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Mineral Nutrition

Essential Mineral Elements

As we know, carbohydrates, proteins, and fats provide organisms with the energy needed for growth and development. Are there any other nutrients essential for survival? Yes! They are called ‘Minerals’. Minerals are also equally important for organisms to perform functions necessary for life. Let’s learn about them in more detail.

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Minerals in Plants

Julius von Sachs, a German botanist, was the first to grow plants to maturity in a nutrient solution in the complete absence of soil. This technique of growing plants in a soil-free, nutrient solution is ‘Hydroponics’. The essential minerals were identified by adding or omitting an element in the nutrient solution or using varying concentrations. Today, hydroponics is commercially used as a technique to grow tomatoes, lettuce, and seedless cucumbers.

Minerals

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Essential Mineral Elements

So far, 105 elements have been discovered, of which more than 60 exist in plants. But, are all these necessary for plants? How do we know which minerals are essential for plants and which are not? There are certain criteria to determine this. Let’s learn what they are.

Criteria For Essentiality

  • The element must be critical for the growth and development of the plant. The plant can not complete its life cycle or produce seeds in the absence of the element.
  • The requirement for the element must be specific and not replaceable by another element. This means the deficiency of one element cannot be compensated by supplying any other element.
  • The element must have a direct role in the metabolism of the plant.

According to the above-mentioned criteria, only a few mineral elements were found to be absolutely necessary for plant growth and development. Based on the quantity in which these minerals are required, they are further classified as follows.

Macronutrients

These are present in plant tissues in large (macro) amounts (i.e. more than 10mmole/kg of dry matter). Macronutrients include carbon, hydrogen, nitrogen, oxygen, sulphur, phosphorus, calcium, potassium and magnesium. Carbon, hydrogen and oxygen are absorbed from CO2 and H2O while the others are absorbed from the soil.

Micronutrients

These are needed in small (micro) amounts (i.e. less than 10mmole/kg of dry matter). Therefore, they are also called trace elements. Micronutrients include iron, copper, manganese, molybdenum, chlorine, nickel, zinc and boron.

All the 17 elements mentioned above are essential elements. In addition to these, elements such as sodium, silicon, selenium and cobalt are also important for higher plants.

Read more about Metabolism of Nitrogen here

Classification Of Essential Mineral Elements

Essential mineral elements can also be classified as follows based on their diverse functions:

  • As components of biomolecules and hence structural elements of cells. Examples – carbon, hydrogen, nitrogen and oxygen.
  • As components of energy-related chemical compounds in plants. Examples – phosphorus in ATP and magnesium in chlorophyll.
  • As activators or inhibitors of enzymes. Examples -Zinc is an activator of alcohol dehydrogenase while molybdenum activates nitrogenase during nitrogen metabolism. Magnesium activates multiple enzymes involved in photosynthesis.
  • As elements that can alter the osmotic potential of a cell. Example – Potassium is very important in the opening and closing of stomata.

Role Of Macro And Micronutrients

The table below shows the diverse functions the essential minerals perform in plants.

Element Absorbed as Points to remember
Nitrogen Mainly as NO3, some as NO2 and NH4+
  • Required in the greatest amount by plants.
  • Needed by all parts of a plant, especially meristematic tissues and metabolically active cells.
  • Major constituent of nucleic acids, proteins, vitamins and hormones.
Phosphorus Phosphate ions (either H2PO4 or HPO42-)
  • Is a constituent of cell membranes, all nucleic acids and nucleotides, and certain proteins.
  • Required for all phosphorylation reactions.
Potassium Potassium ion (K+)
  • Required in large amounts in meristematic tissues, leaves, buds and root tips.
  • Helps to maintain an anion-cation balance and maintains the turgidity of cells.
  • Involved in protein synthesis, activation of enzymes and opening and closing of stomata.
Calcium Calcium ions (Ca2+)
  • Required by meristematic and differentiating tissues.
  • Used in the synthesis of cell wall during cell division (calcium pectate in the middle lamella).
  • Needed during mitotic spindle formation.
  • Required for normal functioning of cell membranes.
  • Activates certain enzymes and regulates metabolic activities.
Magnesium Magnesium ion (Mg2+)
  • Is a constituent of the ring structure of chlorophyll.
  • Activates the enzymes of respiration and photosynthesis.
  • Involved in RNA and DNA synthesis.
  • Helps to maintain ribosome structure.
Sulphur Sulphate (SO42-)
  • Present in amino acids – cysteine and methionine.
  • It is the main constituent of several coenzymes, vitamins (Biotin, Thiamine) and ferredoxin.
Iron Ferric ions (Fe3+)
  • Required in large amounts by plants.
  • An important constituent of proteins like ferredoxin and cytochromes.
  • Important for electron transfer (reversibly oxidized from Fe2+ to Fe3+)
  • Activates catalase enzyme and is needed for chlorophyll formation.
Manganese  Manganous ions (Mn2+)
  • Activates enzymes involved in respiration, photosynthesis and nitrogen metabolism.
  • Very important in splitting water to release oxygen during photosynthesis.
Zinc Zinc ios (Zn2+)
  1. Activates various enzymes, especially carboxylases.
  2. Needed in the synthesis of auxin.
Copper Cupric ions (Cu2+)
  • Needed for overall plant metabolism.
  • Associated with enzymes involved in redox reactions (reversibly oxidized from Cu+ to Cu2+).
Boron BO33- or B4O72-
  • Required for calcium uptake and use.
  • Needed for membrane functioning, pollen germination and carbohydrate translocation.
  • Essential for cell elongation and differentiation.
Molybdenum Molybdate ions (MoO22+)
  • Component of enzymes involved in nitrogen metabolism – nitrogenase and nitrate reductase.
Chlorine  Chloride anion (Cl)
  • Helps to determine the solute concentration and anion-cation balance in cells.
  • Essential to split water and release oxygen during photosynthesis.

Deficiency Symptoms of Essential Elements

When a plant gets limited amounts of an essential mineral, its growth becomes retarded. The concentration of the essential element below which the growth of a plant is retarded is the ‘critical concentration’. Below this concentration, the plant is said to be deficient in that particular element.

Understand more about Components of Food here in detail

Deficiencies are indicated by morphological changes in plants. These are called ‘deficiency symptoms’. These symptoms differ from mineral to mineral and disappear when sufficient levels are provided. However, if the deficiency continues then it leads to plant death.

The part of the plant that shows the effect of the deficiency depends on the mobility of the mineral. For example, the deficiency of nitrogen, potassium and magnesium first appears in the old leaves after which they are mobilised to younger leaves.

But for minerals that are immobile or not transported out of the mature organs, the deficiency first appears in the young leaves. For example, calcium is the structural component of the cell and therefore is not easily mobilised. Plants show the following types of deficiency symptoms:

  • Chlorosis (loss of chlorophyll) due to lack of N, K, S, Fe, Mg, Mo, Mn and Zn.
  • Necrosis (tissue death, particularly leaf tissue) due to lack of Ca, Mg, K and Cu.
  • Inhibition of cell division due to lack of N, K, S, and Mo.
  • Delayed flowering due to low levels of N, S, Mo.

Note: Deficiency of one element can cause multiple symptoms or same symptoms can be caused by the deficiency of multiple elements. Mineral deficiencies that affect crop yield are provided through fertilizers. Macro and micronutrients are important parts of fertilizers.

Toxicity Of Micronutrients

There is a narrow range of concentration at which the micronutrients are optimum. Just like a little less than the critical concentration leads to deficiency, little more can cause toxicity. The mineral concentration that reduces the dry weight of plant tissue by 10% is considered toxic.

The critical concentration varies for each micronutrient and the toxicity level for any element also varies for different plants. Therefore, toxicity symptoms are difficult to identify. Moreover, the excess of one element may inhibit the uptake of another element. For example, the excess of manganese may cause deficiencies of iron, calcium and magnesium.

Absorption Of Elements

The mechanism of absorption of mineral elements involves two phases:

  • The first phase involves the rapid uptake of ions into the ‘free space’ or ‘outer space’ of cells called the ‘apoplast’. This usually occurs through ion channels, transmembrane proteins that act as selective pores and is therefore passive (does not need energy).
  • The second phase involves the slow uptake of ions into the ‘inner space’ of plants called the ‘symplast’. This uptake requires energy and therefore is an active process. The entry of ions into cells is ‘influx’ and exit of ions from cells is ‘efflux’.

In summary, mineral elements are pulled up from the soil along with water through the xylem. Soil acts as a reservoir for minerals. Weathering and breakdown of rocks enrich the soil with inorganic salts and dissolved ions. In addition to minerals, soil also provides water and air; and holds beneficial microbes like nitrogen-fixing bacteria.

Solved Example For You

Question: Which mineral element is crucial for photosynthesis in plants?

  1. Calcium
  2. Magnesium
  3. Molybdenum
  4. Potassium

Solution: The answer is ‘b’. Magnesium is a major component of chlorophyll and activates multiple enzymes involved in photosynthesis.

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