Manganese [Mn, 25]
195
The most common symptoms of Mn toxicity are Fe chlorosis and brown spots
on leaves, and in some plants also browning of roots. Most plants are affected by
Mn contents around 500 mg/kg, but some plants, and especially hyperaccumulators,
may contain Mn within the range 1,000–10,000 mg/kg, without toxic symptoms.
Interactions between Mn and some metals may have an impact on the phytoavailability and biochemical functions. The most important is an appropriate Fe:Mn ratio,
which should range from 1.5 to 2.5. Both metals have an impact on their absorption and metabolic functions. Cobalt became unavailable to plants under excess Mn.
Also, other trace metals, which are especially important in the case of Cd and Pb,
are likely to be fixed by Mn compounds. The interaction of Mn and P is related to
both the variation in the Mn–phosphate solubility, and the Mn influence on P metabolic reactions. Depending on soil conditions, P fertilizers may either aggravate Mn
deficiency in some crop plant or increase Mn uptake by other plants. These phenomena are related to soil pH and soil-sorption capacity. Elevated Si contents in plants
may reduce Mn toxicity. Increased Mn levels may induce Ca and Mg deficiencies in
some plants. Antagonistic interactions between Mn and K, Na, and N have also been
observed. Most of these interactions, however, are very variable, depending on plant
characteristic and soil properties.
There is a very wide variation in Mn contents among plant species, grown in the
same soil and site, from 30 mg/kg in Medicago truncatula to about 500 mg/kg in
Lupinus albus. Worldwide contents of Mn in grass range from 17 to 334 mg/kg and
in clover from 25 to 119 mg/kg.
Food plants also contain variable amounts of Mn, with the highest being in beet
(both sugar and red) roots (36–113 mg/kg) and the lowest in apples (1.3–1.5 mg/kg).
A remarkable variation in Mn contents among plant species, stage of growth, different organs, and various ecosystems is noticed. A relatively small variation in Mn
contents is seen in cereal grains, with an average from 18 to 48 mg/kg, throughout
the world. Highest Mn amounts are reported for wheat grains (16–103 mg/kg) and
for oats grains (17–121 mg/kg). Relatively high Mn contents (10–42 mg/kg) are in
nuts and almonds (Kabata-Pendias and Mukherjee 2007).
The common mushroom, chanterelles (Cantharellus cibarius) grown in the
mountains of Poland contains higher amounts of Mn, mean 32 mg/kg, than those
grown in the Baltic Sea coast, mean 21 mg/kg (Falandysz et al. 2012).
26.6 HUMANS
The human body contains about 12 mg of Mn, which is stored mainly in the liver,
kidneys, bones, and pancreas (Emsley 2011). Normal ranges of Mn levels in the body
fluids are (in μg/L) 4–15 in the blood; 1–8 in the urine; and 0.4–0.85 in the serum.
Average tissue concentrations are typically between 0.1 and 1 mg Mn/kg FW. Mn
concentration in the liver is slightly higher, 1.2–1.7 mg/kg FW, and the lowest concentrations are in bones and fat, about 0.1 mg/kg FW (ATSDR 2012b).
Manganese is an essential dietary mineral for mammals. Its biological function acts as both a constituent of metalloenzymes and an enzyme activator. It is a
component of metalloenzymes such as superoxide dismutase, arginase, and pyruvate carboxylase, and is involved in amino acid, lipid, and carbohydrate metabolism