All plants require manganese for growth and reproduction. Manganese is the element chosen by nature to catalyse oxygen evolution in photosynthesis. When there is a lack of manganese, the structure of the chloroplasts is markedly impaired. Manganese also plays a role in lignin synthesis and in thephenolic acid metabolism, helping to suppress both leaf and root diseases. Some plants are particularly susceptible to manganese deficiency.
Manganese content in the soil can vary widely. It is only 50 ppm in some localities, but can reach 10,000 ppm in unleached alkali soils. Only the divalent cation (Mn+2) which is soluble and mobile in the soil, is easily available for absorption by plant roots. The tetravalent cation (Mn+4) is virtually insoluble. Both leaf-manganese and soil-manganese analyses are used to determine manganese deficiencies or toxicity.
Manganese deficiency in most crops is indicated by an overall chlorosis of the leaf, which leaves only the main veins and midrib outlined in dark green healthy tissue. The degree to which plants can absorb manganese is more important than its simple chemical presence in the soil.
Manganese sulphate is considered the most efficient source of manganese for fertilizer production, although manganese monoxide is also used. These are added in regions deficient in manganese. Manganese sulphate and other compounds can also be used in solution to spray on foliage.
It has been demonstrated that a fungal infection of wheat can develop as a result of manganese deficiency. To correct it efficiently, manganese must be brought to the roots of the plant. The hypothesis is that manganese has a direct effect on the pathogen through toxicity, and strengthens the host plant by modifying its resistance and susceptibility.
It should also be noted that an excess of absorbable manganese which sometimes occurs in acid soil, can be toxic to some species. The problem is generally solved by adding lime.