Introduction
Plants absorb water, minerals, and other nutrients from the soil which is carried to the other parts of the plant by plant tissues (mostly xylem). Abundant minerals are present in the soil that are absorbed by the roots of the plants and transferred to the other parts of the plant. This is the way, all other living beings like humans and animals receive nutrients when we consume different parts of the plant. Essential mineral elements are a mix of macronutrients like calcium, phosphorus, sodium, carbon, etc. and micronutrients like iron, manganese, zinc, copper, etc. that are necessary for the growth of plants and animals.
What are Mineral Elements?
Numerous inorganic elements are essential for growth and various other life processes by humans as well as animals. Thus, these mineral elements should be necessarily taken in our diets in adequate amounts. A deficiency in any of the minerals leads to an abnormality in the metabolic functioning of the body of the organism.
As of now, approximately 105 minerals have been discovered out of which about 60 to 65 minerals are absorbed by plants from the soil. Different plant species absorb different minerals. Some of the essential minerals required by plants for their growth and development include calcium, nitrogen, potassium, phosphorus, magnesium, sulfur, etc.
Soil is the basic medium from which almost all varieties of plants derive their essential nutrients. However, to provide more to the field of horticulture, agriculture, etc. plants are also grown on mineral nutrient media without soil. This technique used nowadays to grow plants without soil is also known as Hydroponics. Not all minerals absorbed by plants are necessary for them, so it is important to identify the essence of minerals to different types of plants. Various techniques can be employed to do this.
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The Criteria to Determine Essential Minerals to the Plant are as follows-
Minerals absorbed from soil by roots of plants are mineral ions dissolved in soil medium. They support plant life through their entire life cycle and also for the production of seeds.
A few minerals like magnesium and phosphorus are crucial for the growth, development, and reproduction of plants.
Mineral deficiency like deficiency of nitrogen, iron, copper, sulfur leads to various problems like stunted growth, poor development, insufficient chlorophyll in leaves, cell senescence, etc. in plants.
Mineral potassium greatly influences the quality of fruits and vegetables.
Proper metabolism in plants is dependent on minerals like potassium, sodium, magnesium, and other elements.
Essential Mineral Elements in Plants
There are two categories of minerals essential for plant growth. They are-
1. Macrominerals
Minerals like Phosphorous, carbon, sodium, nitrogen, potassium, silicon, magnesium, hydrogen, calcium, oxygen, selenium, and sulfur are required in abundance for proper plant growth and are also found abundantly in plant tissues. Therefore, they are known as macrominerals.
2. Microminerals
A few minerals like Iron, boron, nickel, manganese, copper, zinc, cobalt, chlorine, molybdenum, etc. are required in lesser quantities for proper growth by the plants and hence they are called microminerals. They are also called trace elements.
Absorption of Mineral Elements in Plants
Plants absorb the mineral elements from the soil by either of the two mechanisms-
1. Active Mineral Absorption
The type of absorption that takes place against the concentration gradient and the one which is active is known as Active transport. Active transport requires energy to take place. Inactive transport roots of the plants pick up the mineral ions and move them across the cell membrane.
2. Passive Mineral Absorption
Passive transport is a type of transport that takes place along the concentration gradient and hence it does not require energy. Transmembrane proteins act as selective pores to carry out the movement of mineral ions into the apoplast through the ion channels in this type of transport.
Functions of Mineral Elements in Plants
Formation of Plant Body
Various components of the plant body show the presence of various mineral elements, indicating that these elements play a vital role in the formation of the plant body. For Example- Nitrogen and sulfur make up the compound protein, iron is found in cytochromes, phosphorus in nucleic acids, and magnesium is found in chlorophyll, which is food made by plants.
Determining the Osmotic Potential of the Plant Cell
Osmotic potential is crucial in maintaining the turgidity of the cell and mineral elements present in the cell sap of plants determine the osmotic potential gradient.
Maintaining pH levels
Minerals absorbed by plants also influence the pH levels of the cell sap in plants.
Permeability of the Cytoplasmic Membrane
Mineral elements have cations and anions which influence the permeability of cytoplasmic membranes in plants.
Catalytic Effects
Bringing about the catalytic effects in the plants with the help of mineral elements such as copper, iron, zinc, etc.
Balancing Toxic Mineral Elements
Various mineral elements like potassium, magnesium neutralize the toxic effects of other minerals. Thus, mineral elements help to maintain the ionic balance.
Phloem Tissue Transport
Translocation of organic substances in the phloem tissue is carried out by elements like boron and potassium.
FAQs on Essential Mineral Elements
1. What are essential mineral elements for plants, and what criteria determine their essentiality?
An essential mineral element is one that is indispensable for a plant to complete its life cycle. According to the criteria established by Arnon and Stout, an element is considered essential if:
- The plant cannot complete its life cycle or set seed in the absence of the element.
- The element's function cannot be replaced by another mineral element.
- The element is directly involved in the plant's metabolism, for example, as a component of an essential molecule or an enzyme cofactor.
2. How are the 17 essential mineral elements for plants classified?
The 17 essential elements are classified into two main categories based on the quantitative requirements of the plant:
- Macronutrients: These are required in large amounts (generally >10 mmole kg⁻¹ of dry matter). They include Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, and Sulfur.
- Micronutrients: Also known as trace elements, these are required in very small amounts (generally <10 mmole kg⁻¹ of dry matter). They include Iron, Manganese, Copper, Molybdenum, Zinc, Boron, Chlorine, and Nickel.
3. What are the major roles of macronutrients like Nitrogen, Phosphorus, and Potassium in plants?
Macronutrients form the building blocks of a plant's body and are crucial for its primary functions.
- Nitrogen (N) is a major component of proteins, nucleic acids, chlorophyll, and hormones. It is essential for vegetative growth.
- Phosphorus (P) is a constituent of cell membranes, certain proteins, all nucleic acids, and energy-carrying molecules like ATP. It is vital for energy transfer reactions.
- Potassium (K) is involved in maintaining the anion-cation balance in cells and is essential for protein synthesis, opening and closing of stomata, and enzyme activation.
4. What is the importance of micronutrients or trace elements in plant life?
Despite being required in small quantities, micronutrients are just as critical as macronutrients. Their primary importance lies in their role as cofactors for enzymes, which are proteins that catalyse metabolic reactions. For example, Zinc (Zn) is an activator for carboxylases, while Iron (Fe) is a crucial constituent of proteins involved in electron transport, like cytochromes, and is essential for chlorophyll formation.
5. How do plants absorb mineral ions from the soil?
Plants absorb minerals from the soil primarily through their roots using two main mechanisms:
- Passive Transport: This process does not require energy. Ions move from a region of higher concentration to a region of lower concentration across the cell membranes through ion channels.
- Active Transport: This process requires metabolic energy (ATP) to move mineral ions into the root cells against their concentration gradient. Specific pump proteins in the membrane actively transport ions from the soil into the plant.
6. Why are some essential minerals required in large amounts (macronutrients) while others are needed only in trace amounts (micronutrients)?
The difference in required quantity is based on the element's function. Macronutrients are the primary structural components of organic molecules that make up the plant's body, such as proteins (requiring Nitrogen and Sulfur) and nucleic acids (requiring Nitrogen and Phosphorus). They constitute the very fabric of the plant. In contrast, micronutrients typically function as catalysts or cofactors for enzymes. Only a tiny amount is needed to activate thousands of enzyme molecules and facilitate countless biochemical reactions.
7. What is the difference between mineral deficiency and mineral toxicity in plants?
Mineral deficiency occurs when the concentration of an essential element is below the critical concentration, leading to impaired growth and visible symptoms like chlorosis (yellowing of leaves) or necrosis (death of tissue). Mineral toxicity occurs when an element is present in an amount that is too high, inhibiting growth. A key point is that for micronutrients, the concentration range between adequate and toxic is very narrow. For example, an excess of manganese can induce deficiencies of iron, magnesium, and calcium.
8. How does the principle of mineral essentiality apply to the agricultural technique of hydroponics?
Hydroponics is the technique of growing plants in a nutrient solution without soil. This method relies completely on our understanding of mineral essentiality. In hydroponics, a solution is prepared that contains all 17 essential elements in their correct proportions and concentrations. This allows for precise control over plant nutrition, helping to identify deficiency symptoms and determine the optimal nutrient levels for maximum yield, a process that is much harder to control in soil.
9. Can a plant absorb elements that are not essential for its survival? If so, why does this happen?
Yes, plants can and do absorb elements that are not essential for their survival. The mineral absorption process by roots is not perfectly selective. While there are specific transport proteins for essential ions, other elements with similar chemical properties (like charge and ionic radius) can also be taken up. For example, plants can absorb heavy metals like lead or cadmium from contaminated soil, which are non-essential and often toxic. This is why the criteria for essentiality specify that the element must be indispensable, not just present in the plant tissue.
10. Why is nitrogen often a limiting nutrient for plant growth despite being the most abundant gas in the atmosphere?
Although the atmosphere is about 78% nitrogen gas (N₂), plants cannot use it in this form. The two nitrogen atoms in N₂ are held together by a very strong triple covalent bond, which plants lack the enzymes to break. Plants can only absorb nitrogen in the form of nitrate (NO₃⁻) or ammonium (NH₄⁺) ions from the soil. The conversion of atmospheric N₂ into these usable forms, a process called nitrogen fixation, is carried out by specific soil bacteria, making the availability of usable nitrogen in the soil a critical limiting factor for plant growth.
