Criteria of essentiality and Categorisation of Essential elements

Introduction:

• It has been observed that out of 118 discovered elements more than 60 are found within the plants.
• Plants can accumulate elements based on their habitat and locality.
• It is necessary for us to know which mineral is essential for plant growth and internal functions and which of them are not.
• The elements, i.e. macro and micronutrients can be divided into various categories based on their functions.
• They can be divided into categories are Structural elements, energy-related compounds, enzyme activators and inhibitors, and osmotic regulators.

Topics covered-

• Criteria for essentiality
• Macronutrients and Micronutrients
• Categorization of elements on the basis of their diverse functions

Criteria for essentiality
 

Introduction:

• All the diverse mineral elements present in plants are not really necessary for plants.
• Some plants that grow close to nuclear test sites also have radioactive strontium in them.
• Some plants have been known to exhibit selenium while some exhibit gold traces.
• The elements that are termed necessary or essential for plants need to have a specific structural and physiological role.
• These elements need to be an essential component without which the processes and life cycle is incomplete.
• The criteria for essentiality was given by Arnon and Stout in the year 1939.

Detailed explanation:
 

Criteria for essentiality-

1. The element must be absolutely necessary for supporting normal growth and reproduction amongst the plants. In the absence of the element, the plants are not able to complete their life cycle or set the seeds.

2. The requirement of the element must be specific and not replaceable by another element. In other words, every element has an important role assigned and the lack of one element cannot be fulfilled by another.

3. The element must be directly involved in the metabolism of the plant.

4. Disorders and problems in various processes might be observed due to the lack of certain essential elements.

5. The missing elements need to be replenished in order to protect the plant from further deterioration.

• Based upon the criteria of essentiality, 17 elements have been found to be essential for plant growth and metabolism. They are C, H, O, N, P, K, Ca, Mg, S, Fe, B, Mn, Cu, Zn, Mo, CI and Ni. All the rest are known as non-essential elements.

- Beneficial Elements:

• Cobalt, silicon, sodium, selenium etc. are found to be useful in various metabolic activities in plants but are not highly essential. Hence, they are known as non-essential or beneficial elements.
• Silicon: It is found in the grasses and cereal plants and their scarcity can cause the plants to have stunted growth as well as leaf necrosis.
• Sodium: Accumulation of Na+ at high concentrations in the cytoplasm results in deleterious effects on cell metabolism, e.g., on photosynthetic activity in plants. In plants, it can be accumulated at high concentrations in vacuoles, where it is used as an osmoticum.
• Selenium: It is of metabolic importance in cyanobacteria and in some plants, being involved in their antioxidative processes. Selenium can increase the tolerance of plants to UV-induced oxidative stress, delay senescence, and promote the growth of ageing seedlings.
• Cobalt: It can delay the senescence of leaf, increase the drought resistance ability in seeds, regulates the alkaloid accumulation amongst medicinal plants, and inhibitor of ethylene biosynthesis

Macronutrients and Micronutrients
 

Introduction:

• On the basis of criteria of essentiality, only a few elements are found to be essential for plant growth and metabolism.
• Essential elements are further divided into two broad categories based on their quantitative requirements (i) Macronutrients, and (ii) Micronutrients.
• Excessive consumption or the deficiency of both the macro and micro-nutrients have a negative impact on health.
• Therefore, it is important to have a balanced diet that includes an equal and required quantity of both macro-nutrients and micro-nutrients.

Detailed explanation:
 

Macronutrients

• The essential elements which are generally present in plant tissues in large amounts (in excess of 10 mmole Kg –1 of dry matter) are known as Macronutrients or macro elements.
• Nine of the essential elements are called macronutrients because plants require them in relatively large amounts.
• Six of these are the major components of organic compounds forming a plant’s structure: carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulfur.
• The other three macronutrients are potassium, calcium, and magnesium.
• Carbon, hydrogen and oxygen are obtained via water and carbon dioxide which are absorbed by plants through roots and stomata respectively.
• Of all the mineral nutrients, nitrogen contributes the most to plant growth and crop yields nitrogen is such an essential macronutrient that is needed by plants in the largest quantity. Nitrogen is taken up majorly in the form of NO3- form but also as NO2- or NH4+ also. Nitrogen is a key constituent of proteins, nucleic acids, hormones etc. It is necessary for the meristems and metabolically active cells.
• Phosphorus is taken up in the form H2PO4- and HPO42- through the soil. It is a key component of the phosphorylation reactions. It also forms cell walls, nucleic acids, nucleotides.

Micronutrients

• Micronutrients are also known as trace elements that are needed by plants in very minute quantities.
• The essential elements which are needed in very small amounts (less than 10 mmole Kg –1 of dry matter) are known micronutrients.
• There are 8 microelements including iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.
• Micronutrients function in plants mainly as cofactors, nonprotein helpers in enzymatic reactions.
• Iron, for example, is a metallic component of cytochromes, the proteins in the electron transport chains of chloroplasts and mitochondria.
• It is because micronutrients generally play catalytic roles that plants need only tiny quantities.
• The requirement for molybdenum, for instance, is so modest that there is only one atom of this rare element for every 60 million atoms of hydrogen in dried plant material. Yet a deficiency of molybdenum or any other micronutrient can weaken or kill a plant.
• Zinc is involved in the activation of carboxylases and also aids in the biosynthesis of auxin.
• Copper absorbed in the form Cu2+ is required for overall plant metabolism.
• Their essentiality was known only when extra pure salts were added in the hydroponics culture experiments.
   

Categorization of elements on the basis of their diverse functions
 

Introduction:

• Essential elements can also be grouped into four broad categories on the basis of their diverse functions.
• These categories are: Structural elements, Components of energy-rich compounds, Enzyme activators and inhibitors, Maintaining osmotic potential.

Detailed explanation:
 

A) Structural elements-

• These are all those essential elements that are responsible for the formation of all sorts of biomolecules.
• All these biomolecules are responsible for the formation of key components in cells and the plant body.
• Carbon, hydrogen, oxygen and nitrogen form the basis of all biomolecules.
• These elements form the basic organic compounds such as sugars, amino acids, lipids, nucleic acids.
• Nitrogen is an extremely essential element contributing to the formation of nucleic acids, proteins and chlorophyll.

B) Components of energy rich compounds-

• Magnesium, Phosphorus are two elements that are related to energy conversion and energy currency respectively.
• Magnesium is a key component of chlorophyll which is found in the chloroplasts and helps in converting the light energy of the sun into chemical energy during photosynthesis.
• Phosphorus is the primary component of ATP or adenosine triphosphate, the energy currency in all living systems.

C) Enzyme activator and inhibitors-

• Various essential elements play an important role in activating or inhibiting an enzyme.
• Magnesium in the form Mg2+ acts as the activator element for ribulose bisphosphate carboxylase- oxygenase and phosphoenolpyruvate carboxylase. These two enzymes are highly crucial for photosynthetic carbon fixation.
• Zinc in the form of Zn2+ helps in the activation of the enzymes such as alcohol dehydrogenase, carboxypeptidase and carbonic anhydrase..
• Molybdenum (Mo) activates nitrogenase for the purpose of nitrogen metabolism.
• Manganese in the form of Mn2+ helps in the photolysis of water during light reaction of photosynthesis.

D) Maintaining osmotic potential-

• Essential elements can also act as regulators of the osmotic potential of a cell.
• Osmotic potential is controlled by inorganic salts such as Cl-, K+, nitrate, sulphate etc.
• Osmotic potential is required for water absorption and maintenance of cell turgidity.
• Besides this, mineral elements are also involved in other important functions such as-
  i.Potassium plays an important role in the opening and closing of stomata and turgor movements.
  ii.Buffer activity against pH changes is shown by phosphates, weak acids and their salts.

Frequently Asked Questions: FAQs
 

Q1. Which elements are said to be essential elements based on the essentiality criteria?
Answer:
Criteria for essentiality-

1. The element must be absolutely necessary for supporting normal growth and reproduction amongst the plants. In the absence of the element, the plants are not able to complete their life cycle or set the seeds.
2. The requirement of the element must be specific and not replaceable by another element. In other words, every element has an important role assigned and the lack of one element cannot be fulfilled by another.
3. The element must be directly involved in the metabolism of the plant.

Q2. What are the various categories that elements are grouped into on the basis of their diverse functions?

Answer: All the essential elements are divided into 4 separate groups based on their functions-
i. Structural elements
ii. Components of Energy-related compounds
iii. Enzyme activators and inhibitors
iv. Osmotic regulators

Q3. Which elements are key to energy-storing compounds in biological systems?
Answer: Magnesium, Phosphorus are two elements that are related to energy conversion and energy currency respectively.

• Magnesium is a key component of chlorophyll which is found in the chloroplasts and helps in converting the light energy of the sun into chemical energy during photosynthesis.
• Phosphorus is the primary component of ATP or adenosine triphosphate, the energy currency in all living systems.

Q4. How essential elements are responsible for the maintenance of osmotic potential?
Answer:

• Essential elements can also act as regulators of the osmotic potential of a cell.
• Osmotic potential is controlled by inorganic salts such as Cl-, K+, nitrate, sulphate etc.
• Osmotic potential is required for water absorption and maintenance of cell turgidity.
• Besides this, mineral elements are also involved in other important functions such as-
  i.Potassium plays an important role in the opening and closing of stomata and turgor movements.
  ii.Buffer activity against pH changes is shown by phosphates, weak acids and their salts.

Q5. What are macronutrients with examples?
Answer:

• The essential elements which are generally present in plant tissues in large amounts (in excess of 10 mmole Kg –1 of dry matter) are known as Macronutrients or macro elements.
• Nine of the essential elements are called macronutrients because plants require them in relatively large amounts.
• Six of these are the major components of organic compounds forming a plant’s structure: carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulfur.
• The other three macronutrients are potassium, calcium, and magnesium.

Q6. What are micronutrients with examples?
Answer:

• Micronutrients are also known as trace elements that are needed by plants in very minute quantities.
• The essential elements which are needed in very small amounts (less than 10 mmole Kg –1 of dry matter) are known micronutrients.
• There are 8 microelements including iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.
• Micronutrients function in plants mainly as cofactors, nonprotein helpers in enzymatic reactions.