Functions of Mg :- Visual symptoms :- Predisposing factors:- Corrective measures :- Magnesium is a small and strongly electropositive divalent cation and its rate of uptake is depressed by other cations and pH. Magnesium facilitates substrate binding to enzymes, regulates cellular pH and cation anion balance. A major function of Mg⁺² is its role as the central atom of the chlorophyll molecule. When the supply of Mg⁺² is optimal for growth, between 10 and 20% of the total Mg⁺² of leaves is localized in the chloroplasts, and less than half of it is bound to chlorophyll. Magnesium is required for RNA polymerases and hence for formation of RNA and in response to magnesium deficiency, the net synthesis of RNA immediately stops. Magnesium deficiency particularly affects the size, structure and function of chloroplasts. Synthesis of ATP has absolute requirement for Mg⁺². Magnesium requirement for optimal plant growth is ~0.5% of the dry weight of the vegetative parts. Magnesium deficiency impairs transport of photosynthates from source (leaves) to sink, such as root and hence, shoot root ratio increases. When magnesium supply is in excess of its requirement, it gets accumulated in vacuoles. Magnesium deficiency symptoms appear first in the lowermost leaves of the plant. Leaves lose their normal colour at the tips, margins and between the veins. The veins and the tissue close to them retain the normal colour. Magnesium deficiency starts from the base of the plant. In the individual leaf, it appeared at the tip and margin and proceeded towards the base and center. Lower leaves become chlorotic between the principal veins. The veins and the tissue close to them retain normal green colour. Mg deficiency is seen in strongly acidic light sandy soils where magnesium can be easily leached away. Excess concentration of other cations especially potassium and Ammonium ions may lead to decrease in Mg uptake. Magnesium deficiency in the light soil nurseries can be corrected by the foliar application of Epsom salt (MgSO₄, 7H₂O). In the field crop grown in light textured soils, application of dolomite (CaCO₃, MgCO₃) is recommended for preventing the occurrence of Mg deficiency.

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