Manganese (Mn) is a micronutrient required in small amounts for fundamental processes like photosynthesis and enzyme activation. Plants absorb manganese as the divalent ion (\(text{Mn}^{2+}\)), playing a specific role in splitting water molecules within the chloroplasts. However, the concentration window between sufficiency and toxicity is exceptionally narrow for many plant species. Manganese toxicity occurs when the plant accumulates an excessive amount of the element, leading to internal damage that severely inhibits growth and yield.
Recognizing the Visible Symptoms
Manganese toxicity presents with symptoms that affect different ages of foliage. The most diagnostic sign is the appearance of small, dark brown or reddish-brown necrotic spots that develop primarily on the older leaves. These spots often begin near the main veins and are caused by the accumulation of oxidized manganese compounds within the leaf tissue. In some susceptible plants, the veins on the underside of the older leaves may appear to darken or blacken, which is the result of manganese oxide deposits in the vascular tissue.
The excess manganese can also induce a secondary nutrient deficiency, which manifests as interveinal chlorosis, or yellowing between the veins, on the younger leaves. This yellowing occurs because the high concentration of manganese interferes with the uptake and utilization of other nutrients, particularly iron. Plants experiencing manganese overload may also exhibit stunted growth, crinkled or cupped leaves, and an overall reduction in biomass production. The appearance of these symptoms can vary significantly depending on the plant species and the severity of the toxicity.
Environmental Conditions Causing Toxicity
The primary factor driving manganese toxicity is a low soil \(text{pH}\), typically falling below 5.5. In acidic conditions, the concentration of hydrogen ions (\(text{H}^{+}\)) increases, causing mineral manganese oxides to dissolve rapidly. This releases large amounts of the soluble \(text{Mn}^{2+}\) ion into the soil solution. For every unit the \(text{pH}\) decreases, the amount of available manganese can increase by ten to a hundred times, quickly pushing its concentration into the toxic range.
Poor soil drainage and waterlogging are significant factors that contribute to manganese overload. Saturated soil conditions deplete the available oxygen, creating an anaerobic environment that favors chemical reduction reactions. Under these oxygen-deprived conditions, microorganisms and chemical processes convert insoluble manganese oxides (\(text{Mn}^{4+}\)) into the more soluble and mobile \(text{Mn}^{2+}\) form, making it highly accessible for root uptake. The presence of high organic matter content can also exacerbate the problem, as the decomposition process can supply electrons that further aid in the reduction of manganese oxides.
Internal Damage Mechanisms
Once absorbed in excess, manganese disrupts the plant’s internal chemistry through multiple mechanisms. A major pathway involves antagonism with other positively charged ions, such as calcium (\(text{Ca}^{2+}\)), magnesium (\(text{Mg}^{2+}\)), and iron (\(text{Fe}^{2+}\)). The overabundance of \(text{Mn}^{2+}\) in the root zone competes for the same uptake sites, effectively blocking the absorption and transport of these essential nutrients to the shoots, leading to induced deficiencies. This competition explains the interveinal chlorosis on young leaves.
Manganese also causes direct cellular damage, particularly by generating oxidative stress within the leaves. Excess \(text{Mn}^{2+}\) ions accumulate inside the chloroplasts, interfering with the electron transport chain and promoting the formation of damaging reactive oxygen species. This photo-oxidative stress destroys chlorophyll and chloroplast structure, which directly impairs photosynthesis and leads to yellowing observed in the foliage. Furthermore, the accumulation of manganese in the leaf cell walls (apoplast) can trigger the formation of dark, necrotic lesions as the element is oxidized and precipitates.
Managing and Reversing Manganese Overload
Correcting manganese toxicity involves addressing the underlying environmental conditions that make the element excessively available. The most effective long-term solution is to raise the soil \(text{pH}\) through the application of liming materials, such as agricultural or dolomitic lime. Applying lime neutralizes soil acidity, which drastically reduces the solubility of manganese and converts it back into less available oxide forms. For the \(text{pH}\) correction to be effective, the lime must be thoroughly incorporated into the soil profile, which may take several years to achieve.
Improving soil aeration and drainage is another management step to mitigate the risk of toxicity. Preventing waterlogged conditions ensures that oxygen remains available in the soil, which favors the less soluble oxidized forms of manganese. Growers should also review fertilizer programs to ensure that no acid-forming fertilizers or excessive manganese supplements are being applied. In cases of severe toxicity, a temporary measure can involve foliar applications of antagonistic nutrients, like calcium or iron, to help counteract the immediate competitive effects of manganese while the long-term soil adjustments take effect.