Mineral Toxicity: Causes, Symptoms, Effects

Mineral toxicity is a condition in which vital minerals present in a living body is excessively in high amounts, resulting in various health issues. For instance, manganese is a micronutrient that helps plants produce oxygen through photosynthesis.

Excess manganese in plants can prevent other mineral elements including calcium, magnesium, iron, and phosphorus from being absorbed, translocated, and utilised.

All inorganic elements or inorganic compounds that are essential for life are referred to as mineral nutrients. Water, sodium, potassium, chloride, calcium, phosphate, sulphate, magnesium, iron, copper, zinc, manganese, iodine, etc. are some of the examples of various minerals required for life. The proper amount of each of the necessary micronutrients should be stored. As a result, it may be assumed that too much leads to poisoning and too little leads to a deficiency. Let’s understand what is mineral toxicity, its causes, symptoms, effects and discuss some important questions.

 Key Takeaways: Mineral toxicity, mineral concentration, vital minerals, photosynthesis, nutrient absorption, deficiency, toxic effects 

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What is Mineral Toxicity?

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Mineral toxicity is the condition in which the concentration of any of the minerals in the body is excessively high and has an unfavourable effect on health. Toxicity is defined as a mineral ion concentration in tissues that lowers the dry weight of tissues by 10% or more. The micronutrients [boron (B), zinc (Zn), manganese (Mn), iron (Fe), copper (Cu), molybdenum (Mo), and chlorine (Cl)] are seven essential plant nutrients. In most plants, they constitute less than 1% of the total dry weight. 

In all plants, mineral elements serve as electron carriers, enzyme activators, osmoticum for turgor and development, charge balance maintenance, structural elements, and more. A delicate balance between more and less must be maintained. An excess of one element can prevent the uptake of another. The formation of brown spots surrounded by chlorotic veins, for example, is a common indication of manganese toxicity. Manganese competes for absorption with iron and magnesium, as well as binding with enzymes. Manganese also inhibits calcium transfer at the shoot apex, so an excess of manganese can lead to iron, magnesium, and calcium shortages.

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Causes of Mineral Toxicity

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The mineral deficiencies and toxicities in plants are caused by

1. Plants require a variety of nutrients to grow and function properly. Plant cells are made up of them, and they help plants photosynthesize to produce important sugars, among other things. A nutrient deficiency occurs when a plant does not have enough of a certain nutrient to form cells, DNA, carbohydrates, or any of the other critical components of a plant.
2. When a plant has too much of one nutrient, it may be unable to absorb enough of another, or the excess nutrient may make the plant sick. 
3. Plants may experience nutrient toxicities and shortages as a result of insufficient or excessive nutrient levels in the soil.
4. Plants' symptoms of nutrient toxicity or inadequacy are frequently caused by issues with one or more nutrients' absorption. 
5. Plants toxicity can be due to something interfering with the plant's ability to absorb the nutrient (for example, too little water hinders calcium uptake)

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Importance of Different Minerals in Plants 

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Mineral elements play a variety of roles in plants, including electron transport, enzyme activation, osmoticum for turgor and development, charge balance maintenance, structural components, and more.

The various functions of the essential minerals in plants are as follows:

1. Nitrogen

• Mostly absorbed as NO₃^–, NO₂^– and NH₄⁺
• Plants require the most of this nutrient.
• This nutrient is required by all parts of a plant, especially meristematic tissues and metabolically active cells.
• Nucleic acids, proteins, vitamins, and hormones all include it.

2. Potassium (k)

• Large amounts of potassium ion (K⁺) are required in meristematic tissues that can be divided, into leaves, buds, and root tips.
• Aids in the maintenance of an anion-cation balance and cell turgidity.
• Involved in protein synthesis, enzyme activation, and stomata opening and shutting.

3. Calcium (Ca²⁺) 

• Meristematic and differentiating tissues require calcium.
• During cell division, it's used to make the cell wall (calcium pectate in the middle lamella).
• Required for the development of the mitotic spindle.
• Essential for the proper functioning of cell membranes.
• Regulates metabolic activity and activates specific enzymes.
• Is a component of chlorophyll's ring structure.

4. Phosphorus 

(H₂PO₄– or HPO₄^2–) Phosphate ions (H₂PO₄– or HPO₄^2–) 

• Is found in cell membranes, as well as all nucleic acids and nucleotides, and some proteins.
• All phosphorylation reactions require it.

5. Magnesium Magnesium ion (Mg²⁺):

• Activates respiration and photosynthesis enzymes;
• Plays a role in RNA and DNA synthesis.
• Assists in the maintenance of ribosome structure.

6. Sulphur Sulphate (SO₄^2-)

• It is found in cysteine and methionine, two amino acids.
• It's found in a variety of coenzymes, vitamins (Biotin, Thiamine), and ferredoxin.

7. Iron Ferric ions (Fe³⁺)

• Plants require high levels of iron ferric ions (Fe³⁺).
• A component of proteins such as ferredoxin and cytochromes.
• Electron transfer (reversibly oxidised from Fe²⁺ to Fe³⁺) is important.
• Catalase enzyme activation is required for chlorophyll synthesis.

8. Manganese (Mn²⁺)

• Activates enzymes involved in respiration, photosynthesis, and nitrogen metabolism. 
• Essential for dividing water and releasing oxygen during photosynthesis.

9. Zinc (Zn²⁺)

• Activates several enzymes, including carboxylases.
• Required for auxin synthesis.

10. Copper (Cu²⁺)

• Copper is required for general plant metabolism.

• Associated with redox reaction enzymes (reversibly oxidised from Cu⁺ to Cu²⁺)

• Required for calcium uptake and use.

11. Boron BO₃^3- or B₄O₇²

 Boron is required for membrane function, pollen germination, and glucose transfer, as well as cell elongation and differentiation.

12. Molybdenum (MoO₂²⁺)

• Nitrogenase and nitrate-reducing enzymes contain molybdenum (MoO₂²⁺), which helps in metabolism

12. Chlorine (Cl^–)

• helps in finding the salt concentration and the alkali -metal balance in cells.
• Required for photosynthesis to divide the water and release oxygen.

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Symptoms and Effects of Mineral Toxicity

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Essential elements can also cause deficiency symptoms if they are not supplied to the plant through external ways. The following list shows the symptoms and effects of these various minerals:

Nitrogen (N) 

Symptoms of Toxicity

• Wilting
• Leaf Desiccation
• Root Tip Burn 
• Reduced K, Ca, Mg uptake

Effects of Nitrogen (N) Deficiency

• Growth of lateral shoots is slowed and restricted. 
• Plants show overall chlorosis (light greening of the entire plant) and yellowing of older leaves, which progresses to younger ones.
• Older leaves become necrotic and prematurely defoliate.

Phosphorus (P)

Toxicity Symptoms

Excess P in the soil can lead to iron and zinc deficiency in the plant.

Effects of Deficiency 

• Deeper green foliage
• Purple (or red) pigmentation in numerous species
• Fewer and longer roots, more root mass at initially, less when deficiency is severe.
• Chlorosis of the lower leaves necrosis

Potassium (K)

Toxicity Symptoms

 Potassium K deficiency can lead to calcium (Ca), magnesium (Mg), and nitrogen (N) deficiency

Effects of Deficiency 

• The margins of the leaves become chlorotic, then necrotic.
• Tip and marginal burn begin on mature leaves. 
• The lower leaves begin to turn yellow. 
• Weak stalks and plants readily lodge. 
• Growth is slow.

Magnesium (Mg)

Toxicity Symptoms

Ca shortage can be caused by high Mg levels.

Effects of Deficiency 

• Older leaves develop interveinal chlorosis, which spreads to younger leaves. 
• The chlorotic interveinal yellow patches normally appear in the leaf's core, with the margins turning yellow last. 
• Leaves curl upward along the borders.

Calcium (Ca)

Toxicity Symptoms

Calcium abundance can lead to Magnesium or Boron deficiency.

Effects of Deficiency 

• Young leaves show patchy chlorosis with a light green tint.
• New leaf tips burn brown or black
• Developing points die back.
• Stem and root growing points stop developing. 
• Roots that are short and thickened due to poor root growth.

Manganese (Mn) 

Symptoms of Toxicity

• Reduced growth rate 
• Necrosis along the main veins 
• The signs of toxicity begin on the lower leaves and progress up the main stem.
• The leaves shrivel up and die back to the stem.

Effects of Deficiency 

• New leaf interveinal chlorosis with some green near veins, 
• Grey or tan necrotic patches in chlorotic zones.

Boron 

Symptoms Toxicity

• A yellow-tinged ring appears along the leaf margins.
• The chlorotic zone of the leaf becomes necrotic and grey and the majority of the leaf remains green.

Effects of Deficiency

• The death of terminal buds causes the development of lateral buds.
• Results in a 'witches broom' look.

Molybdenum (Mo) 

Symptoms Toxicity

Plants usually don't show any symptoms, other than Iron deficiency

Effects of Deficiency 

• Interveinal chlorotic patches appear on older leaves, which become cupped and thicker. 
• As the shortage increases, chlorosis spreads to younger leaves.

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Prevention of Plant Toxicity

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• When nutrients are supplied to the soil by people, they often build up in the soil to the point where they cause harmful consequences in plants.
• Inadequate fertilizer application is one obvious way nutrients can enter the soil, but neighboring mining and heavy traffic can increase the amount of some nutrients in the soil while also introducing hazardous pollutants. 
• Some individuals are unaware that some pesticides, including those permitted in organic gardening, can induce nutritional toxicities.
• For example, copper-based fungicides can cause dangerous quantities of copper to accumulate in the soil
• If you need to use a pesticide on your home, make sure you read the instructions thoroughly and investigate the product's probable adverse effects before using it to avoid these problems.

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Things to Remember

• Mineral toxicity is a situation in which the concentration of any mineral in the body is abnormally high and has a negative impact on health.
• Toxicity is defined as a mineral ion concentration in tissues that reduces tissue dry weight by 10% or greater.
• Plant toxicity can occur as a result of something interfering with the plant's capacity to absorb nutrients (for example, too little water hinders calcium uptake)
• Nutrient toxicity or deficiency symptoms in plants are typically caused by problems with one or more nutrients absorption.
• When nutrients are added to the soil by humans, they frequently build-up to the point where they are detrimental to plants.