Mineral Nutrition

SYNOPSIS

• Absorption, distribution and metabolism of various mineral elements by plants is called mineral nutrition.
• Methods to Study the Mineral Requirement of Plants

• Hydroponics: Technique of growing plants in a nutrient solution.
• Aeroponics: Technique of growing plants without a growing medium.
• Sand Culture: Sand is used as rooting medium and nutrient solution is added to it.
  
  
• Physiological Role and Deficiency Symptoms of Macronutrients and Micronutrients
  
  
  

  Macronutrients are used in relatively large amounts by plants for their growth.
  Macroelement	Physiological Role	Deficiency Symptoms
  Nitrogen	Absorbed mainly as NO3−, NO2−, NH4+. Chief constituent of proteins, nucleic acids, amino acids, purines.	Yellowing of leaves and development of chlorosis.
  Sulphur	Absorbed in the form of SO42− ions. Determines protein structure.	Young leaves become chlorotic, anthocyanin pigmentation.
  Phosphorus	Absorbed in the form of H2PO4− or HPO42− ions. Acts as an activator of some enzymes.	Disruption of general metabolism. Abnormalities in the shape and size of chloroplasts.
  Calcium	Absorbed in the form of Ca2+ ions. Essential for chromatin or mitotic spindle organisation.	Margins of younger leaves show chlorosis.
  Potassium	Absorbed as K+ ions. Regulates stomatal movement.	Weak stalks are developed.
  Magnesium	Absorbed in the form of Mg2+ ions. Helps in photosynthesis.	Yellowing of leaves. Tips and margins of leaves turn upwards.
  Micronutrients are required by plants in minute quantities.
  Microelement	Physiological Role	Deficiency Symptoms
  Iron	Absorbed in the form of Fe3+ ions. Plays an important role in electron transport systems.	Development of characteristic chlorotic spots and the veins remain green.
  Boron	Absorbed as BO33− or B4O72−. Regulates carbohydrate metabolism.	Deformation, discolouration and disorganisation of meristematic tissue.
  Manganese	Absorbed in the form of Mn2+ ions. Plays a role in photo-oxidation of H2O and release of molecular O2.	Chlorosis and necrosis in the interveins of leaves.
  Copper	Absorbed in the form of Cu2+ ions. Plays a key role in the electron transport chain in photosynthesis.	Distortion and chlorosis in leaves followed by necrosis of the tips of young leaves.
  Zinc	Absorbed in the form of Zn2+ ions. Plays an important role in the synthesis of auxins.	Shortening of internodes resulting in stunted plant growth. Suppression of seed formation.
  Microelement	Physiological Role	Deficiency Symptoms
  Molybdenum	Absorbed in the form of MoO22+ ions. Required for nodulation in legumes, synthesis of tannins and reduction of nitrates to nitrites.	Development of chlorosis along with poor leaf growth. Reduction in nitrogen fixation in symbiotic plants.
  Chlorine	Absorbed in the form of Cl− anions. Essential in the transport of electrons from water to photo oxidised chlorophyll.	Wilted leaves, which then become chlorotic and necrotic.
  Nickel	Catalyses the hydrolysis of urea to CO2 and NH4.	Development of necrotic spots at the tips.

• The process of conversion of free nitrogen (N₂) to ammonia (NH₃) to make it available for uptake by plants is called nitrogen fixation.
• Rhizobium fixes atmospheric nitrogen in symbiotic association with the roots of leguminous plants.
  
• Nodules act as sites for nitrogen fixation because they contain the enzyme nitrogenase and pigment leghaemoglobin essential for fixing atmospheric nitrogen.
• Nitrogenase is a Mo–Fe protein which catalyses the conversion of atmospheric N₂ to NH₃.
  
  
  
• At physiological pH, ammonia is protonated to form ammonium (NH₄⁺) ions.
• In legumes, ammonia is assimilated into amino acids, amides or ureides.