Macronutrients in plants

 Macronutrients Macronutrients are required in relatively large amounts by plants. These are carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium, magnesium and silicon. Carbon, Hydrogen and Oxygen: They are not minerals in origin, but are important structural constituents of protoplasm and other organic compounds. Of them, carbon is taken in as CO2 from air. Hydrogen is released from water during photosynthesis. Oxygen is generated within the plant during photosynthesis and is also absorbed in molecular form from air.

 Nitrogen: This is the mineral element required by plants in greatest amount. The chief source of nitrogen is the soil. It is absorbed as NO2 –, NO3 –, or NH4 + . This is required by all parts of a plant, NO2 –, NO3 – particularly the meristematic tissues. 

Functions: Nitrogen is an essential constituent of different proteins, nucleic acids, vitamins and hormones. Symptoms of nitrogen deficiency: The common symptom of nitrogen deficiency is chlorosis, which is yellowing of leaves. The symptom appears first in mature leaves and lastly in young leaves. Due to reduction in protein, plant growth remains stunted and lateral buds remain dormant. As a result cereals show wrinkling of grains. The nitrogen deficient plants develop purplish colouration due to the synthesis of pigment other than chlorophyll, like anthocyanin.

 Phosphorus: Plants absorb phosphorus from soil in the form of phosphate ions (either as H2PO4 – or HPO4 2–). 

Functions: Phosphorus is a constituent of plasma membrane, certain proteins, all nucleic acids and nucleotides, and is required for all phosphorylation reactions. It plays an important role in the energy transfer reactions. 

Symptoms of phosphorus deficiency: Deficiency of phosphorus causes delay in seed germination, stunted growth and results in the development of purple or red spots on leaves, premature fall of leaf and flower buds. 

Potassium: It differs from all other macronutrients in not being a constituent of any metabolically important compound. It is absorbed as potassium ion (K+). In plants, this is more abundant in meristematic tissues, buds, leaves and root tips. 

Functions: Potassium is involved in many of the physiological processes like respiration, photosynthesis, chlorophyll and protein synthesis and stomatal movement. Symptoms of potassium 

deficiency: The deficiency of potassium induces scorched leaf tips, shorter internodes, dieback, chlorosis in inter-veinal areas, loss of apical dominance, bushy habit, loss of cambial activity, plastid disintegration and increase in rate of respiration. Calcium: Plant absorbs calcium from the soil in the form of calcium ions (Ca2+). 

Functions: Calcium is required for the formation of calcium pectinate, which is the constituent of the middle lamellae in the cell walls. Meristematic and differentiating tissues require it. Calcium is also used in the mitotic spindle during cell division. It accumulates in older leaves. It is involved in the normal functioning of cell membranes. It activates certain enzymes and plays an important role in regulating metabolic activities. Symptoms of calcium deficiency: The deficiency of calcium results in stunted growth, necrosis of young meristematic regions, such as root tips or young leaves.

 Magnesium: It is absorbed by the plants in the form of divalent Mg2+. Functions: It activates enzymes in respiration and photosynthesis, and the synthesis of DNA and RNA. Magnesium is a constituent element of chlorophyll and maintains ribosome structure. 

Symptoms of magnesium deficiency: Magnesium is readily mobile and when its deficiency occurs, it is apparently transferred from older to younger leaves, where it can be reutilised in growth processes. As a result, deficiency symptoms develop first in older leaves. Deficiency of magnesium induces chlorosis between the leaf veins and necrotic or purple coloured spots on older leaves. Magnesium deficiency may cause premature leaf abscission. Sulphur: Plants obtain sulphur in the form of sulphate (SO4 2–). Sulphur is present in two amino acids, cysteine and methionine, and is the main constituent of several coenzymes, vitamins (thiamine; biotin; CoA) and ferredoxin.

 Functions: Sulphur increases the root development. It also increases the nodule formation in legumes. Symptoms of sulphur deficiency: The deficiency of sulphur causes chlorosis of younger leaves, stunted growth and anthocyanin accumulation. These symptoms are essentially similar to those of nitrogen deficiency because sulphur and nitrogen are constituents of proteins.