There are mainly six nos. of essential plant nutrient elements defined as micronutrients in plants. Those are: boron (B), zinc (Zn), manganese (Mn), copper (Cu), molybdenum (Mo), chlorine (Cl)]. They constitute in total less than 1% of the dry weight of most plants. Let us, we have some brief discussion about them:
Chlorine as Micronutrients in plants
Chlorine is absorbed as Cl - ion and possibly remains in this form without becoming a structural part of organic molecules. It helps in keeping anion-cation balance in cells and is essential for oxygen evolution in photosynthesis.
Chlorine deficiency symptoms include wilted leaves, which may ultimately attain a bronze color, and stunted roots thickened near the tips.
Manganese as Micronutrients in plants
As a bivalent manganous ions Manganese is absorbed largely. It is known to activate many enzymes involved in fatty acid synthesis and nitrate reduction, nucleic acid formation and the enzymes of the Kreb's cycle that decarboxylate and oxidise iso-citric acid. It is directly involved in photosynthesis reaction in which water is broken and oxygen is released.
Manganese as Micronutrients in plants, by deficiency of which is characterized by the appearance of chlorotic and necrotic spots in interveinal region of the leaf. Its deficiency also has a marked effect on chloroplasts which lose chlorophyll and starch grains, turn yellow-green in color and finally disintegrate.
Zinc as Micronutrients in plants
Plants use zinc in the form of Zn++ which are adsorbed to the soil surface and no organic matters in transferrable form. Zinc activates several enzymes, especially carboxylases, carbonic anhydrase, and dehydrogenases. It is also involved in biosynthesis of the plant auxin, indole-3- acetic acid. Moreover, zinc also plays an important role in protein synthesis.
The first sign of zinc deficiency appears in the form of interveinal chlorosis of mature leaves, starting at tips and margins. In severe zinc deficiency, leaves become smaller and internodes are shortened, resulting in stunted growth of the plant. The most easily recognized symptom of zinc deficiency is distorted appearance of leaves.
Boron as Micronutrients in plants
Boron is largely absorbed from soil in the form of soluble borate and tetra-borate ions. It is intricate in protein synthesis, nitrogen, calcium and potassium metabolism and carbohydrate conveyance within the plant. It also plays an important role in cellular differentiation and growth, fertilization, active salt absorption, hormone metabolism, fat metabolism, water relations and photosynthesis.
Symptom of boron deficiency is the death of the shoot tip. Leaves develop a thick coppery texture and sometimes curl and become quite brittle and roots are stunted. A general disintegration of internal tissue results in deviations such as heart rot of sugar beet, internal cork formation in apples, brown heart of turnips and rolling of leaves in potatoes.
Molybdenum as Micronutrients in plants
Molybdenum occurs to a large extent in soils in the form of molybdate ion and is active in hexavalent state. It occurs in the soil as three forms—dissolved, exchangeable and non-exchangeable. It plays an important role in gaseous nitrogen fixation and nitrogen metabolism.
The first noticeable symptom of molybdenum deficiency is chlorotic interveinal mottling of lower leaves. It is followed by marginal necrosis and in-folding of leaves, and under more severe condition, mottled areas may become necrotic.
Copper as Micronutrients in plants
Copper is mostly absorbed by plants as cupric or cuprous ions. Copper undergoes alternate oxidation and reduction as it acts as an electron carrier and part of certain enzymes like phenolases, laccase and ascorbic acid oxidases. It is an essential part of plasto-cyanin and polyphenol oxidase. It may also play catalytic role in nitrogen obsession.
Copper deficiency reasons necrosis of the tips of young leaves that proceeds along the margins and gives the leaf a withered appearance under more severe conditions. Leaves may be eventually lost and the whole plant appears wilted. Copper deficiency also leads to interference in respiratory activities. Withering of tips of young leaves of oats/citrus is common visual copper deficiency symptom.