Mineral Nutrition: Definition, Essential elements, Beneficial elements, Macronutrients, Micronutrients, Importance of Macro and Microelements, Toxicity of Micronutrients
Definition
Mineral nutrition refers to how different inorganic substances or minerals are absorbed, distributed, and metabolised by plants for their physiology, structure, and reproductive processes. Van Helmont conducted the initial investigation on inorganic or mineral nutrition in 1648.
Criteria for Essentiality of Elements
The Nutrients or Mineral elements that are necessary for a plant’s proper growth are referred to as Essential Nutrients or Essential Elements. 17 elements are considered vital. The term “essential mineral element” was first proposed by Arnon and Stout in the year 1939. They established that an element must satisfy the following three requirements to be considered essential.
1. Without the mineral element, a plant must be unable to finish its life cycle.
2. No other mineral element may perform the function of the element.
3. The element must be required for a specific metabolic phase, such as an enzymatic reaction, or it must be directly involved in plant metabolism, such as being a part of an enzyme that is an essential plant component.
Beneficial elements
Beneficial elements are those that promote growth but are not needed for all plant species or may become essential for some species under particular circumstances. Crop plants’ genetic potential cannot be maximised if the agricultural production system does not have advantageous components.
According to the essentiality criterion, mineral elements are required for specific metabolic processes in plants. Therefore, depending on the demand for a nutrient element to generate maximum plant growth, the nutrient is referred to as either Macronutrient or Micronutrient.
Macronutrient
The Macronutrients are required in larger quantities and are available in plant tissues in amounts ranging between 0.2 and 4.0 % (on a dry weight basis).
Micronutrient
While the amount of micronutrients in plant tissue is less than 0.02% and ranges from 5 to 200 ppm.
According to their needs, the macronutrients are further divided into primary macronutrients, which include nitrogen, phosphorus, and potassium, and secondary macronutrients, which include calcium, sulphur, and magnesium. Another classification of nutrients into metals (K, Ca, Mg, Fe, Mn, Zn, Cu, Mo, Ni) and non-metals is (N, S, P, B, Cl). However, rather than physiochemical characteristics, the most common classification is based on the quantity of mineral element requirements.
Importance of Macro and Microelements
The Macro- and Micronutrient elements can be divided into four groups as shown below (Malik and Srivastava 1982) for ease of understanding.
1. Nitrogen(N) and Sulphur(S) have covalently linked components of biological matter that are present in reduced form
2. Phosphate, borate, and silicate are examples of oxyanions that appear as P, B, and Si.
3: K, Na, Mg, Ca, and Cl play distinct roles in enzyme conformation and catalysis in addition to being engaged in osmoregulation and ionic equilibrium (e.g. metalloprotein complexes)
4: Metalloproteins or structural chelates of Fe, Cu, Mo, and Zn are present and also participate in oxidation-reduction (redox) reactions (first three elements)
Nitrogen
Nitrogen makes up around 80% of the atmosphere on Earth, but plants cannot access the dinitrogen (N2), which is the most stable form of atomic N. However, both free-living and symbiotic microbes can fix atmospheric N2 into the ammoniacal form (NH4 +), which is subsequently either directly absorbed by plants or transformed into nitrate (NO3 ) by nitrifying bacteria. The preferred way that N is absorbed varies on the kind of soil and the plant species. Paddy is an example of a plant that can absorb NH4 + since it is acclimated to low pH and damp soil conditions. Most plants prefer NO3 as the main type of nitrate in aerobic soils with higher pH. Moreover, there is growing evidence that organic N substances such as amino acids, which are abundant in soil, can potentially serve as significant N sources.
Symptoms of Deficiency and Excess
Widespread chlorosis that begins in the lower leaves, caused by the loss of chlorophyll, is the N deficiency sign. Chlorosis that forms in a characteristic “V” shape at the leaf tip is a sign of an N deficit. Younger leaves exhibit this later yellowing, and in cases of severe N shortage, the older leaves drop off. The synthesis of anthocyanin pigment causes the plant to turn a pale green colour, and the leaf petiole and veins to turn purple. Due to inadequate protein synthesis, plant development is generally weak and stunted.
Excessive N fertiliser causes plants to produce an abundance of foliage and dark-green leaves, but it also causes their root systems to become underdeveloped, resulting in a low root/shoot ratio.
Phosphorus
Nearly 90% of Phosphorus is fixed in the soil as calcium/magnesium phosphates or aluminium/iron phosphates, depending on the pH of the soil. These fixed or non-labile forms of P are useless to plants. The labile fraction of insoluble phosphorus exchanges with the soil solution. Solution P, the inorganic P that is slowly released from the labile fraction into the soil solution, can take several years. Only this type of phosphorus is available to plants for absorption. P insufficiency is a widespread issue as a result. Rock phosphates are the raw material used to make phosphatic fertilisers, P is regarded as a nonrenewable resource that will run out in the next 50 to 60 years. Although the pH of the soil solution affects the form of phosphorus (P), at average soil solution pH, H2PO4 is the only form of phosphorus (Pi) that plants prefer to absorb.
Symptoms of Deficiency and Excess
Phosphorus deficiencies result in reduced plant development and frequently dark green foliage. The buildup of carbohydrates and sugars in the leaves is what gives them their dark green colour. Shoot growth is significantly more suppressed than root growth in the presence of a P shortage, resulting in a greater root/shoot ratio. In extreme circumstances, the roots also turn purple (observed in hydroponically grown maize plants). Reduced cell division and enlargement have an impact on leaf growth, resulting in smaller leaves. Older leaves are the first to exhibit chlorosis because P is highly mobile among plant-developing tissues. Delays in plant maturity also occur. P toxicity in plants caused by excessive P treatment delays the development of reproductive organs in plants. This might be the case because P and N work in concert, increasing the absorption of N when P is present.
Potassium
Potassium makes up about 2.3% of the crust of the Earth. Clayey soils contain high levels of K because it is mostly bonded in primary minerals or found in secondary clay minerals. Examples of minerals that include potassium as K2O include alkali feldspar (4–15%), muscovite or K mica (7–11%), biotite or Mg mica (6–10%), and illite (4–7%). Between 0.1 and 1 mM K+ is the typical range of K concentration in soil solutions. There are three different types of soil K: soil solution-present (easily accessible to plants), soil colloids such clay minerals-adsorbed with exchangeable K, and K as a structural component of soil minerals. K deficiency is often infrequent, although extra K supplies frequently encourage plant development.
Symptoms of Deficiency and Excess K
The symptoms of a K deficit first show up on older leaves because, like N and P, K is easily redistributed in plant tissue. Chlorosis that eventually develops into necrotic lesions on the apex of the leaf and spreads downward on the margins is the typical sign of K deficiency. The juvenile leaves also turn chlorotic in conditions of severe K insufficiency. Maize has weak stalks and limited root development due to the absence of lateral roots. The plant is generally more susceptible to lodging because the vascular bundle’s lignification is impaired. Other signs of K deficiency include rolling of the leaves and shortening of the internodes, which result in stunted growth.
Sulphur
Though soils contain both inorganic and organic forms of sulphur, organically bonded sulphur is the main source of soil sulphur. The C/N/S ratio in soil organic matter is roughly 125:10:1.2, and the organic S is primarily found as phenolic and choline sulphates as well as lipids and amino acids. Inorganic salts predominate in saline and sodic soils. In an aerobic soil environment, inorganic S is largely available as sulphate (SO4 2), which is the preferred form for plant uptake. FeS, FeS2, and H2S have reduced forms of inorganic S that can be found in wet environments. Sulphate reduction under waterlogged (anaerobic) conditions is carried out by bacteria belonging to the genus Desulfovibrio leading to the formation of H2S. By chemotrophic S bacteria like Beggiatoa and Thiothrix, the H2S is converted into elemental S. The range of the total S concentration in soil is 0.005 to 0.04%.
Symptoms of Deficiency and Excess S
In contrast to nitrogen, Sulphur deficiency symptoms start to show up in the youngest leaves. The entire leaf, including the vascular bundles, is chlorotic (veins). When there is a severe lack of S, protein synthesis is inhibited, which also significantly reduces the amount of chlorophyll in the leaves. Plants lacking in S develop an overgrowth of soluble organic nitrogen and nitrate. High SO4 2 – concentrations in the nutrient medium are rather indifferent to plants. The symptom of high S is a decline in growth rate and dark green leaves. High levels of SO2 are hazardous to plants because they bleach the chlorophyll, which results in necrotic symptoms in the leaves.
Calcium
The lithosphere is rich in calcium, which makes up around 3.64% of the Earth’s crust. Al-silicates (feldspars and amphiboles), Ca phosphates, and Ca carbonates are minerals that include calcium. Low soil pH accelerates the development of Ca-deficient soil, which results from severely weathered soils followed by leaching. It is possible for the Ca2+ adsorbed on soil colloids to exchange with the soil solution, resulting in “free” Ca2+ that combines with other elements like P to create insoluble compounds, reducing its availability. Most soils contain enough quantities of Ca2+ in soil solution, and their exchange sites are well enough saturated with Ca2+ to adequately supply the crop demand
Symptoms of Ca Deficiency
Growing tips and the youngest leaves will twist and distort, which is a symptom of severe Ca shortage. Necrosis at the leaf margin happens at a later stage. Additionally, a calcium deficit results in the collapse of the affected tissues, including the petioles and higher portions of the stems, and the breakdown of cell walls. Ca2+ levels in rapidly expanding tissues may fall below a critical level, resulting in the emergence of illnesses including “bitter pit” in apples, “black heart” in celery, and “blossom-end rot” in tomatoes. Plasma membrane permeability is influenced by Ca, therefore when Ca is lacking, the membrane leaks. Ca also mediates the conversion of starch to sugar, so its lack causes leaves to become overgrown with starch. Under Ca shortage, wheat roots have fewer mitochondria.
Magnesium
The Greek word “Magnesia” is the source of the word “magnesium.” The Mg concentration in soil ranges from 0.05 to 0.5%. The addition of Mg to the soil comes from secondary clay minerals and easily weatherable ferromagnesian minerals such as biotite, serpentine, hornblende, and olivine. Similar to Ca2+, Mg2+ also exists in soil solution at a very high concentration of 2 to 5 mM. Due to the hydrated ion’s tiny (0.428 nm radius) size and weak adhesion to soil particles, magnesium leaching loss is considerable and ranges from 2 to 30 kg per hectare. Such losses cause recurrent magnesium deficiencies in crops.
Symptoms of Mg Deficiency
The interveinal chlorosis of elder leaves is the first sign of an Mg deficit. When Mg was removed from the growth media for maize seedlings, purple colour and necrotic patches appeared on the leaves. Dicotyledonous plants, such as grapes, beans, potatoes, and sugar beet, exhibit chlorosis as well as stiff, brittle, and twisted intercostal veins in Mg2+-deficient leaves. Additionally, protein synthesis is negatively impacted, which hinders plant growth. Other ultrastructural alterations in Mg2+-deficient leaves include irregularly shaped grana with a decrease in their number, accumulation of starch grains in chloroplasts, deformation of the lamellar structure, underdeveloped mitochondrial cristae, and a decrease in the contents of chlorophyll and carotenoids. These indications of ultrastructural disorganisation contribute to visual Mg deficient symptoms. Due to a magnesium deficiency, maize roots exhibit enhanced endodermis and hypodermis suberisation.
Physiological Functions of Micronutrients
Iron
The Earth’s crust contains about 5% iron by weight, and soils almost universally contain iron. Ferromagnesian silicates like olivine, augite, hornblende, and biotite are the main sources of Fe in nature. In soils, there is very little plant-available Fe present. The pH of the soil solution heavily influences how soluble Fe is in the soil. While the solubility is at its lowest at higher pH (7.4–8.5), soils’ availability of iron is at its highest in low pH or acidic conditions. Additionally, Fe2+ and Fe3+ concentrations in aerated soils kept in the physiological pH range are below 1015 M. When the soil is wet or depleted, The anaerobic bacteria that utilise Fe oxides as electron acceptors in respiration cause the reduction of Fe3+ to F2+.
Symptoms of Deficiency and Excess Fe
Fe is relatively immobile within plant tissue, therefore symptoms of a visual shortage first manifest on developing young organs. Young leaves exhibit interveinal chlorosis, and in extreme cases, the chlorophyll is lost, causing the leaf to turn white and eventually dry out. In contrast, mature leaves may not show chlorosis at all. Protein synthesis is hampered by a lack of Fe. Toxicity is a significant issue in systems where rice is grown in wetlands due to excessive Fe uptake. The appearance of tiny brown patches that subsequently expand into a uniform brown colour, known as bronzing, is one of the indications of Fe toxicity in rice. In the range of 300–1,000 g Fe per g dry weight, the Fe concentration in rice leaves continues to be too high.
Copper
Cu2+, a divalent cation of copper, is present in the soil in concentrations of 5 to 50 ppm. Cu in soil solution, however, is present at very low concentrations (108 to 60108 M). More than 98% of the copper in the soil is bonded to organic matter, which is a key determinant of copper mobility in soil. Cu is absorbed as either a divalent cupric (Cu2+) ion in aerated soils or a monovalent cuprous (Cu1+) ion in waterlogged soils.
Symptoms of Deficiency and Excess
Young leaves that frequently turn dark green and are twisted with necrotic patches are visible signs of Cu deficiency. On the other hand, in cereals, the leaf tips get white and the leaves are curled and narrow. Internode development is suppressed, resulting in a bushy look and stunted growth. Pollen grain viability is compromised, which inhibits the development of panicles. The lack of two crucial Cu-containing enzymes, phenolase and laccase, impairs lignin formation. Nearly half of the total copper concentration in plants is located in the chloroplasts, which is crucial for photosynthetic processes. Cu deficiency symptom in citrus is called ‘die back’ because new leaves die away.
Above 20–30 g Cu per g of dry weight is the crucial toxicity limit of copper in the leaves. Cu poisoning symptoms are linked to Cu’s capacity to displace other metal ions, such as Fe, from physiologically active locations. Thus, the symptom of Fe shortage in plants will appear in place of the sign of Cu toxicity. Inhibition of root growth more so than shoot growth is one of the other signs of toxicity. There are some Cu-tolerant species termed metallophytes, which can withstand Cu levels as high as 1,000 μg Cu per g dry weight.
Zinc
About 80 ppm of Zn make up the lithosphere, while between 10 to 300 ppm of Zn are often found in soil. Due to Zn2 similar +’s ionic radius to that of Fe2+ and Mg2+, these elements may be replaced by Zn2+ via isomorphous substitution in the structure of minerals. Organic matter in soil reacts with zinc to generate soluble and insoluble Zn-organic complexes. While the inorganic complexes are formed from humic acids, the soluble Zn-organic complexes are mostly linked to amino, organic, and fulvic acids. Zn is absorbed by plants as a divalent cation.
Symptoms of Deficiency and Excess Zn
The most obvious signs of Zn deficiency in dicotyledonous plants are stunted growth caused by a reduction in leaf size known as “little leaf” and a sharp reduction in internode length, or “rosette.” The expanding branch apex dies, a ‘die back’ characteristic of severe Zn deficiency. Additionally, the combination of these symptoms and chlorosis results in a “mottled leaf.” The critical toxicity levels of Zn in the leaves vary between 100 and more than 300 μg Zn per gramme dry weight. Suppression of root extension, chlorosis in immature leaves, and inhibition of photosynthesis are signs of Zn toxicity.
Boron
Boron concentrations in the soil are between 20 and 200 ppm. About 3-4% B is found in the main mineral tourmaline. In soil solution, depending on soil pH, only the monomeric species B(OH)3 and B(OH)4 are present.
Symptoms of Deficiency and Excess Boron
Boron deficiency results in the death of the root and shoot tips, which stunts (rosette) plant growth. The leaves grow brittle, twisted, and have a thick, coppery feel. Boron also has an impact on nitrate assimilation, pollen fixation, pollen tube growth, and blossom retention. Root extension is prevented, and the tips of the roots swell and get discoloured. Fruit illnesses including “heart rot” in sugar beets, “water core” in turnips, and “browning” in cauliflower are brought on by the fleshy tissue disintegrating in fruits. On mature leaves, typical B toxicity symptoms like marginal or tip chlorosis and necrosis manifest. Depending on the plant species, the critical B toxic content in tissue ranges from 100 to 1,000 mg per kg of dry weight.
Manganese
Pyrolusite (MnO2) and manganite [MnO (OH)] are the two main rock minerals that contain Manganese. The total Mn content of soil ranges from 200 to 3,000 ppm. Mn is found in oxidation states II, III, and IV in biological systems; states II and IV are rather stable, while states III are unstable. Mn(II) is the predominant form of iron in plants, but Mn(III) and Mn(IV) can be easily formed by oxidation (IV). This characteristic enables Mn to play a significant part in redox processes. Mn2+, a divalent cation, is the cation that plants prefer to absorb. Like Fe, Mn availability rises when there is standing water. Ca2+, Mg2+, Zn2+, and Fe2+ are some examples of other divalent cations that Mn2+ can remove from their active uptake site.
Symptoms of Deficiency and Excess Mn
Small yellow patches and interveinal chlorosis are the first noticeable signs of Mn shortage on younger leaves. Mn shortage causes lamellar and chloroplast disorganisation at the ultrastructural level. The interepidermal tissue contracts, the cell volume decreases, and cell walls take centre stage. Symptoms of Mn shortage include “marsh spot” in pea cotyledons and “grey speck” in oats. Brown patches on older leaves surrounded by chlorotic areas are an indication of Mn poisoning. Inadequate levels of other mineral nutrients like Fe, Mg, and Ca also result from excess Mn. In maize, the critical toxicity level ranges from 200 to 5,300 ppm (Foy et al. 1988).
Molybdenum
Molybdenum concentrations in soil typically range from 0.6 to 3.5 ppm. It manifests as molybdate oxyanion, MoO4 2 – in soil. Additionally, some soil molybdenum is found in organic form.
Symptoms of Deficiency and Excess Molybdenum
N deficiency symptoms, which result in chlorosis in younger leaves and reduced growth, are common in plants with low levels of Mo, notably legumes. In some dicotyledonous species, such as cauliflower, the size of the leaf blades is drastically reduced, and the leaf blade formation is irregular. Only the midrib of the leaf is present with such a severe Mo deficit, giving it the nickname “whiptail” for its appearance of a whip. Yellow spots, or localised chlorosis and necrosis along the major veins of mature leaves, are typical in citrus. Because the essential quantities for deficiency and toxicity levels differ by a factor of 104 (0.1-1,000 g Mo per gramme dry weight), Mo toxicity typically does not occur. However, if it is ever absorbed above the dangerous levels, shoot tissue becomes a golden-yellow colour and leaves become malformed.
Chlorine
Since chlorine can be found in a variety of sources, including soil reserves, irrigation water, rain, fertilisers, and air pollution, it is widely distributed in nature. Therefore, chloride toxicity rather than its lack is of more significance in crop production. It can be found in soil as the monovalent anion Cl in an aqueous form. Since it is mobile and not adsorbable to soil particles, it is quite vulnerable to leach loss.
Symptoms of Deficiency and Excess Cl−
Reduced leaf surface area, withering at the leaf edges, interveinal chlorosis of older leaves, and constrained and heavily branching root systems are all signs of a cl shortage. The youngest leaf curls under severe Cl shortage, followed by shrivelling and necrosis. In palm plants, besides withering and premature senescence of leaves, frond fracture and stem breaking are characteristic Cl− deficient signs. Agriculture faces a more serious issue with chlorine toxicity. Plants that have been grown in salt-affected soil exhibit indications of Cl poisoning. Bronzing of leaf tips and margins, early yellowing, and leaf abscission are typical signs of Cl poisoning. Some crops, such as sugar beet, barley, maize, spinach, and tomatoes, are quite tolerant of chlorine toxicity, whereas tobacco, beans, citrus, potatoes, and legumes are very vulnerable.
Nickel
Cobalt, Fe, and Ni are all related chemically. In biological systems, it can also be found in Ni(I) and Ni(III) oxidation states, and it can form stable complexes with cysteine, citrate, and Ni enzymes. Ni easily displaces other heavy metals (like Fe) from physiologically significant locations by forming chelates. The typical soil Ni level is less than 100 ppm. It comes from the weathering of igneous rocks that are extremely basic (serpentine mineral)
Symptoms of Deficiency and Excess Ni
Instead of a deficiency, Ni toxicity is a significant issue in agricultural plants. Toxic quantities of urea may accumulate in conditions of Ni shortage. However, the signs of Ni toxicity were quite similar to those of Fe insufficiency. Chlorosis results from acute Ni poisoning. Pale yellow stripes can be seen running the length of the leaf in cereals. The whole leaf may turn white and necrosis occurs at leaf margins. Similar to Mn insufficiency, Ni toxicity in dicots results in chlorotic areas between leaf veins.
Toxicity of Micronutrients
Toxic concentration is defined as one that inhibits the dry weight of tissue by 10% or more. Each plant has a different level of toxicity. Example: Manganese toxicity in soybeans occurs at concentrations above 600 g/g, whereas symptoms of toxicity in sunflower plants appear only at concentrations above 5300 g/g.
An element’s overabundance can occasionally limit the absorption of another element.
Magnesium and iron are competitors for nutritional absorption with manganese. Additionally, it challenges magnesium for enzyme binding. Therefore, a hazardous excess of manganese causes a magnesium and iron shortage.