Manganese

Manganese either stimulates, or is a component of many enzymes and, therefore, greatly affects the metabolism of the plant. It affects photosynthesis, synthesis of fatty acids and carbohydrate metabolism.  Manganese deficiency mainly occurs in organic and carbonate rich soils, because of manganese fixation. A high pH value and adequate soil aeration decrease the concentration of manganese ions. 

Manganese occurs mainly in oxide form but also occurs in silicates. Mn²⁺ ions are released into the soil solution during weathering of silicates. These can then be adsorbed onto the positive surfaces of the cation exchangers. In addition to the clay mineral content of the soil, the pH value and the redox potential of the soil are of particular importance for this binding.

With a decreasing pH value and decreasing redox potential comes an increase in the concentration of plant available Mn ions. A low redox potential occurs during low oxygen concentrations in the soil i.e. compaction, flooding, standing water). In contrast, a high pH value and adequate soil aeration decreases the concentration of Mn-ions.

Manganese deficiency mainly occurs in organic and carbonate rich soils, because of manganese fixation. Humus rich and podsol sandy soils are rather Mn poor as the manganese is less fixed.

Manganese is taken up by the plant only as Mn²⁺-ions. This process can be inhibited by high concentrations of Mg²⁺-, Ca²⁺-, Cu²⁺- and iron ions. Manganese either stimulates, or is a component of many enzymes and, therefore, greatly affects the metabolism of the plant.

Functions of manganese in the plant:

• Directly affects photosynthesis by assisting the synthesis of chloroplasts.
• Important role in synthesis of fatty acids.
• Affects energy budget by regulating carbohydrate metabolism.
• Reduction of nitrates in plants is only possible if sufficient manganese is present.
• Increases growth of secondary roots.
• Stimulates growth due to effect on elongation of cells.
• Similar to copper, manganese is important for immobilisation of free oxygen radicals.
• Manganese and magnesium increase the concentration of valuable ingredients such as citric acid and vitamin C. They increase the quality of frozen vegetables and the resistance of potatoes against discoloration during processing to mash and potato powder.

Manganese excess in the plant

An oversupply of manganese can cause manganese excess. It occurs on acidic soils because these soil solutions mainly contain Mn2+ ions which are easily taken up.

Consequences of Manganese excess: 

• Older leaves, leaf bases and stems show black-brown spots as a consequence of MnO2 deposits. Later on they show chlorotic margins.
• Induced deficiency of iron, magnesium, and/or calcium can occur and add to the symptoms exhibited by the plant.

• Chlorotic spots appear between the leaf veins of the middle and youngest leaves. This is due to disturbed chloroplast formation.
• Chlorotic and necrotic streaks form on gramineae.
• Particularly characteristic are the deficiency symptoms in oats. This is referred to as dried spot disease.
• Dirty gray streaks or dots can be seen on the basal part of the leaf. The entire water balance is disturbed.
  Manganese deficient plants have low cell volume. Cell elongation growth and lateral root formation are disturbed.