covers 30% of the earth crust. The disorder is due to deficiency of active Fe in the leaves and induces imbalance in the metabolic processes. Iron deficiency directly affects the photosynthetic mechanism causing a dramatic reduction in crop yield. Interveinal areas of the leaves become chlorotic and veins usually remain green. Deficiency is common in alkaline soil (lime induced chlorosis) due to immobilization of iron compounds. In general, when iron values are 50 ppm or less in the dry matter, deficiency is likely to occur, the sufficiency range seems to be from 10-250 ppm Fe. It is taken up by in divalent form (Fe++). Iron compounds generally become unavailable in alkaline soils, so foliar application of Fe compounds are generally beneficial.
MANGANESE: Manganese is a constituent of pyruvate carboxylase and a number of enzymes concerned with the metabolism of N, synthesis of chlorophyll and physiological reactions in plants and such enzymes are nutrite reductase, malic dehydrogenase and oxalo-succinic decarboxylase. It is involved in the plant's respiratory process. It controls the redox potential in plant cells during the phases of light and darkness. It can replace Co++ or Mg++ under certain cases.
Manganese deficiency symptoms are first seen on the new leaves. Chlorotic between the veins of young leaves, characterized by the appearance of chlorotic and necrotic spots in the interveinal areas. The normal contents of Mn in plants range from 50-200 ppm on a dry weight bases. Values less than 25 ppm in plant leaves are usually deficient and those over 400 ppm are considered excessive. Other symptoms are grey speck of rats, pahala blight of sugarcane, speckle yellow of sugarbeet and marsh spot of peas.
Plants take up Mn in bivalent (Mn++) form. Total Mn content of soil vary from a trace to as high as 10% or even more. However, the total Mn contents of soil between 200-300 ppm are most common. The critical level is ranged from 1-2 ppm. Soil application of MnSO4 on calcareons soils ranged from 5-10 kg/ha for crop plants. In foliar spray of 0.5 kg Mn/ha may be used in combination with other trace elements.
ZINC: Next to N and P, Zn is an element, which limits plant growth, if present in inadequate amounts. Thus, it is one of the essential nutrient elements needed in sufficient and balanced amount for the normal growth of plants. The element has functions in biosynthesis of tryptophane and indole acetic acid and acts as an activator of a number of enzymes. A few enzymes in which the Zn is associated are carbonic anhydrase, ribonuclease, alcohol dehydrogenase and pyridine nucleotide dehydrogenase, glutamic dehydrogenase, L - lactic acid dehydrogenase, D - glyceral dehyde - 3 - phosphate dehydrogenase, malic dehydrogenase, D - lactic dehydrogenase, D - lactic cytochromec, reductase and aldolase. Zinc plays an important role in nucleic acid and protein synthesis, assists the utilization of P and N in plants and is involved in auxin plant hormone production. In the deficiency of Zn, plants do not grow properly well due to reduction in enzymes activities. The auxin depression due to Zn deficiency has been studied since 1940 by a large number of researchers and at approximately the same time the depression of protein content due to Zn deficiency was revealed. Zinc deficiency can be caused in several ways:
The pH value of the soil is the most important influence on Zn availability. A one unit rise in pH makes Zn one-hundred times less soluble. Thus, Zn tends to be less available in alkaline soils then in acid soils. But, submerged acid soil may develop zinc deficiency because the pH of acid soils tends to rise after they are submerged. Another cause of Zn deficiency is obviously the level of available Zn in soil. Depending on the method of analysis the critical level is 1.0-1.5 ppm Zn. High organic matter content accentuates Zn deficiency in soils that have light pH. At high pH values, organic matter facilitates the formation of chemical complexes that tie up Zn.
Continuously submerged soils tend to develop Zn deficiency, because the concentration of Zn in soil solution decreases, at larger the soil is submerged. The decrease is fastest in nutrient and calcareous soil. Soils with a high content of available and water soluble P tend to have low concentration of water soluble Zn, regardless of pH or the content of total or available Zn. Zinc deficiency has been detected in most of the alkaline calcareous soils of Pakistan. Up to 85% of the soils were found to be either deficient or within the marginal range of Zn. In soil, Zn is usually present in the concentration range 10-300 ppm, zinc occurs a number of different minerals, on exchange sites of clay minerals and organic matter or adsorbed on solid surface. Zinc interacts with soil organic matter and both soluble and insoluble Zn organic complexes are formed. The levels of Zn in plat material are low and the Zn requirement of plants is correspondingly small. The form in which Zn is translated from roots to shoots is generally of Zn++. Zinc accumulates in root tissue especially Zn supply is high. Plants suffering from Zn-def often show chlorosis. The interveinal areas of the leaf. These areas are pale green, yellow or white.
COPPER: Copper is an important element for the growth and development of plant. Copper is known to be associated with a number of enzymes, such as cytochrome oxidase, abcorbicacid oxidase, phenolase, laccase and polyphenol oxidase. It generally promotes the formation of vitamin A in plants. The normal range of Cu in many plants is from 5-20 ppm. When the Cu concentration in plants is less than 4 ppm in dry matter, deficiencies are likely to occur. The characteristic symptoms of Cu deficiency in crop first appear in the leaf tips. The leaf tips become white and the leaves are narrow and twisted. Top leaves develop necrotic spots and brown areas, followed by withering and death of short tips. The total contents in soil generally vary between 10-200 ppm with an average value of around 50 ppm. Copper is thought to be involved in the oxidation of soluble divalent iron (Fe++) to trivalent iron (Fe+++) state and manganese to manganic salts and soluble sulphide to more insoluble sulphides.
Copper is generally taken by plant in bivalent form. The soil organic matter inmobilizes Cu in soils. Copper compounds such as copper sulphate may be broadcasted to various crops on copper deficient and soils at the rate of 2.0-5.0 kg/ha copper may become toxic if very frequent spraying of Cu containing fungicides e.g. Boreaux mixture carries out.
BORON: Boron is associated with meristematic activity, auxin, cell division, cell growth and membrane function, protein synthesis, lignin synthesis and pectin metabolism, maintaining correct water relations within the plant, sugar translocation, fruit processes and phenolase inhibition. Much of the B in the plant is located in the cell wall. It is also associated with the uptake of Ca and its utilization by plants and in the regulation. Boron deficiency is one of the most widespread trace element problems in arable crops. There are large variations in susceptibility between crops. Cereals and grasses contain only 2-5 mg B kg-1 dry matter and very rarely show B deficiency. The most sensitive crops are the root crops, sugar beet, turnips and swedes; others often affected include red beet, carrots, cauliflower, cerely, lucerne, red clover, kale, cabbage, grapes and tomatoes. Many fruit and forest trees and flower crops are also susceptible. Boron has low mobility in the plant, so B deficiency symptoms appear in growing points and reproductive organs. The symptoms are often very severe, and may include death of the growing point and quality losses in harvestable organs. Mild boron deficiency can cause quality problems in susceptible crops, especially root vegetables and flowers, in the absence of visual symptoms in the leaves. When symptoms are visible it is often too late to obtain a worthwhile response to application of B, so it is important to apply B before growing susceptible crops on soils low in B. The optimum range of B in leaf tissue of most crop is from 20-100 ppm. Toxicities may occur when B level in most of the crop plants exceed 25 ppm.
Boron deficiency symptoms are heart rot of sugarbeet, brown rot in turnip, drought spot or corky pit of apples, brown spotting of apricot and browning of cauliflower curd. In tomato, corky patches and uneven ripening and in turnip terminal bud break down, leaves are curled, rotted purplish yellow. In severe cases, the central tissue may break down and root become hollow. Morphological characters of a plant such as root length, surface area, fineness and intensity of root hairs combined to strongly influence P uptake, because soil P is supplied to plants mainly by diffusion and P diffusion co-efficient is very low.
Boron is taken up by plants in form of borate (BO3---) ion. Plant needs continuous low supply. Boron is present in tourmaline mineral. Its solubility decreases with increase in pH of the growth medium. Boron as sodium tetraborate is the most commonly used B fertilizer in soil. The normal range of B application on the soil is 1-2 kg B/ha of area.
Foliar application of boron directly on young developing fruit can be highly beneficial, but is less effective in other cases, because of poor translocation of boron from the leaves. Boric acid and solubor can be used as foliar spray. The concentration of B in a spray used for majority crop is 0.2-0.5%. For sugar beet, the concentration can be as high as 2.5 percent.
MOLYBDENUM: Molybdenum is less subject to unavailabilities than Zn, B, Mn, Cu and Fe in the soils of Pakistan. Molybdenum is necessary for nitrogen metabolism. It is involved in iron and phosphate metabolism. It is also necessary for chlorophyll and N fixation by leguminous crops. Molybdenum is an essential component of nitrate reductase and nitrogenase, which controls the reduction of inorganic nitrate to ammonium and help in fixing N2 to NH3. It is also associated with aldehyde oxidase and xanthine dehydrogenase in maize roots. Molybdenum is required in the synthesis of ascorbic acid and is implicated in making iron physiologically available within the plant. It also involves in P metabolism in plants and reduces ethane to ethylene. It is considered an antidote of excession Ca, Mn, Zn, Ca, B and Si in plants. Molybdenum concentration in plants range from less than 0.1 ppm to greater than 300 ppm.
Plants usually respond to Mo, if they contain less than 0.1 ppm Mo. High Mo in lungs (15 ppm) can be problem for cattle, which is known as molybdenosis or teart disease. Molybdenum deficiency resembles N-deficiency and show up as general yellowing and stunting growth. Yellow spot disease of citrus, bean, scald and whiptail of cauliflower are the well known name associated with Mo deficiency. Molybdenum deficiency symptoms are likely to occur, when leaf Mo concentration are less than 0.2 ppm on a dry-weight basis. In soils with low Mo, soil application of 1 to 2 Ibs of sodium molybdate or foliar application of 2 ounces per acre usually provide complete control of deficiencies. The availability of Mo increases as the pH rises, so Mo deficiency is most likely to occur in acid soils, and can often be cured by application of lime. Molybdenum is taken up by plants as MoO42-, so behaves in the soil rather like phosphate (PO43-). Molybdate is strongly adsorbed by ferric sides and the adsorption is pH dependent.
SODIUM: Sodium is beneficial to a variety of higher plant species, especially when the K content of the medium is limited. Also, sodium is required for the growth of blue-green algae and is reported to be essential for such salt-loving species as Halogeton.
COBALT: cobalt is an essential trace element required in the concentration range of 0.0001 to 0.005 ppm in nutrient solutionly legumes and non-legumes. Cobalt is required for N-fixation by soybean plants. It is also required by alfalfa plants to fix nitrogen in symbiosis with root nodule endophytes. It is also involved in B12-coenzymes.
Inorganic elements required by plants function as structural components of fats, carbohydrates, proteins, intermediary metabolites, nucleic acids, and coenzymes. Many of the mineral cation and anion participate in metabolism as cofactors for citric acid cycle enzymes, glycolytic enzymes, electron transport enzymes of both respiratory and photosynthetic processes; also, in the enzyme involved in nitrate reduction, and in a variety of other enzymes that participate in metabolic sequences. Several examples have been presented in which mineral deficiency such as iron or molybdenum resulted in metabolic lesions that have been satisfactorily interpreted on the basis of detailed knowledge of the biochemical role of metals in enzymes of important metabolic pathways.