Mineral Nutrition

Learning Outcomes:

1. Understand the role of essential elements in plant nutrition.
2. Identify deficiency symptoms associated with specific mineral nutrients.
3. Comprehend methods of mineral absorption and translocation in plants.
4. Explore biological nitrogen fixation and its significance.

Plants, like all living organisms, require a combination of macromolecules and inorganic substances for their growth and development. The primary focus of this chapter is the importance of inorganic plant nutrition, the methods used to determine essential elements, deficiency symptoms, and biological nitrogen fixation. Understanding these aspects is crucial for managing plant health and agricultural productivity.

Methods to Study the Mineral Requirements of Plants

In 1860, Julius von Sachs demonstrated that plants could grow to maturity in a nutrient solution devoid of soil, a method known as hydroponics. This technique allowed the identification of essential minerals by immersing plant roots in nutrient solutions and adjusting the concentration or presence of particular elements to observe growth effects.

1. Hydroponic Method: Plants are grown in a nutrient solution, where roots are immersed, and different minerals are added or removed to identify which are essential.
2. Commercial Use: Hydroponics has been widely used for commercial production of crops like tomatoes, seedless cucumbers, and lettuce. Proper aeration of the solution is necessary for optimal growth.
3. Purified Water: Purified water and nutrient salts are essential for precise studies to avoid contamination and ensure accurate results in mineral requirement identification.

Essential Mineral Elements

Plants absorb minerals primarily through their roots, and over 60 elements have been found in various plants. Not all of these elements are necessary for plant growth, and scientists have established criteria for determining essentiality.

1. Criteria for Essentiality: An element must be necessary for growth and reproduction, must have a specific role that cannot be replaced by another element, and must be involved in the metabolism of the plant.
2. Classification of Essential Elements: These elements are divided into macronutrients and micronutrients based on the amount required by plants.

Macronutrients

• Carbon, Hydrogen, Oxygen: Derived from CO₂ and H₂O, these are vital for the structural components of plants.
• Nitrogen: Essential for proteins, nucleic acids, vitamins, and hormones. It is absorbed primarily as nitrate ions (NO₃⁻) and is crucial in metabolically active cells.
• Phosphorus: Absorbed as phosphate ions (H₂PO₄⁻ or HPO₄²⁻), it is a component of nucleic acids and plays a critical role in energy transfer reactions.
• Potassium: Absorbed as K⁺, it helps maintain ionic balance, is involved in stomatal regulation, and plays a role in enzyme activation.
• Calcium: Absorbed as Ca²⁺, it is crucial for cell division, membrane function, and the formation of the cell wall.
• Magnesium: As a component of chlorophyll, Mg²⁺ activates enzymes involved in respiration and photosynthesis.
• Sulphur: Absorbed as sulfate (SO₄²⁻), it is present in amino acids like cysteine and methionine and is vital for coenzymes and vitamins.

Micronutrients

• Iron: Absorbed as Fe³⁺, it plays a role in electron transport and is a crucial part of proteins involved in electron transfer, such as cytochromes.
• Manganese: Absorbed as Mn²⁺, it is essential for the photolysis of water during photosynthesis.
• Zinc: Zn²⁺ activates various enzymes, including those involved in auxin synthesis.
• Copper: Absorbed as Cu²⁺, it participates in redox reactions and is necessary for enzyme activity.
• Boron: Absorbed as BO₃³⁻ or B₄O₇²⁻, it is essential for calcium uptake, cell wall structure, and carbohydrate translocation.
• Molybdenum: Absorbed as MoO₄²⁻, it is involved in nitrogen metabolism and is a component of nitrogenase.
• Chlorine: As Cl⁻, it helps in osmoregulation and the photolysis of water.

Role of Essential Nutrients

Essential elements play a variety of roles in plant metabolism, enzyme activation, and structural formation.

1. Structural Components: Elements like carbon, hydrogen, and oxygen form the backbone of biomolecules.
2. Energy Compounds: Elements like phosphorus and magnesium are involved in energy-related chemical compounds, such as ATP and chlorophyll.
3. Enzyme Activation: Mg²⁺, Zn²⁺, and Mo²⁺ activate critical enzymes in photosynthesis and nitrogen metabolism.
4. Osmotic Regulation: Potassium helps regulate osmotic potential, influencing stomatal movement.

Deficiency Symptoms of Essential Elements

Deficiency in any essential element leads to specific morphological changes, called deficiency symptoms. These symptoms are related to the element’s role in plant metabolism.

1. Chlorosis: The yellowing of leaves due to the loss of chlorophyll, caused by deficiencies in nitrogen, potassium, magnesium, sulfur, iron, manganese, zinc, or molybdenum.
2. Necrosis: The death of tissue, typically seen in calcium, magnesium, copper, and potassium deficiencies.
3. Stunted Growth: Caused by a lack of nitrogen, phosphorus, or potassium, leading to poor development.
4. Premature Leaf Fall: Results from nitrogen or phosphorus deficiency.
5. Inhibition of Cell Division: Caused by low levels of nitrogen, potassium, or molybdenum.

The mobility of elements within the plant determines where deficiency symptoms appear first. Mobile elements like nitrogen, potassium, and magnesium show symptoms in older tissues, while immobile elements like calcium and sulfur cause symptoms in younger tissues.

Toxicity of Micronutrients

Although micronutrients are required in trace amounts, their excess can lead to toxicity. Toxic levels of an element can cause symptoms like manganese toxicity, which manifests as brown spots on leaves. Manganese also interferes with the uptake of iron and magnesium, creating additional deficiency symptoms. The concentration range between deficiency and toxicity is narrow, making careful management critical.

Mechanism of Absorption of Elements

The absorption of minerals by plants occurs in two main phases:

1. Apoplast Phase: Involves the rapid, passive uptake of ions into the free space of cells, or the apoplast. This process relies on ion channels for selective transport.
2. Symplast Phase: Involves the slow, active uptake of ions into the symplast, requiring energy. Active transport mechanisms involve the movement of ions across cell membranes, consuming ATP.

Translocation of Solutes

Minerals absorbed by the roots are transported to other parts of the plant through the xylem, along with water. Transpirational pull drives this upward movement, and analysis of xylem sap confirms the presence of dissolved mineral salts. Radioisotopes have provided evidence supporting the view that minerals move through the xylem.

Soil as a Reservoir of Essential Elements

Soil serves as the primary reservoir of essential minerals for plant growth. Weathering and rock breakdown release dissolved ions and salts into the soil, providing nutrients to plant roots. Soil also supports nitrogen-fixing bacteria and holds water and air, stabilizing the plant.

Fertilizers are often added to the soil to supplement the availability of essential minerals. These fertilizers contain both macronutrients (N, P, K) and micronutrients (Cu, Zn, Fe, Mn).

Metabolism of Nitrogen

Nitrogen is one of the most abundant elements in living organisms, but plants cannot directly use atmospheric nitrogen (N₂). Nitrogen is a critical component of amino acids, proteins, chlorophyll, and other biomolecules.

1. Nitrogen Cycle: Atmospheric nitrogen is converted to ammonia by lightning, ultraviolet radiation, and industrial combustion. Organic nitrogen from dead plants and animals is converted to ammonia in a process known as ammonification.
2. Nitrification: Ammonia is oxidized to nitrite by Nitrosomonas and Nitrococcus bacteria and then to nitrate by Nitrobacter. Nitrate is absorbed by plants and reduced to ammonia for use in amino acid synthesis.
3. Denitrification: Some bacteria, such as Pseudomonas and Thiobacillus, convert soil nitrate back into nitrogen gas, completing the nitrogen cycle.

Biological Nitrogen Fixation

Certain prokaryotic organisms can fix atmospheric nitrogen into biologically usable forms. This process, known as biological nitrogen fixation, is carried out by nitrogenase, an enzyme present exclusively in prokaryotes.

1. Free-living Nitrogen Fixers: Examples include Azotobacter (aerobic) and Rhodospirillum (anaerobic), along with cyanobacteria like Anabaena and Nostoc.
2. Symbiotic Nitrogen Fixation: The most well-known symbiotic relationship is between Rhizobium bacteria and leguminous plants. **Rh

izobium** forms nodules on the roots of legumes where nitrogen fixation occurs.

Nodule Formation

Nodule formation in legumes involves several steps:

1. Bacterial Colonization: Rhizobia multiply around the roots and attach to root hairs.
2. Infection Thread: The bacteria invade root hairs, forming an infection thread that carries them into the root cortex.
3. Nodule Development: The bacteria are released into the root cells, which differentiate into nitrogen-fixing cells, establishing vascular connections for nutrient exchange.
4. Nitrogen Fixation: Inside the nodules, nitrogenase converts atmospheric nitrogen to ammonia. The presence of leg-haemoglobin in the nodules helps maintain an anaerobic environment for the enzyme, which is sensitive to oxygen.

Table: Macronutrients and Micronutrients Comparison

Nitrogen Fixation and ATP Requirement

The nitrogenase enzyme requires significant amounts of energy to fix nitrogen. For each molecule of ammonia produced, 16 ATP molecules are consumed. The energy is provided by the host plant through respiration.

MCQ

Question: Which enzyme is crucial for biological nitrogen fixation?
Answer: Nitrogenase