Mineral Nutrition Biology Notes - Hydroponics, Essential Elements, Deficiency Symptoms, Nitrogen Cycle and Biological Nitrogen Fixation

All organisms require macromolecules (carbohydrates, proteins, fats, etc.), water, and minerals for growth and development.

Methods to Study the Mineral Requirements of Plants

• The technique of growing plants in a defined nutrient solution without soil is known as hydroponics.
• It was demonstrated by Julius von Sachs (Germany, 1860).
• The nutrient solution is aerated for optimum growth.
• Hydroponics is used to identify essential elements required for plants and their deficiency symptoms.
• In hydroponics, plant roots are immersed in nutrient solutions, and an element is added, removed, or provided in varied concentrations.
• Hydroponics is used for commercial production of vegetables such as tomatoes, seedless cucumbers, and lettuce.

Essential Mineral Elements

• More than 60 elements are found in different plants.
• Some plant species near mining sites accumulate selenium, gold, etc. Plants near nuclear test sites may take up radioactive strontium.
• Techniques exist to detect minerals at very low concentrations (10^-8 g/mL).

Criteria for Essentiality of an Element

• An element is essential if it is needed for normal growth and reproduction. Without it, plants cannot complete their life cycle or set seeds.
• The requirement for an element must be specific; its deficiency cannot be compensated by another element.
• It must be directly involved in plant metabolism.

Only 17 elements are absolutely essential for plant growth and metabolism.

Based on quantitative requirements, essential elements are divided into two types: Macronutrients and Micronutrients.

i. Macronutrients

• They are present in plant tissues in large amounts (more than 10 mmole kg^–1 of dry matter).
• They include carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium.
• Carbon, hydrogen, and oxygen are mainly obtained from CO₂ and H₂O. Others are absorbed from the soil as mineral nutrition.

ii. Micronutrients (Trace Elements)

• They are needed in very small amounts (less than 10 mmole kg^–1 of dry matter).
• They include iron, manganese, copper, molybdenum, zinc, boron, chlorine, and nickel.
• Higher plants may also require sodium, silicon, cobalt, selenium, etc.
• Based on functions, essential elements are categorized into four groups:
• Components of biomolecules and structural elements of cells: E.g., carbon, hydrogen, oxygen, and nitrogen.
• Components of energy-related chemical compounds: E.g., magnesium in chlorophyll and phosphorus in ATP.
• Elements that activate or inhibit enzymes: E.g., Mg²⁺ activates RuBisCO and phosphoenol pyruvate carboxylase (critical enzymes in photosynthetic carbon fixation). Zn²⁺ activates alcohol dehydrogenase, and Mo activates nitrogenase during nitrogen metabolism.
• Elements that alter the osmotic potential of a cell: E.g., potassium aids in the opening and closing of stomata.

Role of Macro- and Micro-nutrients

• Essential elements play roles in metabolic processes such as:

• Permeability of cell membranes.
• Maintenance of osmotic concentration of cell sap.
• Electron transport systems.
• Buffering action.
• Enzymatic activity.
• Constituents of macromolecules and co-enzymes.

Nitrogen

• Required in the greatest amount.
• Absorbed mainly as NO₃^–, with some uptake as NO₂^– or NH₄⁺.
• Essential for all plant parts, particularly meristematic tissues and metabolically active cells.
• Major constituent of amino acids, proteins, nucleic acids, chlorophyll, vitamins, and hormones.

Phosphorus

• Absorbed as phosphate ions (H₂PO₄⁻ or HPO₄²⁻).
• Constituent of cell membranes, certain proteins, nucleic acids, and nucleotides.
• Required for all phosphorylation reactions.

Potassium

• Absorbed as potassium ion (K⁺).
• Essential in meristematic tissues, buds, leaves, and root tips.
• Maintains anion-cation balance in cells.
• Involved in protein synthesis, opening and closing of stomata, enzyme activation, and maintaining cell turgidity.

Calcium

• Absorbed as calcium ions (Ca²⁺).
• Required by meristematic and differentiating tissues.
• Used in cell wall synthesis, particularly as calcium pectate in the middle lamella, and during mitotic spindle formation.
• Accumulates in older leaves.
• Involved in cell membrane functioning.
• Activates some enzymes and regulates metabolic activities.

Magnesium

• Absorbed as divalent Mg²⁺.
• Activates enzymes of respiration and photosynthesis.
• Involved in the synthesis of DNA and RNA.
• Constituent of the ring structure of chlorophyll.
• Helps maintain ribosome structure.

Sulfur

• Obtained as sulfate (SO₄²⁻).
• Present in amino acids cysteine and methionine.
• Constituent of coenzymes, vitamins (thiamine, biotin, Coenzyme A), and ferredoxin.

Iron

• Obtained as ferric ions (Fe³⁺).
• Required in larger amounts compared to other micronutrients.
• Main constituent of proteins involved in electron transfer, such as ferredoxin and cytochromes.
• Reversibly oxidized from Fe²⁺ to Fe³⁺ during electron transfer.
• Activates catalase enzyme and is essential for chlorophyll formation.

Manganese

• Absorbed as manganous ions (Mn²⁺).
• Activates enzymes involved in photosynthesis, respiration, and nitrogen metabolism.
• Key role in the splitting of water to liberate O₂ during photosynthesis.

Zinc

• Obtained as Zn²⁺ ions.
• Activates various enzymes, especially carboxylases.
• Needed in the synthesis of auxin.

Copper

• Absorbed as cupric ions (Cu²⁺).
• Essential for overall plant metabolism.
• Associated with enzymes in redox reactions, reversibly oxidized from Cu⁺ to Cu²⁺.

Boron

• Absorbed as BO₃³⁻ or B₄O₇²⁻.
• Required for Ca²⁺ uptake and utilization, membrane functioning, pollen germination, cell elongation, cell differentiation, and carbohydrate translocation.

Molybdenum

• Obtained as molybdate ions (MoO₂²⁺).
• Component of enzymes like nitrogenase and nitrate reductase, which participate in nitrogen metabolism.

Chlorine

• Absorbed as chloride anion (Cl^–).
• With Na⁺ and K⁺, it determines solute concentration and anion-cation balance in cells.
• Essential for the water-splitting reaction in photosynthesis, leading to oxygen evolution.

Deficiency Symptoms of Essential Elements

• Deficiency of an essential element causes retarded growth.
• The concentration below which plant growth is retarded is called the critical concentration. An element is deficient when present below this level.
• Morphological changes due to deficiency are called deficiency symptoms.
• Deficiency symptoms vary from element to element.
• The plant parts showing deficiency symptoms depend on the element’s mobility:
• For mobile elements (e.g., nitrogen, potassium, magnesium), deficiency symptoms appear first in older tissues, as these elements are exported to young developing tissues.
• For immobile elements (e.g., sulfur, calcium), deficiency symptoms appear first in young tissues, as they are not easily released from mature organs.
• This aspect of mineral nutrition is significant in agriculture and horticulture.
• Deficiency symptoms include chlorosis, necrosis, stunted growth, premature fall of leaves and buds, and inhibition of cell division.
• Chlorosis: Loss of chlorophyll, leading to yellowing of leaves, caused by deficiencies of N, K, Mg, S, Fe, Mn, Zn, and Mo.
• Necrosis: Death of tissue, particularly leaf tissue, caused by deficiencies of Ca, Mg, Cu, and K.
• Deficiencies of N, K, S, and Mo inhibit cell division.
• Low levels of N, S, and Mo delay flowering.
• Since different elements may cause similar symptoms, all symptoms are studied to identify the deficient element. Different plants also respond differently to the same element’s deficiency.

Toxicity of Micronutrients

• A moderate increase in micronutrients causes toxicity.
• A mineral ion concentration reducing tissue dry weight by about 10% is considered toxic, with critical concentrations varying among micronutrients.
• Toxicity symptoms are difficult to identify and vary by plant.
• Excess of one element may inhibit the uptake of another. For example, excess manganese induces deficiencies of Fe, Mg, and Ca by competing with Fe and Mg for uptake and with Mg for enzyme binding. Manganese also inhibits Ca translocation to the shoot apex.
• Symptoms of manganese toxicity may appear as deficiency symptoms of Fe, Mg, and Ca. The main symptom is the appearance of brown spots surrounded by chlorotic veins.

Mechanism of Absorption of Elements

The inward movement of ions into cells is called influx, and the outward movement is called efflux.

The process of absorption includes two main phases:

• First phase: Initial rapid and passive uptake of ions into the apoplast (free space or outer space) of cells, usually through ion channels (trans-membrane proteins that function as selective pores).
• Second phase: Slow uptake of ions into the symplast (inner space or cytoplasm) of cells, which is an active process requiring energy.

Translocation of Solutes

Mineral salts are translocated through the xylem along with the ascending stream of water.

Analysis of xylem sap shows the presence of mineral salts, and the use of radioisotopes of mineral elements confirms their transport through the xylem.

Soil as Reservoir of Essential Elements

Weathering and breakdown of rocks enrich the soil with dissolved ions and inorganic salts.

Roles of soil:

• Supplies minerals and holds water.
• Harbours nitrogen-fixing bacteria and other microbes.
• Supplies air to the roots.
• Acts as a matrix that stabilizes the plant.

Deficiency of essential minerals affects crop yield, so fertilizers containing both macro- and micro-nutrients should be supplied.

Metabolism of Nitrogen

Nitrogen Cycle

• Nitrogen is the most prevalent element in living organisms.
• Plants compete with microbes for limited nitrogen in soil, making nitrogen a limiting nutrient for both natural and agricultural ecosystems.
• The conversion of atmospheric nitrogen (N₂ or N≡N) to ammonia is called nitrogen fixation.
• In nature, lightning and UV radiation provide energy to convert nitrogen to nitrogen oxides (NO, NO₂, N₂O). Industrial combustions, forest fires, automobile exhausts, and power-generating stations also produce atmospheric nitrogen oxides.
• Decomposition of organic nitrogen from dead plants and animals into ammonia is called ammonification.
• Some ammonia volatilizes and re-enters the atmosphere, but most is oxidized into nitrate by soil nitrifying bacteria (Nitrosomonas, Nitrococcus, and Nitrobacter—chemoautotrophs). These steps are called nitrification.

• Plants absorb nitrate, which is transported to leaves and reduced to ammonia, forming the amine group of amino acids.

• Nitrate in soil is also reduced to nitrogen through denitrification. It is carried out by bacteria such as Pseudomonas and Thiobacillus.

Biological Nitrogen Fixation

Biological nitrogen fixation is the reduction of N₂ to NH₃ by living organisms in the presence of the nitrogenase enzyme.

Only certain prokaryotic species, called N₂-fixers, possess the nitrogenase enzyme and can fix N₂.

Nitrogen-fixing microbes are of two types:

• Free-living: E.g., Azotobacter and Beijerinckia (aerobic), Rhodospirillum and Bacillus (anaerobic), and cyanobacteria such as Anabaena and Nostoc.
• Symbiotic: E.g., Rhizobium (aerobic).

Symbiotic Biological Nitrogen Fixation

• Legume-bacteria relationship: The most prominent example involves Rhizobium species (rod-shaped) in the roots of legumes such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, and clover beans.
• The most common association on roots is as nodules.
• The microbe Frankia produces N₂-fixing nodules on the roots of non-leguminous plants (e.g., Alnus).
• Rhizobium and Frankia are free-living in soil but can fix atmospheric nitrogen as symbionts.
• The central part of a nodule is red or pink due to leguminous haemoglobin (leg-haemoglobin).

Principal stages in nodule formation:

• Rhizobia multiply and colonize the surroundings of roots, attaching to epidermal and root hair cells.
• Root hairs curl, and bacteria invade the root hair.
• An infection thread carries bacteria into the root cortex, initiating nodule formation.
• Bacteria are released into cells, leading to the differentiation of specialized nitrogen-fixing cells.
• The nodule establishes a direct vascular connection with the host for nutrient exchange.

• Nodules contain the nitrogenase enzyme and leg-haemoglobin.
• Nitrogenase (a Mo-Fe protein) catalyzes the conversion of N₂ to NH₃, the first stable product of N₂ fixation.

• Ammonia synthesis requires high energy input (8 ATP per NH₃), obtained from the respiration of host cells.
• Nitrogenase is highly sensitive to molecular oxygen, requiring anaerobic conditions. Leg-haemoglobin acts as an oxygen scavenger to protect it.
• Rhizobia are aerobic under free-living conditions (when nitrogenase is not operational) but become anaerobic during N₂-fixing events to protect nitrogenase.

Fate of Ammonia

• At physiological pH, NH₃ is protonated to form NH₄⁺ (ammonium) ion. Most plants can assimilate nitrate and NH₄⁺, but NH₄⁺ is toxic and cannot accumulate in plants.
• NH₄⁺ is used to synthesize amino acids in two ways:

a. Reductive Amination:

Ammonia reacts with α-ketoglutaric acid to form glutamic acid.

b. Transamination:

The amino group (NH₂) is transferred from one amino acid to the keto group of a keto acid in the presence of a transaminase enzyme. Glutamic acid is the main amino acid from which NH₂ is transferred to form other amino acids.

• Asparagine and glutamine are important amides in plants, forming structural parts of proteins. They are derived from aspartic acid and glutamic acid by adding another amino group, replacing the hydroxyl part of the acid with an NH₂ radical.
• Since amides contain more nitrogen than amino acids, they are transported via xylem vessels. Additionally, nodules of some plants (e.g., soybean) export fixed nitrogen as ureides, which have a high nitrogen-to-carbon ratio.