Mineral Nutrition. Criteria for Essentiality

Transcription

1 Mineral Nutrition Criteria for Essentiality The element is absolutely necessary for supporting normal growth and reproduction. In the absence of essential elements, plants cannot complete their life cycle or set the seeds. The essential element must be specific and is not replaceable by another element. The element must be directly involved in metabolism. Types of Essential Elements: There are 17 essential elements in plants. Additionally, some other elements; like sodium, silicon, cobalt and selenium are required by higher plants. There are two types of essential elements, viz. macronutrients and micronutrients. Macronutrients: Elements which are present in large amounts in plant tissues are called macronutrients. They are in excess of 10 mmole per kg of dry matter. Carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium and magnesium are the macronutrients. Micronutrients: Elements which are present in small amounts, i.e. less than 10 mmole per kg of dry matter are called micronutrients. Iron, manganese, copper, molybdenum, zinc, copper, boron, chlorine and nickel are the micronutrients. Categories of Essential Elements: a. As components of biomolecules, e.g. carbon, hydrogen, oxygen and nitrogen. These are structural elements of cells. b. As components of energy-related chemical compounds, e.g. magnesium in chlorophyll and phosphorous in ATP. c. Elements which activate or inhibit enzymes, e.g. Mg 2+ activates RUBISCO and phosphenol pyruvate carboxylase. Similarly, Zn 2+ activates alcohol dehydrogenase. d. Elements which alter osmotic potential of a cell, e.g. potassium plays an important role in opening and closing of stomata. Role of Macro- and Micro-nutrients:

2 Nitrogen: Nitrogen is the mineral which is required by plants in the greatest amount. Nitrogen is mainly absorbed as NO 3, but some amounts are also taken up as NO 4 + or NH 4 +. Nitrogen is one of the major constituents of protein, nucleic acids, vitamins and hormones. Phosphorous: This is absorbed by plants in the form of phosphate ions; either as H 2 PO 4 or HPO 4. Phosphorous is a constituent of cell membranes, some proteins, nucleic acids and nucleotides. Phosphorous is also required for all phosphorylation reactions. Potassium: Potassium is absorbed as potassium ion (K + ). This is required in more quantities by the meristematic tissues. Potassium helps in maintaining an anion-cation balance in cells. Potassium is involved in protein synthesis, opening and closing of stomata, activation of enzymes and maintenance of cell turgidity. Calcium: Calcium is absorbed in the form of calcium ions (Ca 2+ ). Calcium is required by meristematic tissues and differentiating tissues. Calcium is utilised in the synthesis of cell wall. Calcium is also required for the formation of mitotic spindle. Certain enzymes are activated by calcium. Magnesium: Magnesium is absorbed in the form of magnesium ions (Mg 2+ ). Magnesium activates the enzymes of respiration and photosynthesis. Magnesium is involved in the synthesis of DNA and RNA. It is a constituent of the ring structure of chlorophyll. It also helps in maintaining the ribosome structure. Sulphur: Sulphur is absorbed in the form of sulphate ion (SO 4 2 ). Sulphur is present in two amino acids; cysteine and mthionine. Sulphur is the main component of several coenzymes, vitamins and ferredoxin. Iron: Iron is absorbed in the form of ferric ions (Fe +3 ). Iron is the micronutrient which is required in the largest amount. Iron is an important component of proteins which are involved in electron transfer chain. Iron plays an important role in the formation of chlorophyll. Manganese: Manganese is absorbed in the form of manganous ions (Mn +2 ). Manganese activates many enzymes which are involved in photosynthesis, respiration and nitrogen metabolism. Splitting of water molecule during photosynthesis is facilitated by manganese. Zinc: Zinc is absorbed in the form of zinc ions (Zn +2 ). Zinc activates various enzymes; like carboxylase. Zinc is required in the synthesis of auxin.

3 Copper: Copper is absorbed in the form of cupric ions (Cu +2 ). Copper is essential for overall metabolism in plants. Copper is associated with certain enzymes in redox reactions. Boron: Boron is absorbed as BO 3 3 or B 4 O 7 2.Boron is required for uptake and utilization of calcium, membrane functioning, pollen germination, cell elongation, cell differentiation and carbohydrate translocation. Molybdenum: Molybdenum is absorbed in the form of molybdate ions (MoO 2 2+ ). Molybdenum is a component of various enzymes; like nitrogenase and nitrate reductase. Chlorine: Chlorine is absorbed in the form of chloride ion. Along with Na + and K +, chlorine helps in determining solute concentration and in anion-cation balance. Chlorine also plays an important role in splitting of water. Mineral Nutrition Deficiency Symptoms of Essential Elements There are different symptoms for deficiency of different elements. When a deficient mineral is provided to the plant, the symptoms disappear. But if the deficiency continues, it may lead to the death of the plant. Appearance of deficiency also depends on the mobility of the element in the plant. Some elements are actively mobilized in plants and are exported to young developing tissues. Deficiency of such elements first appears in the older tissues. For example; the deficiency symptoms of nitrogen, potassium and magnesium are first seen in the senescent leaves. This happens because these elements are mobilized to younger leaves. Some elements are relatively immobile in plants. These elements are not transported out of the mature organs. Deficiency of such elements first appears in younger parts of the plant, e.g. sulphur and calcium. Some deficiency symptoms in plants are; chlorosis, necrosis, stunted plant growth, premature fall of leaves and buds and inhibition of cell division. Chlorosis: Loss of chlorophyll is called chlorosis. This results in yellowing of leaves. Chlorosis is caused by the deficiency of N, K, Mg, S, Fe, Mn, Zn and Mo. Necrosis: Death of tissue; particularly leaf tissue; is called necrosis. Necrosis is caused by the deficiency of Ca, Mg, Cu and K. Deficiency of N, K, S and Mo causes stunted growth because of inhibition of cell division. Deficiency of N, S and Mo delays flowering.

4 Toxicity of Micronutrients If a mineral ion concentration in tissues reaches to a level that it reduces the dry weight of tissues by about 10%, the mineral then becomes toxic. It is difficult to identify the symptoms of toxicity. Sometimes, excess of an element may inhibit the uptake of another element. For example; the symptom of manganese toxicity is the appearance of brown spots surrounded by chlorotic veins. Manganese competes with iron and magnesium for uptake. Manganese also inhibits calcium translocation in shoot apex. Hence, excess of manganese results in deficiency of iron, magnesium and calcium. So, the apparent symptoms of manganese toxicity are in fact the deficiency symptoms of iron, magnesium and calcium. MECHANISM OF ABSORPTION OF ELEMENTS Absorption of minerals takes place in two main phases. In the first phase, passive absorption takes place through apoplast pathway. In the second phase, absorption takes place through symplast pathway. The first phase involves passive transport (facilitated diffusion), while the second phase involves active transport. After that, minerals are transported through xylem. NITROGEN CYCLE Nitrogen is available in limited amount in soil. Plants have to compete with microbes for this form of nitrogen. Hence, nitrogen is a limiting nutrient for plants. Lightning and ultraviolet radiations provide energy to convert gaseous nitrogen into oxides of nitrogen (NO, NO 2 and N 2 O). Atmospheric nitrogen oxides also come from industrial combustions, forest fires, automobile exhausts and power stations. Decomposition of organic nitrogen of dead plants and animals leads to the formation of ammonia. This process is called ammonificaiton. Most of this ammonia is converted into nitrate by soil bacteria, while some of the ammonia vaporizes and re-enters the atmosphere. Conversion of ammonia into nitrate; by soil bacteria takes place in following steps:

5 Biological Nitrogen Fixation (BNF): In this process, the atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase. This can be shown by following equation: N2 + 6H+ + 6e 2NH3 This process is coupled with the hydrolysis of 16 equivalents of ATP. This is also accompanied by the co-formation of one molecule of H 2. In free-living diazotrophs, the nitrogenase-generated ammonium is assimilated into glutamate through the glutamine synthetase or glutamate synthase pathway. Many nitrogen-fixing organisms exist only in anaerobic conditions; because the enzymes responsible for nitrogenase action are highly susceptible to destruction by oxygen. Symbiotic Biological Fixation of Nitrogen The plants of the legume family (Fabaceae) are the major contributors towards nitrogen fixation. The root nodules of these plants harbor the Rhizobium bacteria. These bacteria produce nitrogen compounds which help the plant to grow properly. When the plant dies, the fixed nitrogen is released into the soil. Thus nitrogen becomes available for other plants.

6 Question 1: All elements that are present in a plant need not be essential to its survival. Comment. Answer: An element should fulfill certain criteria to be categorized as essential element. Otherwise, it cannot be considered as essential elements. Out of the numerous elements present in plants, only 17 are considered as essential elements. Question 2: Why is purification of water and nutrient salts so important in studies involving mineral nutrition using hydroponics. Answer: Purification of water and nutrient salt is important to rule out other influencing factors. Use of pure nutrients and water will help in obtaining accurate scientific results. Question 3: Explain with examples: macronutrients, micronutrients, beneficial nutrients, toxic elements and essential elements. Answer: Macronutrients: Elements which are present in large amounts in plant tissues are called macronutrients. They are in excess of 10 mmole per kg of dry matter. Carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium and magnesium are the macronutrients. Micronutrients: Elements which are present in small amounts, i.e. less than 10 mmole per kg of dry matter are called micronutrients. Iron, manganese, copper, molybdenum, zinc, copper, boron, chlorine and nickel are the micronutrients. Beneficial Elements: Apart from the 17 essential elements, many other elements are required by plants. For example; sodium, silicon, cobalt and selenium are required by higher plants. These are called beneficial elements. Toxic Elements: If a mineral ion concentration in tissues reaches to a level that it reduces the dry weight of tissues by about 10%, the mineral then becomes toxic. This shows that any element can become toxic if it crosses a certain threshold in plants. Question 4: Name at least five different deficiency symptoms in plants. Describe them and correlate them with the concerned mineral deficiency. Answer: Following are the five deficiency symptoms and related minerals: a. Yellowing of lower leaves: Magnesium deficiency b. Pale green leaves: Nitrogen deficiency c. Purple leaf tints with bronze or brown leaf edges: Potassium deficiency d. Reddish purple undersides of leaves: Phosphorous deficiency e. White deposits on leaves: Carbon dioxide deficiency

7 Question 5: If a plant shows a symptom which could develop due to deficiency of more than one nutrient, how would you find out experimentally, the real deficient mineral element? Answer: For this, we need to tabulate all the available symptoms in different parts of the plant. Then the symptoms are compared with the symptom table; to arrive at a conclusion about the deficiency of a specific element. Question 6: Why is that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs? Answer: Appearance of deficiency also depends on the mobility of the element in the plant. Some elements are actively mobilized in plants and are exported to young developing tissues. Deficiency of such elements first appears in the older tissues. For example; the deficiency symptoms of nitrogen, potassium and magnesium are first seen in the senescent leaves. This happens because these elements are mobilized to younger leaves. Question 7: How are the minerals absorbed by the plants? Answer: Absorption of minerals takes place in two main phases. In the first phase, passive absorption takes place through apoplast pathway. In the second phase, absorption takes place through symplast pathway. The first phase involves passive transport (facilitated diffusion), while the second phase involves active transport. After that, minerals are transported through xylem. Question 8: What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium. What is their role in N2 -fixation? Answer: Rhizobium bacteria need symbiotic association with legume plants to carry out nitrogen fixation. Root nodules contain the necessary enzymes for nitrogen fixation and thus enable rhizobium to fix nitrogen. The enzyme nitrogenase facilitates the conversion of nitrogen into ammonia which is the first stable product of nitrogen fixation. Ammonia is then converted into glutamic acid. Glutamic acid is then utilised by plants to make amino acids; which are then utilised to make protein.

8 Question 9: What are the steps involved in formation of a root nodule? Answer: Development of root nodules happens in following steps: a. Rhizobium bacteria contact a susceptible root hair and divides near it. b. Successful infection of the root hair results in curling of the root hair. c. The infected thread carries the bacteria to the inner cortex. The bacteria get modified into rod-shaped bacteroids and cause inner cortical and pericycle cells to divide. Division and growth of cortical and pericycle cells lead to nodule formation. d. A mature nodule is complete with vascular tissues. The vascular tissues of the nodule are continuous with those of the root. Question 10: Which of the following statements are true? If false, correct them: (a) Boron deficiency leads to stout axis. Answer: True (b) Every mineral element that is present in a cell is needed by the cell. Answer: Out of all the mineral elements, only 17 are considered as essential elements. (c) Nitrogen as a nutrient element, is highly immobile in the plants. Answer: Nitrogen is highly mobile in plants. (d) It is very easy to establish the essentiality of micronutrients because they are required only in trace quantities. Answer: True