Importance to Plants and Animals

Essentiality of Manganese to Higher Plants

The first reported investigations into the essentiality of manganese by Horstmar in 1851 (12) succeeded in identifying this nutrient as needed by oats, but only where iron was in excess. Further evidence for the essentiality of manganese was not made until some Japanese researchers reported that manganese stimulated the growth of several crops substantially (13,14). These crops included rice (Oryza sativa L.), pea (Pisum sativum L.), and cabbage (Brassica oleracea var. capitata L.), and because of their economic importance, further interest was stimulated (15). Supporting these field results were the physiological and biochemical studies of Bertrand (16-18). His work reported manganese as having a catalytic role in plants, and that combinations with proteins were essential to higher plant life. This reported essentiality of manganese was supported by studies by Maze (19) in solution culture. Studies by McHargue (20,21), where the role of manganese in the promotion of rapid photosynthesis was determined, are regarded as having established that manganese is essential for higher plant growth.

Functions in Plants

Manganese is involved in many biochemical functions, primarily acting as an activator of enzymes such as dehydrogenases, transferases, hydroxylases, and decarboxylases involved in respiration, amino acid and lignin synthesis, and hormone concentrations (22,23), but in some cases it may be replaced by other metal ions (e.g., Mg). Manganese is involved in oxidation–reduction (redox) reactions within the photosynthetic electron transport system in plants (24–26). Manganese is also involved in the photosynthetic evolution of O₂ in chloroplasts (Hill reaction). Owing to the key role in this essential process, inhibition of photosynthesis occurs even at moderate manganese deficiency; however, it does not affect chloroplast ultrastructure or cause chloroplast breakdown until severe deficiency is reached (27).

Importance to Animals

In humans, manganese deficiency results in skeletal abnormalities (28,29). In the offspring of manganese- deficient rats, a shortening of the radius, ulna, tibia, and fibula is observed (30). Manganese deficiency during pregnancy results in offspring with irreversible incoordination of muscles, leading to irregular and uncontrolled movements by the animal, owing to malformation of the bones within the ear (30,31). Animals that are manganese-deficient are also prone to convulsions (32).

In contrast, manganese toxicity induces neurological disturbances that resemble Parkinson's disease, and the successful treatment of this disease with levodopa is associated with changes in manganese metabolism (33,34). In animals manganese is associated with several enzymes (35), including glycosyl transferase (36), superoxide dismutase (37,38), and pyruvate carboxylase (39).

Manganese requirement for humans is 0.035 to 0.07 mg kg^-1, with daily intake representing 2 to 5 mg day^-1 in comparison to the body pool of 20 mg (30,40).