Magnesium and Plant Growth

After reading this article you will learn about the role of magnesium in plant growth, plant physiology and nutrition.

It is evident that magnesium plays an important role in the synthesis of chlorophyll. The proportion of the total Mg bound to chlorophyll depends very much on the supply of Mg²⁺. In most cases, the plant growth is depressed and Mg deficiency occurs when the proportion of Mg in the chlorophyll exceeds 20-25%.

The inhibition of photo synthesis by high Mg content in the “metabolic pool” may found in plants under drought stress. Depending on the magnesium nutritional status, between 6 and 25% of the total Mg is bound to chlorophyll.

Another 5-10% of the total Mg in leaves is also bound to pectate in the cell walls or precipitated as sparingly soluble salts in the vacuole and the rest 60-90% is extractable by water. Various enzyme reactions either require magnesium or promoted by magnesium, like glutathione synthase or phosphoenol pyruvate (PEP) carboxylase.

Most of the magnesium dependent reactions can be categorised by the general type of reaction such as transfer of phosphate and carboxyl groups.

In such reactions, magnesium is preferentially bound to N and phosphoryl groups that are as follows:

The synthesis of ATP (phosphorylation: ADP + Pi → ATP) requires magnesium absolutely as a bridging component between ADP and the enzyme. Such phosphorylation reaction inhibited by the addition of calcium, as because low concentration of calcium is to be maintained in vivo within the chloroplasts.

In addition, the modulation of ribulose bisphosphate (RuBP) carboxylase in the stroma of chloroplasts is also affected by magnesium. In Mg-deficient plants especially in leaves, there is an accumulation of non-structural carbohydrates e.g. starch, sugars. In Mg-deficient leaves, the formation of superoxide radicals (O₂^–) and hydrogen peroxide (H₂O₂) is enhanced.

The requirement of Mg for the optimum growth of plants is about 0.15-0.35% of the dry weight of the vegetative part. Chlorosis of completely matured leaves is the most usual visible symptom of Mg-deficiency which results depression of protein synthesis, proportion of protein and non-protein nitrogen.

Roots of most plants have poor ability to take up Mg²⁺ as compared to K⁺. Such poor uptake potential is related to the lack of a particular uptake mechanism of Mg²⁺ across the plasma membrane. This transport is a passive process mediated by ionophores wherein Mg²⁺ moves down the electro-chemical gradient. Such transport and subsequent uptake is affected by the presence of K⁺ and NH₄⁺ ions.

There are other biochemical functions of plants which are carried out by Mg, of which the activation of ribulose bisphosphate carboxylase is important. The light triggers the import of Mg²⁺ into the stroma of the chloroplast in exchange for proton (H⁺) resulting an optimum condition for the carboxylase reaction (Fig. 21.15).

Magnesium also stabilizes the ribosomal particles in the configuration that is necessary for protein synthesis and also has the stabilizing effect the matrix of the nucleus. Further, magnesium also participates in the transfer process of amino acyls i.e. from amino acyl tRNA to the polypeptide chain.

On an average, the uptake of Mg by crops particularly crops grown on uplands, varies from about 10-25 kg ha^-1 year^-1, being higher uptake in the root crops than that of other cereal crops.

Due to intensive crop cultivation with the simultaneous use of high analysis chemical fertilizers as well as adoption of other modern agro-techniques, the deficiency of Mg recently is occurring in soils and thus affect plant growth and nutrition.

The presence of high amounts of K⁺, NH₄⁺ and Ca²⁺, and Al in acid soils depresses the uptake of Mg. However, the application of Mg is only necessary in soils containing excess amounts of K, Ca and At. The hypomagnesaemia may also be found in animals due to lack of Mg.

The application of Mg for the growth and nutrition of plants is usually recommended to soils containing an exchangeable Mg content of less than 25 mg kg^-1 in soils.