So far, sixteen elements have been firmly vital for the growth and development of superior plants. The mineral nutrient elements play essential roles which can be largely clustered as follows:
- Constituting cellular structures and cellular metabolites 2. In cellular osmotic relationships and processes associated with turgor 3. In energy transfer reactions 4. Reactions catalyzed by enzymes 5. In the reproductive process of plants.
According to plant nutrition requirements, the nutrients are known as macronutrients and micronutrients. Mineral nutrients required for plants in concentration superior to one part per million (ppm) or 1–150 g per kg of plant dry matter are known as macronutrients (C, H, O, N, P, K, Ca, Mg, S), and those needed in concentration lower 1 ppm are micronutrients or 0.1–100 mg per kg of plant dry matter (Fe, Mn, Cu, Zn, Mo, B and Cl). This classification differs depending on plant groups and species
Physiological Roles of Macronutrients
Following carbon, hydrogen and oxygen, nitrogen plays a crucial role as a major structural component of plants. It plays a structural role along with carbon, hydrogen, phosphorus and sulfur. It is used to make a different nitrogenous plant compounds like proteins, nucleotides, porphyrins and alkaloids. Reduction of nitrogen to ammonium (NH4) is essential for its incorporation into organic compounds in order to carry out various metabolic functions. This involves nitrate reductase, which reduces NO3 to NO2 and the additional reduction of NO2 to NH4 via nitrite reductase. Amino acids, Peptides, Amides, Ureides, Amines are the Low molecular weight organic nitrogen compound. The Macromolecular organic nitrogen Proteins,Nucleic acids, Co-enzymes, Secondary products; membrane
Phosphorus has a structural function as a constituent of biomembranes and nucleotides. It is a significant lipid constituent of plant membranes occurring in the form of phospholipids, i.e. phosphatidylcholine. The phospholipids shape the central hydrophobic barriers of the cell membrane.
Potassium is absorbed at a very high level by plants because of high permeability of plant membranes to K+. It is characterized by great mobility within the entire plant, and is translocated to younger tissues. Furthermore, Potassium plays a significant role in the plant’s tolerance, which exposed to different biotic and abiotic stresses, including pathogens, water deficit and osmotic and thermal stress. . In cereals, potassium is known to provide mechanical vigor of the straw. Potassium also boosts plant resistance to disease-causing fungi, nematodes and other microorganisms.
Sulfur is absorbed by plants primarily from the soil in the form of sulphate (SO42-) and is then assimilated into a number of organic mixtures. Sulphate is immediately integrated into sulpholipids, polysaccharides, glucosinolates and certain phytoalexins. Sulphur presents protection toward toxic accumulation of heavy metals by phytochelatins. Sulphur binds to iron to produce iron-sulphur clusters (Fe-S) that form integral part of several iron proteins.
Calcium is found in plant tissues in the form of Ca2+, Ca carbonate, Ca phosphate and Ca oxalate. The uptake of Ca2+ is very slow as it is absorbed only by young root tips. Ca2+ is absorbed only by young root tips
Absorption is a passive process and is depressed due to the presence of K+ and NH4+. Low calcium is kept in the cytoplasm to prevent adverse interactions with other nutrient ions (PO4, Mg2+) and inactivation of enzymes.
Calcium works as a structural component of cell walls because it has a high concentration in the cell wall (apoplasm).
Calcium plays a significant role in cellular extending. Root growth is suppressed by calcium deficiency. The development and direction of the pollen tube are controlled by an extracellular calcium gradient. Calcium is found in very small amounts in the cytoplasm, which is important because calcium inhibits enzymes in cytoplasm and chloroplast.
Despite abundant in soil solution, magnesium (Mg2+) is taken up by plant concentrations considerably lower than the other cationic micronutrients. This is feasible due to strong cation competition in absorbing and lack of magnesium transporters in plasmalemma. Several enzymes are activated by Magnesium. An important role of magnesium involves phloem loading and unloading of sucrose. Magnesium-deficient plants show increased accumulation of sugars in the leaves