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"Feeding & Watering Orchids and the Function of Nutrients in Plant Growth" Please be patient while this page loads! Summary By Mr. M.J. Erasmus. Plant growth requires the input of both energy and matter Energy comes from two forms:
Matter comes in several forms:
Light energy is utilized in the well-known process photosynthesis, the building of chemical compounds through the interaction of light with small molecules. Heat drives transpiration, which results in the upward movement of water and minerals within the plant. As water evaporates from the leaves it is replaced from the roots. Without light and transpiration, plants cannot grow. Andy Easton, New Zealand's famous orchid breeder and grower offers simple advice for growing orchids. "Growing orchids is easy. Just give them the right amount of light, the correct amount of feed, the right amount of water and the proper temperatures. Your orchids will grow like weeds". Invariably, the question is asked, "how much of each of these variables is correct"?
Fertiliser N - P - K Fertilisers are labeled with the quantity of macro elements, nitrogen, phosphorus and potassium sold in the package. In the US, fertiliser labelling must conform to guide lines imposed by each state and all states have relatively similar regulations. Labelling provides the N-P-K values for nitrogen, phosphorus and potassium. Why "K" for potassium? The K comes form the German word for potassium, kalium. K is the internationally accepted symbol for the element potassium. To confuse matters, the N in N-P-K is the percentage of nitrogen that can occur in several forms such as nitrate, ammoniacal nitrogen and urea. P is the percentage of phosphorus as expressed by the molecule P2O5 even when the phosphorus source is not P2O5 and similarly K is the potassium (kalium) percentage as expressed by the molecule K2O. (For this discussion these packaging quirks are not particularly important; however, they point to a lack of cogency in regulations - there is better nomenclature available). In addition to N-P-K, fertiliser contains micronutrients, nutrients needed by plants in very small quantities. Regulation requires that micronutrients exist in certain measurable amounts before they can be listed. Regrettably, these measurable quantities can approach or exceed phytotoxic levels for plants. Micronutrients already exist in our water supplies and as impurities in the constituents of fertiliser, even when they are not listed. A confusing aspect of nomenclature is ratio. A 30-10-10 fertiliser has the same ratio as 15-5-5 fertiliser; however, the former is twice as strong per unit of weight. Feed Ratio Plants are about 90% water and 10% solids. The ratio of nitrogen, phosphorus and potassium in plant tissue once the water is removed is about 3 - 4.5% N, 0.3 - 0.6% P and 3 - 4.5% K (1 - 2% Ca and 0.2 - 0.5% Mg). So, what do plants need to be supplied for growth? Plants are adaptable and are not damaged by moderate amounts of N, P or K, regardless of the ratio. Experimentally, a ratio was determined some decades ago which supported excellent growth in orchids. This ratio is 3 - 1 - 2 ratio. My own experience shows this ratio works well. I have reviewed the fertilising schedules of two, superb commercial odontoglussum nurseries. One uses a ratio of 4-1-2, the other 4-1-4. In other words, nitrogen is added in the largest percentage, with phosphorus significantly lower than nitrogen and potassium somewhere in between. My Recommendations:
What is a good, safe level for nutrient solutions?
Conductivity meters There is an even more sophisticated method of determining feed strength. Pure, distilled water has low conductivity and virtually all compounds relating to feed programmes, dissolved in water, significantly increase the conductivity of the solution (urea is an exception). Until recently, the conductivity of a solution was expressed in the conductivity unit "mhos". The mho is ohm spelled backwards and mathematically the reciprocal of the resistance unit, the ohm, I.e., I/ohm's=mho's. One could use either the ohm or the mho to measure conductivity in the solutions we work with for plant culture; however, the conductivity numbers are easier to interpret than resistivity values. To confuse matters, the international committee on scientific units has officially changed the mho to the Siemen honouring the famous German scientist of that name. In keeping with convention, all international units now honour a scientist and are capitalised. Scaling units, such a milli or micro are written lower case. Example:? Siemens. The tradition in horticulture is to refer to the conductivity of a solution as its "EC" (electrical or earth conductivity). The EC for fertiliser feed stocks will vary depending on each specific crop need. The EC for most horticulture crops falls within a range of 0 -2000 Siemens (in "old" units, an EC of 0-2 or 0-2 millimhos). Fairly accurate, temperature corrected conductivity meters are available for around $50. These meters make determining the conductivity of feed solutions relatively simple. These meters are available in either parts per million (ppm) or Siemens. I recommend the Siemens meter over the ppm meter. I use one with a range of 0 - 1990 Siemens when I measure my feed strength. Why not select a ppm meter? For most purposes, a relatively simple conversion factor can be used to correlate Siemens to ppm, typically something like Siemens x .64=ppm. The hitch is I ppm of one chemical compound does not have exactly the same conductivity as 1ppm of another. Example: I ppm ammonium nitrate does not have the same conductivity of 1 ppm of ammonium sulfate. Thus, when measuring a mixture of different compounds one does not measure true ppm's, only an approximate average equivalent. Although the errors are not large, I prefer using conductivity meters which read in Siemens rather than ppm. Using a conductivity meter a grower can repeatedly mix a particular fertiliser with water to the same target EC. It is most productive to run feed levels high enough for good growth. It is important to avoid excessively high feed concentrations which will result in problems such as leaf tip die-back and root damage. What is a good EC value?
Urea Urea is a cheap form of nitrogen often used in fertiliser. Solutions of urea in water have low conductivity. Urea in its better, low biuret grades makes a good nitrogen source for plants grown at temperatures that are above 16°C (60°F). It is not a good source of nitrogen for cool growing crops, particularly during the winter months because bacteria populations, needed for nitrifying urea, are in low population. Urea requires bacterial action before its nitrogen is available to plants. The function of Macro- and Micronutrients in Plant Growth. There are sixteen elements which are essential for higher plants (Orchids) to complete their live cycle. Nine elements - carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur - are described as macronutrients because large quantities are required to control the process within plant cells and form the compounds necessary for plant growth e.g. sugars, starch, cellulose, lignin, fats, amino acids, plant hormones etc. The first three macronutrients, carbon, hydrogen and oxygen is available to plants in 2 compounds i.e. carbon dioxide and water, and is present in most of the hundreds of compounds formed in plants. Seven elements - iron, zinc, copper, manganese, boron, molybdenum and chlorine - are described as micronutrients (or trace elements) because they are needed in very small concentrations for plant growth. Their function is to assist in the many complex reactions which result in the formation of the previously mentioned substances. The essential elements are generally supplied to orchids as solutions of compounds (see list) such that the living plant tissue contains the nutrient at levels and proportions sufficient to maintain satisfactory growth and development. It is interesting to note that some nutrients can be accumulated within a plant even though their concentrations in the plant tissue are higher than those in the external solution around the roots. Some elements may be present in the natural water used to make up the solution. The rate of growth of the plant is affected by the concentration of each of the particular elements in the leaf tissue mentioned above. At low concentrations (deficient zone) the growth rate is below maximum but rapidly reaches the optimum rate as the concentration of the element in the leaf tissue increases. In the deficient zone the plant exhibits characteristic deficiency symptoms because certain plant functions are affected. Any further concentration increase does not change the rate of growth (adequate zone) and excess nutrient becomes stored in the plant. The nutrient deficiency symptoms which are exhibited by plants include: stunted growth of leaves, stems and roots, deformities of leaves and roots, a yellowing of leaves (chlorosis) and a darkening due to the presence of dead tissue (necrosis). Plants can translocate deficient elements to the new growth areas by transferring elements through the sap-conducting vessels (phloem) from the mature leaves to the new leaves. Easily translocated elements: N, K, P, Mg, Cl, and sulphur (S). Intermediate mobility: Zn, Mn, Cu, Mo, (S). Relatively immobile: B, Fe, Ca. For example, with a magnesium deficiency, yellowing first appears in the old leaves because magnesium is readily transferred (traslocated) to the growing leaves. With immobile iron and calcium the deficiency symptoms are first observed in the new leaves. 1. Photosynthesis This process takes place in the chloroplasts of cells in the green leaves of plants exposed to sunlight. Carbon dioxide (enters the leaf through pores called stomates) and water (taken in through the roots), utilising light energy absorbed by the green chlorophyll, are ultimately converted mainly to glucose and oxygen in a series of about fifty reactions. This is the most important process on earth, as the sun's energy is used to replace carbon dioxide in the air with oxygen, while at the same time forming energy rich substances necessary not only for plant growth but also, directly or indirectly, for food and fuel of all animal species. (enzymes) carbon dioxide + water + light = glucose + oxygen 6CO2 + 6H2O + energy = C6H12O16 + 6O2 2. Respiration The formation of larger molecules - starch, cellulose, proteins, vitamins, DNA, RNA, hormones - requires a supply of readily available energy. This is supplied by the process called respiration that takes place in the mitochondria of plant cells. Energy-rich glucose is broken down by oxygen in some fifty distinct reactions, mainly to carbon dioxide and water with the release of energy. Much of the energy is lost as heat, some of which maintains the temperature such that the growth of the plant is stimulated even through hours of darkness. The remaining energy is used to form the large molecules mentioned above, often through other energy-rich compounds such as sugar-phosphates, particularly ATP (adenosine triphosphate). (enzymes) glucose + oxygen = carbon dioxide + water + energy C6H12O16 + 6O2 = 6CO2 + 6H2O + energy 3. Protein Synthesis Nitrogen provided to the plant as nitrate is converted first to nitrite and then to ammonium ions which are used to synthesise amino-acids. With the genetic information provided by DNA from the nucleus and with the aid of RNA, twenty different amino-acids are combined in ribosomes in the cytoplasm in different ways to form the wide variety of proteins found in plants. In these three plant processes and others, the macro- and micronutrients have important functions, some of which will be referred to for each separate element. Nitrogen (N) Nitrogen is generally made available to plants in fertiliser solution as ammonium ions, nitrate ions or urea. Atmospheric nitrogen, can be used by some plants, but not orchids, so this process will not be considered. Most of the nitrate ions that enter the roots from the surrounding solution, are carried upwards through the sap-conducting vessels (the xylem) into the cytoplasm of the leaves. Excess nitrate is stored in the vacuoles. For their main function as a nitrogen nutrient, nitrate ions are first converted to nitrite ions in the cytoplasm with the aid of an enzyme (nitrate reductase) containing molybdenum and iron. These nitrite ions are then transported to the chloroplasts where an iron-containing enzyme (nitrite reductase) catalyses their conversion to ammonium ions. Because of their toxicity to plants, even in quite low concentrations, the ammonium ions are rapidly converted to several nitrogen compounds, one of which (glutamine) undergoes further enzyme-controlled reactions to produce many different amino-acids. Finally in ribosomes (mainly in the cytoplasm) twenty of these amino-acids are combined together in the genetically coded sequences defined by DNA and guided by RNA to form the wide variety of proteins required for their structural and enzymatic roles in the plant. If ammonium ions are present in fertilisers, their toxic effect on the plants is avoided because bacterial action on the root surface converts the ions to amino acids that are then trans located from the roots to the leaves for protein synthesis. Many simpler nitrogen compounds are synthesised in plants from nitrate and ammonium ions. These include compounds (guanine, adenine, cytosine, thymine and uracil) required for the formation of DNA and RNA (see Phosphorus). An adequate supply of nitrogen encourages the formation of the growth hormones cytokinin, auxin and gibberellic acid but a reduction in its availability affects the concentration and balance of these hormones and encourages the formation of the senescence hormone, abscisic acid. Under the latter conditions, especially with a higher than normal potassium ion concentration, it appears that nitrogen in the form of ammonium ions may favour flower bud initiation. With a nitrogen deficiency the growth of plants is retarded as fewer structural and enzyme-functioning proteins are produced. Under these conditions nitrogen compounds are trans located from the mature leaves to the regions of new growth. This leads to a rapid senescence of older leaves. They first turn pale green, then yellow overall and finally to a tan colour as they die. The roots of nitrogen-deficient plants are generally more extensive than normal. With excess nitrogen the leaves become darker green, wider and longer but reduced in thickness. This sometimes causes the plant to become droopy. The roots are generally smaller and more bunched than normal. Phosphorus (P) Phosphorus is supplied to plants as water-soluble compounds such as calcium dihydrogen-phosphate. It enters the roots as phosphate ions. DNA (Deoxyribose Nucleic Acid) and RNA (Ribonucleic Acid), which are responsible for replication of a plant in the same form through generations, are phosphorus compounds composed of long sequences of four different building blocks called nucleotides. Each nucleotide in DNA contains a phosphate group, a sugar (deoxyribose) and one of four nitrogen compounds (adenine, guanine, cytosine and thymine) while those in RNA are similar but sugar is ribose and uracil is present instead of thymine. Nucleotides also serve another equally important function in plants by acting as chemical agents for the storage and transfer of energy between substances during reactions. The most important, adenosine triphosphate (ATP), containing three phosphate groups, participates in a great number of reactions during photosynthesis, respiration and other plant processes. Many other complex phosphorus compounds are involved in the 50 or more reactions which give rise to the final products of both photosynthesis and respiration. For example in utilising the light energy stored during photosynthesis carbon dioxide must first react with a suitable compound to begin the process, function is performed by ribulose bis phosphate. When reserves of protein are being deposited during the formation of seeds (bulbs and tubers) most of the phosphate is converted to phytic acid, a compound with six phosphate groups. Phytic acid becomes associated with calcium, magnesium, potassium, zinc and iron as phytates. These substances regulate the deposition of starch during the development of the above-mentioned storage organs of the plant. When growth begins again, the proteins release amino-acids that are then trans located with other mineral nutrients (phosphate, calcium and magnesium) to the developing roots and shoots. Phosphorus-deficient plants are retarded in growth because fewer phosphorus-containing compounds essential for photosynthesis and respiration are formed. The plants are often darker green than normal and a reddish colouration is occasionally seen due to the pigment, anthocyanin. A deficiency also affects the plant hormone balance thereby delaying flower initiation and decreasing the flower count. Excess phosphorus hastens the maturity of a plant. Root growth is more extensive than normal. Potassium (K) Potassium ions play an essential role, not by being part of the structure of plants, but by exerting a regulatory function on reactions and processes. The uptake of potassium ions by roots from the surrounding solution is highly selective - other non-essential ions such as sodium ions are left in solution. The potassium ions readily move uncharged through sap-conducting vessels to all parts of the plant, carrying other ions such as nitrate ions with them. Potassium ions activate over 50 enzymes in plants. This activation has been likened to a "lock-and-key" effect. For example the enzyme, starch synthetase, assists in the rapid conversion of glucose to starch in amyloplasts but only if potassium ions are present to modify the shape of the enzyme (activate). Similarly these ions activate the enzyme, nitrate reductase, as it converts the nitrate ion to nitrite ion in a step towards synthesis of proteins. Potassium ions play an important role in the steps involved in the conversion of amino-acids to proteins. These ions also protect plants from disease by promoting the growth of thicker outer walls of leaves. In addition they regulate the level and balance of growth and flower initiation (senescence) hormones in plants. The latter effect due to a higher than normal potassium ion concentration and lower nitrogen presence was mentioned earlier to favour flower-bud initiation. The most important function of potassium ions in plants is in the control of the movement of water into, and out of, cells by a process called osmosis. The vacuoles of cells contain a high concentration of dissolved substances of which potassium ions are the most prominent positively charged ion. This results in water moving through the cell-wall membrane into the concentrated solution of the vacuole thereby building up a pressure within the cell. This pressure can be maintained because of the rigidity of the cell wall. The resultant turgidity of the cells plays a major part in producing the rigidity of the plant as a whole. This same mechanism is observed in two other important plant processes: a) The outer surface of leaves contains holes (stomatal pores) whose size can be increased by the swelling of adjacent sausage-shaped guard cells. When light falls on a leaf, potassium ions move from cells surrounding the stomata into the two guard cells. Water then enters the more concentrated solution and the resultant pressure build-up causes guard cells to bow outwards. This leaves an opening through which carbon dioxide from the air can enter for photosynthesis to occur in the chloroplasts of neighbouring cells. The oxygen which is formed moves out through this opening. At night the potassium ions move out of the guard cells and the stomate closes. The non-growth hormone, absecisic acid assists in bringing about this closure. b) Cell extension occurs because potassium ions accumulate in the central vacuole of the cell with other substances and water moves in by osmosis. As the vacuole swells to occupy 80-90% of the whole cell volume, the cell wall extends uni-directionally under the influence of the growth hormone, auxin. Potassium-deficiency symptoms are first seen in the older leaves because potassium ions are readily trans located to the younger leaves. A potassium deficiency also results in a loss of turgidity of cells so that the plants "wilt" because less water enters the vacuoles by osmosis. Plants lacking potassium are more susceptible to fungal diseases, probably due to an accumulation in plant tissue of unused sugars and amino-acids which provide the food for the attacking organism. Under these conditions the roots are readily affected by rotting. Some potassium-deficient plants are more susceptible to frost damage. An excess of potassium can affect the uptake and availability of magnesium and calcium to a plant. Calcium (Ca) Calcium can be supplied to plants as calcium nitrate, calcium dihydrogen phosphate and calcium sulphate (gypsum). Small quantities are present in most natural waters and may be slowly released from the potting mixture if marble chips (calcium carbonate) or shellgrit (calcium carbonate) or dolomite chips (calcium carbonate and magnesium carbonate) are present. Calcium enters the roots as calcium ions and any excess is stored in vacuoles. A significant portion of calcium is located as calcium pectate between the cellulose walls of adjacent plant cells and within the outer plasma membrane surface. Its presence not only strengthens the cell wall but also determines the membrane permeability. If insufficient calcium is available the membranes lose their ability to allow the required chemicals to pass in and prevent leakage outward of substances contained within the cell. Calcium pectate also helps to make the plant less susceptible to fungal diseases. Calcium is required in regions of active cell division (for wall formation) particularly in the meristematic zones of roots and leaves. It is also needed for root extension and its absence causes the process to cease within a few hours. Pollen tube growth is dependent on the presence of calcium, the concentration of which is highest in the growing tip. Calcium plays a part in the movement of sugars and amino-acids within a plant. Calcium ions are believed to fulfill an important role in gravitropism - the tendency of primary roots to grow downwards with gravity and the leaves and stems upwards against gravity. With a calcium deficiency the growth of the plant is stunted. New leaves are limited in their development, the tips tend to die, a paleness is shown along the edges with some twisting and perhaps inward curling, and black necrotic spots may appear. The effect is most noticeable in the meristematic area. Roots are particularly sensitive to a calcium deficiency. Growth is severely affected - young roots and root hair may die and older roots turn brown. A lack of calcium may also affect the flowering of the plant. Magnesium (Mg) Magnesium is generally supplied to plants as water-soluble magnesium sulphate (Epson Salts) but some may be received in small amounts if dolomite (MgCO3, CaCO3) Magnesium is the central atom in the green pigment chlorophyll found in the chloroplasts of leaf cells. In the complex process, photosynthesis, certain wavelengths of light are absorbed by the chlorophyll to begin the many reactions which ultimately replace carbon dioxide from the air with oxygen while forming the high energy sugar, glucose. Magnesium ions activate some of the enzymes involved in these reactions especially those in which ATP is used as a source of energy. One such reaction, that between carbon dioxide and ribulose biphosphate, has previously been referred to. Magnesium ions play a major role in activating those enzyme-controlled reactions using ATP that occur during plant respiration. In this process glucose and oxygen react to release energy: a) mostly as heat to maintain plant life throughout day and night; b) while the remainder constitutes to be entrapped in still more ATP (Adenosine Tri Phosphate) and other high-energy chemicals which ultimately lead to the formation of many important plant substances including chlorophyll, amino-acids and nucleotides. c) Magnesium also plays a part in the formation of DNA and RNA; in the activation of the enzymes that assist in the conversion of ammonium ions to amino-acids and, together with calcium, as phytate during the storage of starch in seeds and bulbs. A deficiency of magnesium leads to a reduction in photosynthesis as insufficient chlorophyll is produced. A lack of magnesium ions also affects protein synthesis so that the rate of plant growth is reduced. Sulphur (S) Sulphur is generally received by plants as sulphate ions. These enter the roots from the surrounding solution and most are conveyed unchanged in the sap-conducting vessels (xylem) to the leaves. In the chloroplasts sulphate ions are converted to sulphide ions, using energy supplied by ATP with the assistance of an iron-sulphur enzyme, ferredoxin. A similar reaction occurs in the roots. Most of these sulphide ions are rapidly converted into the two sulphide-containing amino-acids, cysteine and methionine, that are required with other essential amino-acids for protein synthesis. Traces of cysteine are used to form an important sulphur compound (coenzyme A) that is involved in the step during respiration in which carbon dioxide is formed. The sulphur-containing vitamin B1 (thiamine) coupled with phosphate takes part in this reaction. Small amounts of methionine are converted to a compound (S-adenosylmethio-nine) from which can be formed lignins to strengthen cell walls, brightly coloured compounds (anthocyanins) and chlorophyll. Two iron-sulphur proteins and ferredoxin (in which they are similar) participate in the initial reactions of photosynthesis in which absorption of light by chlorophyll releases oxygen. (A manganese-chloride protein and a copper-containing protein are also involved.) A sulphur deficiency in plants, although not very common, affects protein synthesis and, to a lesser extent, photosynthesis. In most plants sulphur is not easily trans located. As a result, the deficiency symptoms are a light green to yellow chlorosis over the whole of new leaves including the veins. The leaves may not grow to their normal size. Iron (Fe) Many fertiliser mixtures contain this micronutrient in the form of pale green Iron (II) sulphate crystals. However, unless the pH of the nutrient solution is below 5, very little iron is absorbed through the roots because Iron (II) ions, react with water producing an almost insoluble substance. Iron (III) chelates are far more effective. They dissolve in, but are unaffected by, water and on contact of the nutrient solution with the root surface they are reduced to Iron (II) ions which are immediately bound to certain chemicals in the sap (e.g. citric acid) and carried throughout the plant. in plant growth iron mainly functions as an electron-transfer agent by alternating between the Iron (II) and Iron (III) states - ferrous and ferric states. Several iron-sulphur proteins are present in plant cells, the most important being ferredoxin. In photosynthesis this compound plays a key role, in the presence of chlorophyll, in releasing oxygen from water. In the reaction leading to protein synthesis ferredoxin transfers electrons to reactants in enzyme-controlled reactions which convert nitrite ions to the amino-acid, glutamine. It also takes part in the conversion of sulphate ions to sulphide ions in the series of reactions that ultimately form the two sulphur-containing amino-acids, cysteine and methionine. Other iron-proteins also act as enzymes for reactions during plant growth. One such hemeprotein (peroxidase) catalyses the formation of lignin in cell walls. Other hemeproteins (cytochromes) provide electrons for the final steps of respiration which Oxygen forms water in a reduction reaction. Yet another protein of this type (catalase) is involved in the synthesis of chlorophyll and also plays a part in a reaction releasing oxygen during photosynthesis. Manganese (Mn) Manganese is usually supplied to plants as Manganese sulphate. It is absorbed through roots from the nutrient solution as manganese ions, which are then transported in the sap of the xylem to the growing leaves. Manganese fulfils its main function during the first stage of photosynthesis. In the presence of chlorophyll in the chloroplasts of leaf cells a Manganese-chloride protein brings about the light-induced reaction in which water is split with the release of Oxygen. A Manganese deficiency restricts the formation of chlorophyll. This results in a yellow mottling on both young and old leaves. From its role in photosynthesis, an inadequate supply of Manganese reduces the amount of sugars and indirectly affects the formation of amino-acids and therefore protein synthesis. An excess of Manganese can reduce the uptake of Magnesium and Calcium. The symptoms observed are the appearance of dark purplish-brown spots. Copper (Cu) Copper is usually present in fertiliser mixtures as blue crystals of copper sulphate and enters the roots as either copper ions, or in a chelated form (e.g. with amino-acids). About 50% of plant copper is present in chloroplasts where it performs most of its functions, generally as an electron-carrier between the copper (II) and copper (I) states. During photosynthesis a copper-protein (plastocyanin) together with another enzyme (plastoquinone) transfer electrons to reactants linking two series of reactions that take place in chloroplast membranes. Although plastoquinone contains no copper it requires a copper enzyme (laccase) for its formation. In most plants a copper-zinc protein (copper-zinc dismutase), rather than the corresponding manganese compound, assists in the removal of the highly reactive oxygen spcies, O²¯, formed during photosynthesis as previously mentioned (see Manganese). In the membrane of chloroplasts another copper enzyme (phenolase) plays a part in synthesising both lignin and the brown substances which exude when plant tissue is damaged thereby preventing fungal growth. Reference has previously been made to the part played by the copper-iron enzyme (cytochrome oxidase) in the final step of the respiration process. Copper is essential for the development of the anthers and ovaries of flowers. A copper-deficiency results in younger leaves being stunted in growth due to reduced photosynthesis. At the same time these leaves tend to wilt and become distorted because insufficient lignin is deposited in the walls of the sap-conducting cells. The growing tip may eventually die. A copper-deficiency results in the non-viability of pollen. Excess copper may affect the uptake of iron by plants. Zinc (Zn) This micronutrient is present in fertilisers as zinc sulphate and enters the plants through the roots as zinc ions. One of the most important functions of zinc is in the synthesis of the growth hormone, auxin, in meristems and new leaves. Zinc activates, or is present in, many different types of enzymes. It is essential for protein synthesis. Zinc enzymes are also responsible for the transport of certain sugars as phosphates between chloroplasts and the cytoplasm. Boron (B) Boron is present in fertiliser mixtures either as sodium borate (Na2B4O7)3BO3) Plants can tolerate only a narrow range in concentration (about tenfold) between deficiency and toxicity limits - about 0.5 to 6 ppm of boron in the nutrient solution given to the plants. Many functions of boron are not fully understood but unlike other micronutrients this element does not occur in, or activate, enzymes. Boron is an important constituent in cell-walls and plasma membranes of growing tissue including the roots. Molybdenum (MO) Very small amounts of ammonium molybdate are necessary in fertiliser mixtures. The dry tissue analysis of healthy plants shows only about one part of molybdenum in ten million is present, much lower than for any other essential micronutrients. Molybdenum enters the plant as molybdenum ion. Molybdenum is found in a few enzymes but its most important function is in the initial stage of protein synthesis. A molybdenum-iron protein (nitrate reductases) assists in the conversion of nitrate to nitrite in the cytoplasm of leaf-cells. This element is also required for the production and viability of pollen. A molybdenum deficiency in plants is rare. Symptoms are mottled, pale yellow colourations between the veins. Chlorine (Cl) Chloride ions are widely present in natural waters and after entering the roots readily move unchanged throughout the plant. Most plants absorb up to 100 times very small amount of chloride that is essential for optimal plant growth. These chloride ions assist in achieving a charge balance with positive ions in plant solutions and, by entry into vacuoles, help to maintain the turgidity of cells brought about by osmosis. Chloride ions activate as enzyme (asparagine synthetases) for one of the series of reactions involved in the conversion of ammonium ions to amino-acids. References:
Plot 26 Waterklooflandbouhoewes
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