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:
- As components of biomolecules, e.g. carbon, hydrogen, oxygen and nitrogen. These are structural elements of cells.
- As components of energy-related chemical compounds, e.g. magnesium in chlorophyll and phosphorous in ATP.
- Elements which activate or inhibit enzymes, e.g. Mg2+ activates RUBISCO and phosphenol pyruvate carboxylase. Similarly, Zn2+ activates alcohol dehydrogenase.
- 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:
Nitrogen: Nitrogen is the mineral which is required by plants in the greatest amount. Nitrogen is mainly absorbed as NO3 –, but some amounts are also taken up as NO4+ or NH4+. 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 H2PO4– or HPO4–. 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 (Ca2+). 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 (Mg2+). 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 (SO42 –). 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.
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 BO33 – or B4O72 –.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 (MoO22+). 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.