Functions, Mode of Absorption and Deficiency Symptoms of Micronutrients

Micronutrients are needed only in trace amounts but are just as essential to metabolism as macronutrients, and several also play specific roles in individual crops. Iron sits at the boundary between the macro- and micronutrient categories by the amount a plant needs, but functionally it is essential for chlorophyll and carotenoid synthesis and is a component of cytochrome, ferredoxin and flavoprotein, also activating catalase and peroxidase; it is absorbed as ferrous (Fe2+) and ferric (Fe3+) ions, and fruit trees are especially sensitive to its deficiency, which causes interveinal chlorosis, short slender stalks, and inhibited chlorophyll formation. Manganese activates carboxylases, oxidases, dehydrogenases and kinases and is involved in splitting water to release oxygen during photolysis; absorbed as manganous (Mn2+) ions, its deficiency causes interveinal chlorosis, grey spots on oat leaves, and a poor root system. Copper is a constituent of plastocyanin, phenolases and tyrosinase, participates in redox reactions and ascorbic acid synthesis, maintains carbohydrate and nitrogen balance, and is part of oxidase and cytochrome oxidase; absorbed as cupric (Cu2+) ions, its deficiency causes die-back of citrus, reclamation disease of cereals and legumes, chlorosis, necrosis and exanthema in citrus. Zinc is essential for synthesising the auxin indole acetic acid and activates carboxylases, alcohol dehydrogenase, lactic dehydrogenase, glutamic acid dehydrogenase, carboxypeptidases and tryptophan synthetase; absorbed as Zn2+ ions, its deficiency causes little-leaf and mottle-leaf disease (from reduced auxin), interveinal chlorosis, stunted growth, necrosis, and khaira disease of rice. Boron is needed for carbohydrate translocation, calcium uptake and use, pollen germination, nitrogen and fat metabolism, and cell elongation and differentiation; absorbed as borate (BO3 2-) ions, its deficiency causes death of root and shoot tips, premature flower and fruit fall, brown heart of beet root, internal cork of apple, and fruit cracking. Molybdenum is a component of nitrogenase and nitrate reductase and is involved in nitrogen metabolism and fixation; absorbed as molybdate (MoO4 2-) ions, its deficiency causes chlorosis, necrosis, delayed flowering, retarded growth, and whiptail disease of cauliflower. Chlorine, absorbed as Cl- ions, maintains anion-cation balance, supports cell division, and takes part in the photolysis of water; its deficiency causes wilting of leaf tips. Nickel is a cofactor for the enzymes urease and hydrogenase, and its deficiency causes necrosis of leaf tips. Because iron becomes chemically locked up and unavailable in alkaline soils even when it is present, growers correct the …