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In this article we will discuss about the roles of micronutrients elements in plants.
1. Manganese:
Manganese is required in very low concentrations as its high concentrations become toxic to plants. It is abundant in the physiologically active parts of the plant especially leaves and it is relatively immobile.
It is an activator of enzymes involved in respiration and nitrogen metabolism. For example, malic dehydrogenase and oxalo-succinic decarboxylase, the enzymes of the Krebs cycle, in respiration and nitrite reductase and hydroxylamine reductase the enzymes of nitrate reduction in nitrogen metabolism.
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It is also an activator for the enzymes of fatty acids synthesis and of DNA and RNA formation. It is also involved in the destruction or oxidation of indole-3 acetic acid (IAA). It is directly involved in photosynthesis as an electron carrier participating in a reaction by which water is split and oxygen is released (Hill reaction).
It acts as a catalyst in an unknown reaction of chlorophyll formation since chlorosis results from its absence. It plays a structural role in the chloroplast membrane system. Its absence causes a disorganization of the lamellar membranes; chloroplasts lose chlorophyll, become yellow-green and finally disintegrate.
Deficiency Symptoms:
Chlorotic and necrotic spots appear in the interveinal areas on younger or older leaves depending upon the species. It causes deficiency diseases such as “Grey Speck” of oats, “Marsh Spot” of peas and “Speckled Yellows” of sugar beet.
2. Copper:
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Copper is largely absorbed as the divalent cupric or monovalent cuprous ion. It is required in a very low concentration or it is highly toxic to plants in high concentration. In plants it exists mainly in the cupric form, although it undergoes alternate oxidation and reduction as it acts as an electron carrier as part of certain enzymes.
It is a constituent of plastocyanin a compound forming part of the photosynthetic electron transport chain, polyphenol oxidase, ascorbic acid oxidase and nitrite reductase. It may also play a catalytic role in nitrogen fixation.
Deficiency Symptoms:
Copper deficiency causes a necrosis of the tip and margin of young leaves, giving it a withered appearance.
Under severe deficiency conditions, leaves may be lost and the whole plant may appear wilted. Its deficiency also causes “Die Back” disease of Citrus (Young leaves die) and “Reclamation” disease of the cereal and leguminous crops.
3. Zinc:
The presence of zinc in traces is essential for the normal metabolism of plants but in high concentration, it is highly toxic.
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It is involved in the synthesis of auxin.
It acts as an activator of carbonic anhydrase, alcohol dehydrogenase and lactic dehydrogenase.
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It also acts as an activator of phosphate transferring enzymes such as hexose-kinase of triosephosphate dehydrogenase. Its deficiency causes accumulation of phosphate in plants.
Zinc plays an important role in protein synthesis, as amino acids and amides accumulate in zinc- deficient plants.
Zinc participates in chlorophyll formation, since interveinalchlorosis occurs in the leaves of several fruit trees.
Deficiency Symptoms:
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Interveinalchlorosis of the old leaves starts at the tips and margins, soon followed by white necrotic spots. Small leaves and shortened internodes result in stunted growth in severe cases. Leaves also appear distorted in shape and twisted in appearance and clustered on short branches known as rosette.
This effect of zinc deficiency on leaves is known as “Little Leaf’ disease and Rosette” of apples. Zinc deficiency adversely affects the production of seeds in beans and peas and the development of fruit in citrus. The trees bear malformed fruits on the branches which die back. Structural abnormalities result in root tips.
4. Boron:
Boron is required in a very small quantity and in most plants it is not readily mobile. It is absorbed from the soils in the form of the soluble borate ions (BO3–). Boron is essential for the translocation of carbohydrates in plants. The transport of sugar is poor in boron-deficient plants. Carbohydrates, ammonium compounds and other soluble nitrogenous compounds accumulate and this suggests a break-down of protein synthesis. The result is the death of stem and root tips, and abscission of flowers.
Deficiency Symptoms:
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Death of the shoot and root tips, leaves develop a thick coppery texture, sometimes becomes curled and brittle: inhibition in flower formation; stunted root growth and root tip becomes swollen and discoloured. In storage or fleshy organs internal tissues disintegrate causing diseases diseases e.g, “Heart Rot” of sugar beet, “Water Core” in turnip and “Browning” of cauliflower.
5. Molybdenum:
Molybdenum functions as an activator (electron carrier) of the enzyme nitrate reductase which converts nitrate to nitrite and ammonium ions, prior to amino acid and protein synthesis. It is also essential for the process of nitrogen fixation. It is involved in the phosphorus metabolism of the plant.
Molybdenum deficiency causes an interveinalchlorosis of lower leaves, followed by marginal necrosis and unfolding of the leaves. Under more severe conditions, mottled areas may become necrotic, causing the leaf to wilt. The dead areas fall off leaving behind petiole and a few pieces of lamina looking like a ‘whip tail’. The condition is known as “Whip tail” disease as observed in cauliflower plants.
Its deficiency also inhibits flower formation and if at all formed, they abscise before setting of the fruits.
6. Chlorine:
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It is absorbed by the plant and retained as chlorine ions. It is essential for the growth of tomato. Its deficiency causes stunted root formation, reduced fruiting and inhibition of photosynthesis as well. It is universally required by the plants. It also stimulates the activity of several enzymes.
