The Role of Manganese in Plant Growth: From Photosynthesis to Stress Resistance

I. The Nutritional Role and Content Characteristics of Manganese in Plants

Manganese (Mn) is an essential micronutrient indispensable for plant growth and development. In 1922, scholar J.S. McHargue confirmed through systematic experiments that manganese is a necessary nutrient for the normal metabolism and growth of higher plants, laying the foundation for manganese nutrition research. Manganese is widely involved in photosynthesis, enzyme activity regulation, material metabolism, and stress resistance processes in plants, playing an irreplaceable role in crop yield and quality formation.

The manganese content in plants varies significantly depending on crop species, plant organs, and growing environments, with a broad overall fluctuation range. Gramineous and leguminous crops exhibit particularly strong manganese accumulation characteristics, and the manganese distribution between grains and straw shows clear differences, as illustrated below.

Table 1. Manganese Content in Several Crops

Crop Type	Grain (mg·kg⁻¹)	Straw (mg·kg⁻¹)
Rice	20 - 250	280 - 900
Wheat	16 - 140	30 - 350
Beans	14 - 80	110 - 130

According to plant nutrition research, the major factors causing large fluctuations in manganese content within crops can be divided into three categories:

1. Ionic Antagonism and Crop Metabolic Regulation

The active uptake of manganese by plants is regulated by their metabolic intensity, while antagonistic interactions among soil cations significantly affect manganese absorption. Magnesium ions (Mg²⁺) compete with manganese ions (Mn²⁺) for root absorption sites, substantially inhibiting the uptake and accumulation of available manganese in plants.

2. Dominant Influence of Soil pH

Soil pH is the key environmental factor determining manganese availability. In alkaline soils (pH > 7), soluble divalent manganese is easily oxidized into insoluble tetravalent manganese, drastically reducing available manganese content in soil. Under such conditions, manganese concentration in crop dry matter is generally below 100 mg·kg⁻¹. In acidic soils (pH < 7), manganese activation is much stronger, significantly increasing manganese accumulation in crops. Under extreme conditions, concentrations may exceed 1600 mg·kg⁻¹, causing manganese toxicity stress.

The general rule is:The higher the soil pH, the lower the availability of manganese, and the lower the manganese uptake by crops.

3. Growth Stage and Organ Distribution Differences

Manganese distribution within plants is uneven and dynamically changes throughout growth stages. For example, manganese concentration in maize decreases continuously as plants age, while manganese mainly accumulates at leaf margins, where concentrations can reach twice those found in the central leaf tissue. In sugar beet, manganese concentration in petioles is only about 50% of that in leaf blades, demonstrating the tissue-specific distribution characteristics of manganese.

Plant roots mainly absorb and utilize divalent manganese ions (Mn²⁺), while manganese mobility within plants is relatively weak. Agronomic studies show that manganese deficiency symptoms usually first appear in intermediate and young leaves rather than newly emerged leaves. In addition, monocotyledonous cereal crops possess stronger internal manganese translocation ability than dicotyledonous crops. Therefore, manganese deficiency symptoms in cereals often appear primarily on older leaves, serving as an important basis for field diagnosis.

II. Core Physiological Functions of Manganese (Supported by Authoritative Research)

1. Regulating Photosynthesis and Delaying Premature Senescence

Manganese is a core component of plant chloroplasts and a critical cofactor of the oxygen-evolving complex (OEC) in Photosystem II (PSII). It directly participates in photolysis of water, electron transport, and oxygen release during photosynthesis. Manganese is also essential for maintaining chloroplast membrane integrity and stabilizing chlorophyll synthesis.

Manganese deficiency directly damages chloroplast thylakoid structures, leading to membrane fusion, disintegration, vacuolization, and accelerated chlorophyll degradation. The photosynthetic system is therefore the first physiological system affected by manganese deficiency. Field symptoms include chlorosis, reduced photosynthetic efficiency, and premature plant aging.

Manganese also maintains an important redox balance relationship with iron. Excessive manganese can inhibit iron reduction and absorption, inducing iron chlorosis. Therefore, manganese application rates must be carefully controlled in agricultural production.

2. Activating Enzyme Systems and Regulating Metabolism

Manganese functions as a structural component or activator for more than 30 key plant enzymes. It is extensively involved in respiration, nitrogen metabolism, and antioxidant defense systems, making it a central factor in crop metabolic regulation.

Manganese-dependent superoxide dismutase (Mn-SOD) removes reactive oxygen species generated during photosynthesis, protecting the photosynthetic apparatus from oxidative damage, stabilizing chlorophyll structure, and delaying leaf senescence.

Within the tricarboxylic acid cycle, Mn²⁺ activates key respiratory enzymes such as citrate dehydrogenase, malate dehydrogenase, and α-ketoglutarate dehydrogenase, ensuring normal respiration and energy synthesis.

In practical diagnosis:

• Magnesium deficiency causes chlorosis in older leaves.

• Manganese deficiency first appears in intermediate and younger leaves.

This distinction is an important field diagnostic feature.

3. Promoting Seed Germination and Vigorous Seedling Growth

Manganese enhances the effect of auxin on coleoptile elongation and accelerates the hydrolysis of starch and proteins within seeds, promoting rapid supply of soluble sugars and amino acids to seedlings. As a result, seed germination rate and seedling vigor are significantly improved.

Seed treatment with manganese fertilizers before sowing wheat, maize, and other cereal crops can effectively promote strong seedling establishment and improve emergence uniformity.

Adequate manganese nutrition also:

• Promotes lateral root formation and elongation

• Improves root absorption networks

• Enhances fruit-setting rates

• Stimulates flower bud differentiation in young fruit trees

• Promotes early fruiting and higher yields

Manganese additionally promotes vitamin C synthesis and strengthens stem mechanical tissues, improving lodging resistance. Manganese deficiency directly suppresses root cell elongation, causing poor lateral root development and reduced root vitality.

4. Regulating Nitrogen, Carbon, and Lipid Metabolism

Manganese directly participates in nitrogen reduction and assimilation processes by activating nitrate reductase, promoting the conversion of nitrate nitrogen into amino acids and proteins, thereby significantly improving nitrogen fertilizer utilization efficiency.

Under manganese deficiency:

• Nitrate nitrogen accumulates abnormally

• Free amino acids increase excessively

• Soluble proteins accumulate

• Nitrogen metabolism becomes disordered

• Plant growth weakens

Manganese also regulates carbohydrate and lipid metabolism. Deficiency significantly suppresses photosynthesis and sharply reduces soluble sugar content in roots. Chloroplast glycolipids and unsaturated fatty acids may decrease by approximately 50%, directly affecting chloroplast structural stability and sustained photosynthetic capacity.

5. Improving Phosphorus and Calcium Utilization

Manganese activates insoluble phosphorus and calcium in soils, increasing available phosphorus and calcium content while promoting root absorption, transport, and assimilation of these nutrients.

Adequate manganese nutrition optimizes phosphorus metabolism, enhances photosynthate transport, and improves grain filling. This has notable positive effects on the yield and quality of cotton, cereals, oil crops, fruits, and vegetables.

6. Enhancing Stress Resistance and Disease Resistance

Through regulation of vitamin C synthesis, strengthening of stem tissues, and activation of antioxidant enzyme systems, manganese enhances crop resistance to:

• Cold stress

• Lodging

• Oxidative stress

Field practices demonstrate that crops with sufficient manganese nutrition exhibit significantly lower incidence of fungal and bacterial diseases. In contrast, manganese-deficient plants show reduced cold tolerance and stress resistance, making them more susceptible to frost injury, premature aging, and disease outbreaks.

III. Classification of Crop Sensitivity to Manganese

Different crops vary significantly in their manganese demand and tolerance, which can generally be classified into three categories:

Highly Sensitive Crops

Legumes, wheat, potato, onion, spinach, apple, and strawberry. These crops readily develop typical manganese deficiency symptoms and require regular manganese supplementation.

Moderately Sensitive Crops

Barley, sugar beet, clover, celery, radish, and tomato. Under normal soils, additional manganese may not be necessary, but supplementation is beneficial in poor alkaline soils.

Low-Sensitivity Crops

Maize, rye, and most forage grasses. These crops possess strong adaptability, and mild manganese deficiency often causes no obvious visible symptoms.

IV. Diagnostic Standards for Manganese Nutrition in Soil and Crops

1. Plant Leaf Nutritional Diagnosis

Leaf manganese concentration is the primary indicator for evaluating crop manganese nutritional status:

• 20–100 mg·kg⁻¹: Normal growth, no manganese supplementation required

• <20 mg·kg⁻¹: Manganese deficiency, requiring prompt fertilization

Additional indicators include:

• Nitrate accumulation in leaves

• Oxalate crystal formation in xylem tissues

• Abnormally elevated peroxidase activity

These can serve as auxiliary diagnostic indicators.

2. Soil Available Manganese Classification

Available manganese is the key indicator of soil manganese supply capacity:

• ≤50 mg/kg: Extremely low, severe deficiency, manganese fertilization required

• 50–100 mg/kg: Low level, crops prone to deficiency

• 100–200 mg/kg: Moderate level, generally sufficient

• 200–300 mg/kg: Rich level, no supplementation needed

• 300 mg/kg: Extremely rich, risk of manganese toxicity

Numerous field experiments confirm that proper manganese fertilization on deficient soils can significantly improve crop physiological performance, stabilize yields, and enhance quality.

V. Soil Types Prone to Manganese Deficiency and Advantages of Supplementation

Crop manganese deficiency is determined primarily by available manganese rather than total soil manganese content. Common manganese-deficient soils include:

• Neutral and alkaline soils

• Over-limed soils

• Coastal saline-alkaline soils

• Sandy soils low in organic matter

• Alluvial soils

• Soils rich in ferrous iron or calcium ions

• Soils with high redox potential

In such soils, manganese mainly exists in insoluble oxide forms that plants cannot easily absorb.

Compared with soil application, foliar application of manganese fertilizers offers significant advantages:

• Absorption efficiency over 8 times higher

• Lower dosage requirements

• Faster response

• Lower cost

• Precise nutrient supply during critical growth stages

Foliar spraying is therefore the preferred method for correcting manganese deficiency in alkaline soils.

VI. Scientific Application Techniques for Manganese Fertilizers

1. Basal Application

Slow-release manganese fertilizers or soluble manganese sulfate may be used. Mixing with organic fertilizers or acidic fertilizers is recommended to reduce soil fixation and improve manganese availability.

Recommended rates:

• Manganese sulfate: 15–60 kg/ha

• Slow-release manganese slag fertilizers: approximately 10 kg/ha

Suitable for field crop application before planting.

2. Foliar Spraying

Foliar application provides the highest utilization efficiency and widest adaptability.

Recommended concentrations:

• Cereals and rice: 0.1%

• Legumes: 0.03%

• Fruit trees: 0.3%–0.4%

Optimal spraying periods:

• Cotton: peak budding to early boll setting

• Winter crops: post-green-up stage in spring

• Fruit trees: initial flowering stage

3. Seed Coating

Suitable for wheat, maize, sugar beet, and similar crops.

Typical rates:

• 2–4 g manganese sulfate per kg of seed

• Up to 5 g/kg for sugar beet

Method: Dissolve manganese fertilizer in a small amount of water, spray evenly onto seeds, air-dry, and sow. This effectively improves seed vigor and stress resistance.

4. Seed Soaking

Use 0.05%–0.1% manganese sulfate solution at a 1:1 solution-to-seed ratio for 12–14 hours.

Seed soaking is not recommended in drought-prone areas because it may reduce emergence rate. Seed coating is preferred under dry conditions.

VII. Common Agricultural Manganese Fertilizers and Their Physicochemical Properties

Currently, agricultural manganese fertilizers are mainly divided into inorganic manganese fertilizers and chelated manganese fertilizers.

Fertilizer Type	Molecular Formula	Mn Content	Characteristics
Manganese Sulfate Monohydrate	MnSO₄·H₂O	29.3%–31.8%	Pale rose crystals, highly water-soluble, fast acting, widely used for basal application, foliar spray, and seed treatment
Manganese Oxide	MnO	66%–70%	Gray-green powder, slightly soluble, slow-release, suitable for soil improvement
Manganese Chloride Tetrahydrate	MnCl₂·4H₂O	27.5%	Rose-colored crystals, highly soluble and hygroscopic, fast acting, unsuitable for chloride-sensitive crops
Manganese Carbonate	MnCO₃	43%–44%	Light brown-white powder, slightly soluble, stable and slow-release, suitable for acidic soils
Manganese Dioxide	MnO₂	55%–60%	Black powder, poorly soluble, extremely slow release, mainly used as soil amendment
Manganese Nitrate Tetrahydrate	Mn(NO₃)₂·4H₂O	21%	Light red crystals, highly soluble, supplies both nitrogen and manganese
Ammonium Manganese Sulfate	3MnSO₄·(NH₄)₂SO₄	26%	Light pink powder, highly soluble, provides both nitrogen and manganese
EDTA Chelated Manganese	C₁₀H₁₂N₂O₈MnNa₂·2H₂O	13%	Near-neutral white powder, extremely soluble, resistant to soil fixation, highly efficient in water-soluble and foliar fertilizers
Manganese Citrate	—	—	Light orange-white powder, highly soluble, residue-free, suitable for precision agriculture and high-value crops
Silicon-Manganese Slag	Composite Silicate System	—	Industrial by-product with long-lasting effect, low cost, and soil improvement benefits

VIII. Conclusion

As a core essential micronutrient for plants, manganese is a critical factor in photosynthetic metabolism, enzyme system regulation, material synthesis, and stress defense. It participates throughout the entire crop growth cycle, from seed germination and vegetative growth to reproductive development, directly influencing crop yield formation and product quality.

Soil pH, ionic antagonism, and crop species are the primary factors affecting manganese availability and uptake. Manganese deficiency in alkaline soils has become one of the most common micronutrient stress problems in modern agriculture.

In practical production, manganese fertilization strategies should be precisely adjusted according to:

• Soil available manganese content

• Crop sensitivity to manganese

• Growth stage requirements

Appropriate methods such as basal application, seed treatment, soaking, and foliar spraying should be selected, with priority given to highly efficient fertilizers such as manganese sulfate and chelated manganese products. Proper manganese nutrition management can effectively prevent premature aging, metabolic disorders, and increased disease susceptibility while improving yield stability, crop quality, and stress resistance.

The Role of Manganese in Plant Growth: From Photosynthesis to Stress Resistance