Manganese is a metallic element with an atomic mass of 54.94; it occurs not only as the cation Mn2+ but also as an anion, e.g., MnO4,thus having an amphoteric nature. A characteristic feature of manganese is its ease of changing its oxidation state (it can be 2-, 3-, 4-, 6-, or 7-valent), and its compounds have a high redox potential. The stimulating effect of manganese on plant growth was recognized at the beginning of the 20th century (Loew, 1903; et al.); in 1922, McHargue recognized this element as essential for plants. Of great significance in the development of research on manganese as a trace element was Sjollema and Hudig's discovery in 1909 that the oat disease known as "gray spot" could be treated by adding manganese sulfate to the soil. However, Dutch researchers were unaware of the nature of this phenomenon at the time. It wasn't until 1929 that Samuel and Piper explained that "gray spot" in oats was caused by a manganese deficiency in the soil. Symptoms of manganese deficiency were also observed in other plants, and it was later concluded that all plants, animals, and humans cannot survive without this element.
Content and availability of manganese in soils
The total manganese content in topsoils varies widely. Generally, a complete (or rather, almost complete) manganese deficiency is rare, because under conditions of sufficient air access (as is usually the case in topsoils), this trace element, like iron, transforms into sparingly soluble oxides, quite resistant to leaching even from very light soils. Almost from the beginning of manganese research, attention has been focused on the complex issue of soil manganese availability. Manganese is found in soil in the form of a wide variety of chemical compounds, with varying degrees of oxidation and availability (generally, the higher the oxidation state, the lower the manganese availability). These compounds are subject to constant redox processes in the soil, which constitute a cycle of transformations.
The main cause of manganese's inability to be absorbed in soil is the formation of manganese complexes with organic matter. It has also been found that higher manganese oxides, commonly considered inaccessible, are absorbed by plants such as oats. Opinions on the process of reducing oxidized manganese compounds are also divided. According to some theories, this reduction occurs primarily through reactions with organic matter, while others believe it is also microbiological in nature. Microbial oxidation of manganese, as well as the reduction processes, depend largely on the pH of the environment. As soil pH increases, up to approximately pH 7.5, microbial oxidation processes become more intense, while the intensity of manganese reduction to the divalent form increases with decreasing pH. Many researchers agree that acidic soils are rich in divalent manganese, which can occur even in large excesses. In addition to pH, the redox potential of the soil and moisture significantly influence manganese absorption. It is assumed that manganese available to plants is:
1) divalent manganese Mn2+, i.e.: manganese soluble in soil solutions and exchangeable manganese,
2) forms of manganese oxides that are easily reduced, e.g. Mn2O3xH2O.
Relationship between Fe and Mn.
Somers and Shive were among the first to note the Fe:Mn ratio in plants. They hypothesized that the level of active iron (Fe2+) in plant tissues is controlled by manganese, which, with its high redox potential, oxidizes Fe2+ to Fe3+. With manganese deficiency, the concentration of divalent iron in the plant may be too high, while with manganese excess, too low. This suggests that a plant can develop normally when its iron to manganese ratio is within certain limits. Somers and Shive determined that for soybeans, the ratio of active iron to active manganese in tissues and nutrient solution should be between 1.5:1 and 2.5:1. When this ratio exceeds 2.5:1, symptoms of manganese deficiency may occur, which are identical to those of iron excess. If the Fe:Mn ratio is narrower than 1.5:1, plants "suffer" from excess manganese, which simultaneously indicates iron deficiency. Numerous scientists have demonstrated that the range of the Fe:Mn ratio necessary for normal plant growth is wider. Furthermore, not only the Fe:Mn ratio but also the absolute amounts of these nutrients are important for plants. It is also believed that the symptoms of excess manganese are not identical to those of iron, and vice versa. Various authors also interpret the essence of the Fe:Mn ratio in plants differently, likely resulting from antagonism in the uptake of these nutrients, or, in conditions of excess manganese, from manganese displacing iron from active sites in enzymes. However, there are also opinions (Nasonem and McElroy) that question any relationship between iron and manganese, arguing that not only excess manganese but also a number of other metals causes iron-free chlorosis, emphasizing that iron and manganese have completely separate physiological functions in plants. Given such controversial views, the issue of the Fe:Mn ratio in plants requires further research.
Distribution and forms of manganese in plants.
Manganese is distributed unevenly throughout the plant, and it is difficult to establish clearer correlations due to the significant variability in manganese content in individual organs. It is assumed that manganese mobility within the plant is limited, but under conditions of deficiency, it was possible to infer the movement of this nutrient from the lower levels of the plant to the upper levels. There is no precise information about manganese compounds in plants. However, it can be assumed with great probability that manganese occurs in plant tissues in varying degrees of oxidation; the most mobile and active form is undoubtedly divalent manganese (Mn2+), which is transformed through oxidation processes into Mn3+ and Mn4+. While manganese 3-valent can be reduced back to Mn2+, manganese 4-valent (Mn4+) is an inactive form and in the case of excessive amounts of this component it precipitates as MnO2in non-assimilating tissues.
Symptoms of manganese deficiency and excess in plants.
The most characteristic symptom of manganese deficiency in plants is typical changes in leaf color. So-called "mottled chlorosis" develops on leaves, spreading across the entire leaf surface between the vascular-sieve bundles (known as "veins"). Because the cells immediately adjacent to these bundles remain normally green, an intricate meshwork is created. Chlorosis primarily affects young, developing leaves. As chlorosis intensifies, necrotic spots appear on leaves, resulting from the death of affected tissues. Changes in leaf color associated with manganese deficiency led researchers to believe that this element participates in chlorophyll synthesis, but direct evidence for this has not yet been obtained. Eltlinge observed that in both manganese deficiency and excess, chloroplasts take on a yellow-green color, gradually lose starch granules, and ultimately disintegrate completely. The simultaneous presence of large amounts of calcium oxalate crystals and fat droplets indicates defective plant metabolism. Manganese deficiency has also been shown to lead to complete disorganization of the lamellar structure of chloroplasts in plants such as spinach.
Symptoms of manganese excess in higher plants are fundamentally different from those of manganese deficiency. Chlorosis appears relatively late, first on older leaves. As the poisoning intensifies, the entire leaf blade, including the veins, becomes discolored. Chlorosis resulting from excess Mn (or lack of Fe) is explained by the destruction of protective protein substances surrounding chloroplasts due to a high redox potential. When manganese is excessive in the plant, it precipitates as MnO2, and this compound is secreted into non-assimilating plant tissues, such as the hair cells on the leaf and stem surfaces, the epidermis, or the root cap. Precipitation of MnO2protects assimilating tissues from poisoning and, as already emphasized, explains the often high tolerance of plants to excessive doses of manganese.
Physiological functions of manganese.
Manganese activates a wide range of enzymatic reactions; the mechanism by which manganese functions in these reactions is not fully understood. Among other things, manganese activates:
a) most citrate cycle enzymes and many other decarboxylases,
b) many peptidases and arginases, most likely including prolidase and leucylaminopeptidase;
c) indoleacetic acid oxidase and is also a specific activator in the peroxidase system.
Manganese plays an important role in photosynthesis, the intensity of which decreases significantly in manganese deficiency. It has been found that manganese plays a specific role in this process in the photosynthetic production of oxygen through water photolysis. It is currently believed that manganese acts as an electron carrier on the oxygen side of the pigment system II, i.e., between water photolysis and photosystem II. However, manganese may also play a role on the reducing side of photosystem II. It also participates in maintaining the lamellar structure of chloroplasts.
Author: Marcin Kołodziejczyk
-Anna Nowotna-Mieczyńska "Physiology of mineral nutrition of plants. PWRiL 1965,
-Lityński T., Jurkowska H "Soil fertility and plant nutrition" PWN 1982,
– Franck B. Salibury, Cleon Ross "Plant Physiology" PWRiL 1975,
-Zurzycki J. Michniewicz M. "Plant Physiology" PWRiL 1979,
-Otis F. Daniel G. Curtis Clark "Introduction to Plant Physiology" PWRiL 1958.
Great article. Always good to be reminded of something. Thank you.
Thank you for your appreciation ;-)