Micronutrients in plants

 Micronutrients, or trace elements, are required in relatively small amounts (equal to or less than 0.1 mg per gram of dry matter) by plants. Iron, manganese, copper, molybdenum, zinc, boron and chlorine have been established as micronutrients.

 Iron: Plants obtain iron in the form of ferric ions (Fe3+). It is required in larger amounts in comparison to other micronutrients. Functions: It is an important constituent of proteins like ferredoxin and cytochromes, involved in transfer of electrons. It is reversibly oxidised from Fe2+ to Fe3+ during electron transfer. It activates catalase. Though iron is not a constituent of chlorophyll, yet it is closely concerned with it and probably it plays a role as a catalyst. 

Symptoms of iron deficiency: Lack of mobility accounts for iron deficiency first developing in younger leaves. Chlorosis of the leaves is a typical symptom of iron deficiency. Initially, the intravenous regions of the leaves become chlorotic and on prolonged deficiency, veins also become chlorotic.

 Manganese: It is absorbed in the form of manganous cation (Mn2+). Functions: This element is an activator for a number of enzymes particularly involved in dehydration in the splitting of water to liberate oxygen during photosynthesis. It is also involved in nitrogen metabolism. 

Symptoms of manganese deficiency: Plants deficient in manganese show chlorosis and grey spots on leaves. 

Zinc: Plants obtain zinc as (Zn2+) ion. Functions: It activates various enzymes, especially carboxylases. It is required for the synthesis of auxin. 

Symptoms of zinc deficiency: The deficiency symptoms of zinc are malformed leaves, inter-veinal chlorosis in leaves, and stunted growth. The deficiency also results in a reduction in size of internodes and consequent rosette type of growth. 

Copper: It is absorbed as cupric ion (Cu2+). 

Functions: Copper is a component of several enzymes, such as polyphenol oxidase, ascorbic acid oxidase and, thus, plays a major role in plant metabolism. It is a component of plastocyanin, a compound involved in the electron transport in photosynthesis. 

Symptoms of copper deficiency: The young leaves exhibit necrosis at the tip, and then in the margins, resulting in the withered appearance. In fruit trees, it causes dieback of shoot, where leaves wither and fall and bark becomes rough and splits, exuding gummy substances. 

Boron: It is absorbed as BO3 3-, B4O7 2-. Functions: Boron is required for uptake and utilisation of Ca2+, membrane function, pollen germination, cell elongation, and cell differentiation. Boron is also involved in the translocation of carbohydrates. Symptoms of boron deficiency: The deficiency of boron results in death of root and shoot tips, loss of apical dominance, abscission of flowers, small size of fruits, absence of root nodules in leguminous plants, and stunted growth.

 Molybdenum: Plants obtain it in the form of molybdate ion (MoO4 2–). Functions: Molybdenum plays an important role in the nitrogen metabolism of plants, especially in the reduction of nitrate. It is a constituent of the enzyme nitrate reductase. 

Symptoms of molybdenum deficiency: Molybdenum deficiency may cause nitrogen deficiency, as it is a component of enzymes involved in nitrogen metabolism. Plants deficient in molybdenum show slight retardation of growth, inter-veinal chlorosis, etc. 

Chlorine: It is absorbed in the form of chloride anion (Cl–). Functions: With Na+ and K+, it helps in determining solute concentration and anion-cation balance in cells. Chlorine may be required for cell division in both leaves and roots. It is essential for water-splitting reaction in photosynthesis, which leads to oxygen evolution. Symptoms of chlorine deficiency: Chlorine deficiency in plants results in wilted leaves, stunted root growth, and reduced fruiting.