'Plant Nutrients: their functions and deficiency'

Transcription

1 'Plant Nutrients: their functions and deficiency' Plant life depends on nutrition from a number of essential sources and these are grouped into three sectors: the principle sources, the macronutrients and the micronutrients. The principal sources, (carbon (C), hydrogen (H) and oxygen (O)), are obtained from carbon dioxide gas in the air and water from the soil and are used by plants in the process of photosynthesis. The plant converts these nutrients photosynthetically. The macronutrients (or major elements) and the micronutrients (or trace elements) are obtained from the soil and are received by plants in an altered form that allows uptake via water-soluble salts. The plant converts nutrients from these two sectors chemosynthetically. All macro and micronutrients are essential for healthy plant growth and it is important to appreciate that the growth rate is dependent on the lowest level of availability of any one of them. Macronutrients are defined as nitrogen (N), phosphorus (P), potassium (K), collectively termed the primary macronutrients, and magnesium (Mg), calcium (Ca) and sulphur (S), collectively termed the secondary macronutrients. Micronutrients are defined as manganese (Mn), iron (Fe) boron (B), zinc (Zn), copper (Cu) and molybdenum (Mb). Some plants may also have a requirement for sodium (Na), chlorine (Cl) or cobalt (Co). The focus of this essay is to describe the functions of the major nutrients and iron in plants and to explain how these functions are related to the symptoms of deficiency of these nutrients in plant tissue. Where relevant, examples of plants that are particularly sensitive to certain deficiencies are given. Plants use each of the macronutrients and iron for a wide variety of purposes. Each nutrient either contributes to or provides the means for several different functions in the plant and each of these functions are usually further defined as being of either a structural nature or of an activator nature. Nutrients may provide either one or both types of function. Structural functions include development of cell components such as proteins, genetic material or cell wall material. Activator functions include the development of enzymes that act as catalysts for chemical reactions in the plant, the development of co-factors that assist the enzymes and the maintenance of chemical balances in the plant to allow transport of substances and control cell turgor. The availability of nutrients to plants is influenced by several factors. Uptake by a plant will be affected by the ionic status of each nutrient, either a cation (+) or an anion (-), depending variously on soil ph, the cation exchange capacity of the soil and the presence of other elements in the soil. The varying combinations of the factors present will have varying influences on both availability and uptake. Nitrogen, for example, is normally available to plants as a negatively charged nitrate but is often absent in a clay soil, also negatively charged, due to rapid leaching caused by the inability of nitrogen

2 ions to attach to clay ions; in turn the plant will be deficient in nitrogen. Iron, for instance, is more easily reduced into a form suitable for plant uptake in low ph (acid) soils, which explains its unavailability in alkaline soils. Some elements in the soil are antagonistic such as potassium and magnesium, where a high level of potassium inhibits uptake of magnesium. Deficiencies of one nutrient in a plant can, therefore, be observed whether or not the deficient nutrient is present in the soil. Nitrogen stimulates leafy, green growth and is incorporated into the structure of chlorophyll, which by turn controls photosynthesis. Nitrogen is essential for carbohydrate use within the plant. Photosynthesis resulting carbohydrates are subsequently synthesised via respiration into the energy needed for growth. Nitrogen stimulates root growth and development allowing uptake of other essential nutrients. It also forms a major part of all amino acids in the structure of proteins, including enzymes, which control most biological processes and in nucleic acids, which control heredity. In many soils, nitrogen is the most important growth-limiting factor. Nitrogen is a mobile nutrient and when supply is limited, is quickly transported from older parts of the plant, especially the leaves, to feed younger parts. Deficiency shows chlorosis (a yellowing of leaves) in older leaves and stunts overall growth. Buds remain dormant and stems, petioles and veins can become red/purple. Brassicas show rapid sensitivity to nitrogen deficiency, as can be seen in young cabbage or wallflower plants, when still seen for sale late in the season and contained in very limited volumes of growing media. Nitrogen induced deficiencies are largely due to failing photosynthesis. Phosphorus is an essential component of adenosine triphosphate (ATP) which is the energy source for the living cells of a plant, particularly in the stimulation of cell division and in assisting photosynthesis. Additionally, it is a component of deoxyribonucleic acid (DNA), which controls heredity and ribonucleic acid (RNA), which directs protein synthesis. Young plants have a high requirement for phosphorus as it stimulates root formation and growth. Phosphorus forms part of fatty cell membrane components (phospholipids). Furthermore, it assists in early ripening and maturity of flowers and fruits including seeds. Phosphorus is also a very mobile nutrient; so, as with nitrogen, older parts of the plant show the first signs of deficiency as phosphorus is transported to younger parts. Stunting, thin stems and poor roots all indicate deficiency as does delayed maturity, sparse flowering and poor seed quality due to retarded cell development. Leaves are often dark, bluish green and veins, stems and petioles may become purple. Older leaves become yellow and senesce. Phosphorus is only available in soluble form between ph6 - ph7 and can become locked up in lime rich soils. Without sufficient ATP energy, of which phosphorus is an essential component, overall plant development is reduced. Phosphorus induced deficiencies are largely due to depleted energy resources. 2

3 Potassium does not play a structural role in plant health. It is an activator of large numbers of enzymes, which control the metabolic pathways in a plant such as energy metabolism, starch and protein synthesis, nitrate reduction, photosynthesis and sugar degradation. Furthermore, it promotes disease resistance and adaptation to environmental stresses such as drought resistance and winter hardiness. It is also important in fruiting and flowering by strengthening stems and improving colour and flavour. In root crops, such as potatoes and carrots it promotes carbohydrate storage. Potassium and magnesium ions mutually interfere with each other's uptake. Again, as with nitrogen and phosphorus, potassium is very mobile in a plant. White spots on leaf margins and a general chlorosis of the tips and margins of older leaves followed by necrosis (death), blue/green leaf colour, stunted growth, flaccidity, weak stems and a rosette or bushy habit are all indications of potassium deficiency. Reduced resistance to drought, pests and diseases may also be seen. Tomatoes frequently display deficiency due to potassiummagnesium ion antagonism. Potassium induced deficiencies are largely due to a breakdown in metabolic pathways within the plant. Magnesium is a constituent of chlorophyll and is essential to protein synthesis and many enzyme reactions. Magnesium is particularly important in its combination with ATP to activate dependent enzymes needed for photosynthesis, respiration and the formation of DNA and RNA. Large quantities of potassium or calcium ions in the soil interfere with magnesium uptake due to antagonisms between them. Deficiency symptoms differ between plant species, although general characteristics are similar. Symptoms are usually localised showing marginal chlorosis or interveinal spotting on older leaves. It is mobile in the plant and chlorosis appears first in older leaves and is progressive. Leaves may also display red, orange or purple pigments, which can often be confused with virus symptoms. Deficiency in Vitis vinifera is common. In extreme cases of deficiency, necrosis is noticeable. Monocots and dicots differ in their symptoms, inasmuch as monocots usually display small, dark green spots at the base of the leaf with chlorosis occurring on the rest of the leaf and necrosis at the tips. Grape, tomato and potato plants are susceptible. Magnesium induced deficiencies are largely due to the deactivation of ATP and reduced photosynthesis. Calcium is required for cell elongation, cell division and membrane stability. It synthesises pectin, which is the 'glue' for bonding cell walls vital to tissue formation in the meristems. It is also thought to be required for stabilisation of newly synthesised membranes - possibly by binding phospholipids to each other or to membrane proteins. Calcium is largely located between cytoplasm and cell walls. It is connected with ripening and senescence due to greater membrane permeability as a result of reduced 'free' calcium in relation to 'bound' calcium. 3

4 Deficiency results in reduction of the growth of meristematic tissue of roots, stems and leaves and has a drastic retarding effect on overall growth. Initially it is witnessed on growing tips, which become deformed and on young leaves, which become chlorotic, distorted, cup-shaped and hook-like. The laminae become crinkled and dried out and the leaf margins become necrotic. Soft stem tissue collapses due to the dissolution of cell walls. Roots become swollen and gelatinous. 'Bitter pit' in apples, 'Blossom end rot' in tomatoes and peppers and 'Cavity spot' in carrots are all symptoms either of complete plant calcium deficiency or of poor distribution of calcium within the plant favouring stems and leaves rather than fruits or roots. Death of plants due to calcium deficiency can be rapid. Calcicole and calcifuge plants show clear differences in their metabolisms. Calcium-rich alkaline soils are unsuitable for calcifuges such as Erica, Rhododendron or Azalea but are ideal for calcicoles such as Clematis, Viburnum, Scabious or Dianthus. Calcium induced deficiencies are largely due to the breakdown of the supporting structures of cells. Sulphur is a constituent of the amino acids methionone, cysteine and cystine, which all occur as free acids. Methionone and cysteine occur as building blocks for proteins whose enzymes regulate photosynthesis and nitrogen fixation. Sulphur's functions are more specialised than that of nitrogen or phosphorus. Other compounds containing sulphur are the vitamins thiamine, biotin and B1 and the coenzyme A, which is essential for respiration and for synthesis and breakdown of fatty acids. Sulphur is an essential ingredient of aromatic oils, which give cabbages and onions their distinctive aromas. As sulphur is an anion, most other plant nutrients - as cations - scarcely affect the absorption of sulphur. Deficiency inhibits protein synthesis and photosynthetic activity. Sulphur is relatively immobile and chlorosis is seen first in younger leaves and then interveinally. Plants become spindly with thin stems and petioles. Growth is slow and maturity is delayed. In field crops, sulphur deficiency can be hard to distinguish from nitrogen deficiency. In savannah type soils or in areas where vegetation has been burned to create arable land use, sulphur deficiency is frequent. It is responsible for 'tea yellows' in tea plantations. Sulphur induced deficiencies are due largely to inhibited photosynthesis. Iron is important in plant enzyme systems, in the structure of chlorophyll and most particularly its green pigments so essential for photosynthesis, in cell wall formation, in lignification and for respiration. In green plants there is a good correlation between the levels of iron and chlorophyll and it is therefore very important for effective photosynthesis. It is possibly also important for protein metabolisation. 4

5 Iron is competitive with the uptake of magnesium, manganese, copper, calcium, potassium and zinc. Most usually iron must be reduced before uptake by plant cells. Iron reduction is ph dependent and is higher at low ph. This explains its unavailability in alkaline soils and the chlorotic iron deficiencies displayed in calcifuge plants such as azaleas, rhododendrons and heathers when grown on alkaline soils. This type of chlorosis is defined as 'lime induced'. Iron is not readily mobile between plant organs and therefore chlorosis is displayed in younger leaves and interveinally at earlier stages. Later on, veins become yellow and young leaves can become white with necrotic lesions. All of these factors are due to a lack of chlorophyll pigments, which are heavily dependent on iron. Iron induced deficiency is largely due to failing photosynthesis. In conclusion, the deficiency of just one major nutrient or iron will have a detrimental effect on plant health regardless of an abundant availability of some or all of the others. As each major nutrient and iron, is individually vital to several areas of plant metabolism, it is clear that malfunctioning of one or more spheres of metabolism will have a 'knock-on' effect on the entire metabolism of the plant; moreover, there is a strong interdependency relationship between most nutrients. Absence or insufficiency of nitrogen, phosphorus and potassium will have the most crucial and damaging effect on plant health Plant science research has established many known links between nutrients and many areas of plant metabolism with proven reasons for these links. It has also identified possible links for which, as yet, no proven reasons or pathways have been established. And such is the intricacy of plant metabolism, that there may, indeed, be many more to discover and establish as fact. References ADAS/MAFF/ARC ( ) Diagnosis of Mineral Disorders in Plants HMSO Brady, N.C. (1998) The Nature & Properties of Soils Prentice Hall Mengel, K. & Kirby, E. A. (1987) Mineral Nutrition of Higher Plants Academic Press Salisbury, F.B. & Ross, C.W. Plant Physiology (4 th Ed.) Wadsworth (1992) Capel Manor (1998) RHS General Certificate Course Notes Capel Manor Capon, Brian (1997) Botany for Gardeners B.T. Batsford Ltd. Bibliography ADAS/MAFF/ARC ( ) Diagnosis of Mineral Disorders in Plants HMSO Brady, N.C. (1998) The Nature & Properties of Soils Prentice Hall Mengel, K. & Kirby, E. A. (1987) Mineral Nutrition of Higher Plants Academic Press Salisbury, F.B. & Ross, C.W. Plant Physiology (4 th Ed.) Wadsworth (1992) Capel Manor (1998) RHS General Certificate Course Notes Capel Manor Capon, Brian (1997) Botany for Gardeners B.T. Batsford Ltd. 5