Mineral Nutrition

Methods To Study The Mineral Requirements Of Plants

Plants obtain carbon and oxygen from the atmosphere in the form of $CO_2$. However, they absorb water and mineral nutrients from the soil. Understanding the mineral requirements of plants is crucial for plant growth and agriculture.

Several methods have been developed to determine the essential mineral elements required by plants and the roles they play. The most commonly used method is Hydroponics.

Hydroponics

Hydroponics is the technique of growing plants in a nutrient solution (aerated water containing dissolved inorganic mineral salts) instead of soil. This method was first demonstrated by Julius von Sachs in 1860.

Procedure:

1. Plants are grown in tanks containing carefully prepared nutrient solutions.
2. The solution is adequately aerated to provide oxygen to the roots.
3. Nutrient solutions are prepared by dissolving known quantities of specific mineral salts in pure water.
4. By varying the composition of the nutrient solution (e.g., adding or removing a specific element), scientists can observe the effect on plant growth.
5. If a plant grows normally in the complete nutrient solution but shows poor growth or specific symptoms when a particular element is removed, that element is considered essential.

*(Image shows a diagram of a hydroponic system with a plant growing in a tank of aerated nutrient solution, highlighting roots submerged in the solution)*

Significance of Hydroponics:

• Allows the study of plant nutrition under controlled conditions, eliminating variability introduced by soil.
• Helps in identifying the essential mineral elements and their specific roles.
• Enables the determination of deficiency symptoms for each essential element.
• Can be used to determine the optimum concentration of mineral nutrients required for plant growth.
• It has commercial applications in the production of vegetables (e.g., tomato, cucumber, lettuce) where soil is unavailable or unsuitable, or where higher yields are desired.

Strictly speaking, water used in hydroponics must be pure (distilled or deionised) to avoid introducing unwanted mineral contaminants. Also, the mineral salts used must be pure. The vessels should be clean to prevent bacterial or fungal growth.

Essential Mineral Elements

Plants require a variety of mineral elements for their survival, growth, and development. Not all mineral elements found in a plant are essential. An element is considered essential if it meets certain criteria.

Criteria For Essentiality

For a mineral element to be considered essential for plant growth, it must satisfy the following criteria:

1. Necessity for Normal Growth and Reproduction: The element must be absolutely necessary for supporting normal growth and reproduction. In the absence of the element, the plants must not be able to complete their life cycle or set seeds.
2. Specificity: The requirement of the element must be specific and not replaceable by any other element. However, in some cases, one element can substitute for another in minor functions (e.g., Magnesium replaced by Zinc, or Potassium replaced by Sodium), but this does not meet the primary criterion for essentiality.
3. Direct Involvement in Metabolism: The element must be directly involved in the metabolism of the plant, either as a component of a biomolecule (like chlorophyll, proteins) or as an activator/inhibitor of enzymes, or involved in energy transfer processes.

Based on these criteria, a set of essential elements have been identified for plants.

Classification of Essential Elements based on Requirement

The essential elements are divided into two broad categories based on the quantity required by plants:

1. Macronutrients: Required in relatively large amounts (usually in excess of 10 mmol per Kg of dry matter).

   These are: Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Sulphur, Potassium, Calcium, Magnesium.

   C, H, O are obtained from $CO_2$ and $H_2O$. The others (N, P, K, S, Ca, Mg) are absorbed from the soil as inorganic ions.

2. Micronutrients (Trace elements): Required in very small amounts (less than 10 mmol per Kg of dry matter).

   These are: Iron, Manganese, Copper, Zinc, Boron, Molybdenum, Chlorine, Nickel.

In addition to these 17 essential elements, some plants may require a few other elements (e.g., Sodium, Silicon, Cobalt, Selenium) for specific functions. These are sometimes called beneficial elements.

Role Of Macro- And Micro-Nutrients

Essential elements play diverse roles in plant metabolism:

Macronutrients:

• Nitrogen (N): Absorbed as $NO_3^-, NO_2^-, NH_4^+$. Required in large amounts, especially by meristematic tissues. Component of proteins, nucleic acids (DNA, RNA), vitamins, hormones, chlorophyll.
• Phosphorus (P): Absorbed as phosphate ions ($PO_4^{3-}$). Component of cell membranes, nucleic acids, nucleotides (ATP), and phosphorylation reactions. Required for root and seed development.
• Potassium (K): Absorbed as $K^+$. Required in more abundant quantities in meristematic tissues (buds, leaves, root tips). Helps maintain turgor pressure, enzyme activation, opening and closing of stomata, protein synthesis.
• Calcium (Ca): Absorbed as $Ca^{2+}$. Component of cell wall (calcium pectate in middle lamella). Required during cell division. Activates certain enzymes. Important for membrane function.
• Magnesium (Mg): Absorbed as $Mg^{2+}$. Component of chlorophyll. Activates enzymes of respiration and photosynthesis. Involved in DNA and RNA synthesis.
• Sulphur (S): Absorbed as sulphate ions ($SO_4^{2-}$). Component of amino acids (cysteine, methionine), proteins, some vitamins (thiamine, biotin), and coenzymes.

Micronutrients:

• Iron (Fe): Absorbed as ferric ions ($Fe^{3+}$). Required in larger amounts compared to other micronutrients. Component of proteins involved in electron transport (e.g., in chloroplasts, mitochondria). Essential for chlorophyll formation (though not a component of chlorophyll). Activates catalase enzyme.
• Manganese (Mn): Absorbed as $Mn^{2+}$. Activates many enzymes of photosynthesis, respiration, and nitrogen metabolism. Involved in splitting of water to liberate oxygen during photosynthesis.
• Copper (Cu): Absorbed as $Cu^{2+}$. Essential for overall metabolism (redox reactions). Component of enzymes involved in redox reactions (e.g., cytochrome oxidase).
• Zinc (Zn): Absorbed as $Zn^{2+}$. Activates enzymes, especially carboxylases. Required for the synthesis of auxin (plant hormone).
• Boron (B): Absorbed as $BO_3^{3-}$ or $B_4O_7^{2-}$. Involved in calcium uptake and utilisation, membrane function, pollen germination, cell elongation, cell differentiation, carbohydrate translocation.
• Molybdenum (Mo): Absorbed as $MoO_2^{2+}$. Component of enzymes nitrogenase (involved in nitrogen fixation) and nitrate reductase (involved in nitrogen metabolism). Essential for nitrogen metabolism.
• Chlorine (Cl): Absorbed as $Cl^-$. Involved in osmoregulation. Essential for water splitting reaction in photosynthesis (along with Mn).
• Nickel (Ni): Absorbed as $Ni^{2+}$. Component of urease enzyme (involved in nitrogen metabolism).

Deficiency Symptoms Of Essential Elements

When an essential element is deficient in the plant, it affects growth and metabolic processes, leading to visual symptoms. Deficiency symptoms appear in specific parts of the plant depending on the mobility of the element.

General Deficiency Symptoms:

• Chlorosis: Loss of chlorophyll leading to yellowing of leaves. Caused by deficiency of N, K, Mg, S, Fe, Mn, Zn, Mo.
  • Deficiency of mobile elements (N, K, Mg) causes chlorosis in older leaves first.
  • Deficiency of immobile elements (S, Fe, Mn, Zn, Mo, Ca, B, Cu) causes chlorosis in younger leaves first.
• Necrosis: Death of tissue, particularly leaf tissue. Caused by deficiency of $Ca^{2+}, Mg^{2+}, Cu^{2+}, K^{+}$.
• Inhibition of cell division: Leading to stunted growth. Caused by deficiency of N, K, S, Mo.
• Delay of flowering: Caused by deficiency of N, S, Mo.
• Premature leaf and bud fall: Caused by deficiency of K, N, Ca.
• Lack of stunted plant growth: Deficiency of any essential element can lead to reduced or stunted overall plant growth.

*(Image shows examples of leaves displaying yellowing (chlorosis), brown spots (necrosis), and comparison of plant height in normal vs deficient conditions)*

Toxicity Of Micronutrients

While micronutrients are essential, they are required in very small amounts. An excess concentration of micronutrients can be toxic to plants.

Mechanism of Micronutrient Toxicity:

• Toxicity occurs when the concentration of a micronutrient in the tissue is above a critical level, which causes a reduction in dry weight of tissue by about 10 percent.
• Toxicity symptoms vary for different micronutrients.
• Excess of one micronutrient can inhibit the uptake or activity of other essential elements, leading to deficiency symptoms of those other elements. For example:
  • Manganese toxicity: Can induce deficiency of Iron, Magnesium, and Calcium. Manganese competes with iron and magnesium for uptake, and also inhibits binding of magnesium to enzymes. Manganese also inhibits calcium translocation in the shoot apex. Symptoms include brown spots surrounded by chlorotic veins.

Therefore, micronutrients need to be provided within a narrow range of concentrations for optimum plant growth.

Mechanism Of Absorption Of Elements

Plants absorb mineral elements from the soil primarily through their roots in the form of ions. The absorption process involves both passive and active mechanisms.

Passive Absorption

• Movement of ions into the root cells or across membranes without the expenditure of metabolic energy (ATP).
• Occurs along a concentration gradient or electrochemical gradient.
• Can occur through diffusion (limited for ions) or facilitated diffusion through ion channels or carrier proteins in the membrane.
• Involves processes like mass flow of water carrying dissolved ions and ion exchange with the soil solution.
• The apoplast pathway in the root cortex (movement through cell walls) is largely passive.

Active Absorption

• Movement of ions into the root cells or across membranes against a concentration gradient.
• Requires the expenditure of metabolic energy (ATP).
• Involves specific membrane proteins (pumps) which act as carrier proteins that use energy to transport ions across the membrane.
• This mechanism allows plants to accumulate ions from the soil even when their concentration in the soil is very low.
• Active absorption is highly specific and subject to metabolic inhibitors.
• Plays a major role in the movement of ions across the plasma membrane of root hair cells and in the loading of ions into the xylem across the endodermis (where the Casparian strip blocks passive apoplast movement).
• The symplast pathway (movement through cytoplasm connected by plasmodesmata) may involve both passive and active steps at the cellular level.

*(Image shows a cell membrane with ion channels (passive), carrier proteins (passive or active facilitated), and pump proteins (active with ATP))*

Phases of Mineral Absorption

Ion uptake by root cells can be considered in two phases:

• First phase (Rapid uptake): Ions move into the outer space (apoplast) of the cells (cell wall and intercellular spaces). This movement is usually passive and rapid.
• Second phase (Slow uptake): Ions move into the inner space (symplast) of the cells (cytoplasm and vacuole). This movement is usually active and slower, involving crossing the plasma membrane.

The selectivity in mineral absorption primarily occurs at the plasma membrane through the action of specific transport proteins and active transport mechanisms.

Translocation Of Solutes

Once mineral nutrients are absorbed by the roots in the form of inorganic ions, they are transported from the roots to other parts of the plant. This movement of absorbed solutes is called translocation.

Pathway of Translocation

• Mineral nutrients are primarily translocated through the xylem vessels.
• The main driving force for this upward movement of ions is the transpiration stream (the bulk flow of water from roots to leaves due to transpiration pull).
• Therefore, the translocation of mineral ions is largely unidirectional, from the roots to the aerial parts (stem, leaves, flowers, fruits).

Unloading and Remobilisation

• Mineral ions are delivered to the various parts of the plant where they are required. These regions act as 'sinks' for the minerals.
• Unloading of ions at the sinks can occur through passive or active transport from the xylem into the surrounding cells.
• Plants have the ability to remobilise certain mineral elements from older, senescing parts (like mature leaves) to younger, growing regions (like shoot apical meristems, young leaves, developing fruits and seeds).
• Elements that are mobile in the plant are easily remobilised (e.g., Nitrogen, Phosphorus, Potassium, Sulphur). Deficiency symptoms of these elements first appear in older leaves.
• Elements that are relatively immobile are not easily remobilised (e.g., Calcium, Boron). Deficiency symptoms of these elements first appear in younger leaves.
• Analysis of xylem sap has shown that most minerals are transported as inorganic ions, but some are also transported as organic compounds (e.g., Nitrogen is often transported as amino acids and amides; Phosphorus and Sulphur are often transported as organic phosphates and sulphates).
• Small amounts of mineral elements are also transported in the phloem along with the translocation of organic solutes (sugars).

Answer:

Soil As Reservoir Of Essential Elements

Soil is the primary natural source of mineral nutrients for plants. It serves as a reservoir of the essential elements required for plant growth.

Composition of Soil:

Soil is a complex mixture containing:

• Mineral particles: Derived from the weathering of rocks. These particles provide many of the essential inorganic elements.
• Organic matter: Decomposing remains of plants and animals. This provides organic nutrients and improves soil structure.
• Water: Held in the pore spaces. Acts as the solvent for mineral ions.
• Air: Present in the pore spaces. Provides oxygen for root respiration and aerobic microorganisms.
• Living organisms: Bacteria, fungi, algae, protozoa, earthworms, insects, etc., which play crucial roles in nutrient cycling and soil structure.

Availability of Minerals in Soil:

• Most mineral elements in the soil are present as inorganic ions, dissolved in the soil water (soil solution). Plants absorb these ions from the soil solution.
• Minerals are derived from the breakdown (weathering) of rocks and the decomposition of organic matter.
• The availability of mineral ions to plants depends on several factors, including:
  • Soil pH: Affects the solubility and form of mineral ions.
  • Aeration: Affects microbial activity and the availability of certain ions (e.g., Iron, Manganese).
  • Water content: Minerals are absorbed from the soil solution.
  • Temperature: Affects root growth and microbial activity.
  • Presence of other ions: Can lead to competition or synergistic effects on uptake.
  • Microbial activity: Soil microbes are essential for nutrient cycling, converting complex organic forms into simple inorganic ions (mineralisation).

Role of Soil Microbes:

• Many soil bacteria and fungi help in making minerals available to plants through processes like decomposition and nitrogen fixation.
• Mycorrhizal associations between fungi and plant roots enhance the absorption of phosphorus and other minerals from the soil.

Soil health and fertility are critical for sustainable plant growth, as they directly impact the availability and uptake of essential mineral elements.

Metabolism Of Nitrogen

Nitrogen is an essential element required by plants in the largest amount. It is a major component of proteins, nucleic acids, vitamins, and hormones. Although nitrogen is abundant in the atmosphere ($78\%$ as $N_2$ gas), plants cannot directly utilise atmospheric nitrogen. They absorb nitrogen primarily from the soil in the form of inorganic ions ($NO_3^-$, $NO_2^-$, $NH_4^+$). The process by which atmospheric nitrogen is converted into usable forms is called Nitrogen Fixation.

Nitrogen Cycle

Nitrogen moves through the atmosphere, soil, and living organisms in a cyclical manner known as the Nitrogen Cycle. The main steps of the nitrogen cycle are:

1. Nitrogen Fixation: Conversion of atmospheric nitrogen gas ($N_2$) into ammonia ($NH_3$). This can be done by:
   • Biological Nitrogen Fixation: Carried out by certain bacteria and cyanobacteria.
   • Atmospheric Nitrogen Fixation: Lightning provides energy to convert $N_2$ into nitrogen oxides.
   • Industrial Nitrogen Fixation: High temperature and pressure are used to synthesise ammonia (Haber process).
2. Nitrification: Conversion of ammonia ($NH_3$ or $NH_4^+$ ions in soil) into nitrite ($NO_2^-$) and then into nitrate ($NO_3^-$). This process is carried out by nitrifying bacteria in the soil:
   • Ammonia is oxidised to nitrite by Nitrosomonas or Nitrococcus. ($2NH_3 + 3O_2 \rightarrow 2NO_2^- + 2H^+ + 2H_2O$)
   • Nitrite is oxidised to nitrate by Nitrobacter. ($2NO_2^- + O_2 \rightarrow 2NO_3^-$)

   Nitrate is the form of nitrogen primarily absorbed by plants.

3. Assimilation: Absorption of inorganic nitrogen ions (nitrate, nitrite, or ammonium) from the soil by plants and their incorporation into organic molecules (amino acids, proteins, etc.).
4. Ammonification: Decomposition of dead organic matter (plants and animals) and excretory products by bacteria and fungi in the soil, releasing ammonia. ($Organic \: Nitrogen \rightarrow Ammonia$).
5. Denitrification: Conversion of nitrate ($NO_3^-$) back into nitrogen gas ($N_2$) by certain bacteria (e.g., Pseudomonas, Thiobacillus) under anaerobic conditions in the soil. This returns nitrogen to the atmosphere, completing the cycle. ($NO_3^- \rightarrow NO_2^- \rightarrow N_2O \rightarrow N_2$).

*(Image shows a diagram illustrating the nitrogen cycle, showing atmospheric N2, biological/industrial fixation, absorption by plants, consumption by animals, decomposition, nitrification, and denitrification)*

Biological Nitrogen Fixation

Conversion of atmospheric nitrogen ($N_2$) into ammonia ($NH_3$) by living organisms (microbes) is called biological nitrogen fixation. This is carried out by bacteria containing the enzyme nitrogenase.

The Enzyme Nitrogenase:

• Nitrogenase is a complex enzyme containing Molybdenum (Mo) and Iron (Fe).
• It is a highly sensitive enzyme that is inactivated by oxygen.
• The reaction catalysed by nitrogenase is:

  $ N_2 + 8e^- + 8H^+ + 16 ATP \rightarrow 2NH_3 + H_2 + 16 ADP + 16 Pi $

  This reaction requires a significant amount of energy (ATP).

Types of Biological Nitrogen Fixation:

• Non-symbiotic (Free-living) Nitrogen Fixation: Carried out by free-living bacteria in the soil.
  • Aerobic: Azotobacter, Beijerinckia.
  • Anaerobic: Rhodospirillum.
  • Cyanobacteria (blue-green algae): Nostoc, Anabaena (in heterocysts).
• Symbiotic Nitrogen Fixation: Carried out by bacteria living in symbiotic association with plants.
  • Symbiotic bacteria with legumes (peas, beans, grams, lentils, groundnut, soybean): Rhizobium bacteria in root nodules.
  • Symbiotic bacteria with non-legumes (e.g., Alnus): Frankia (a filamentous bacterium).

Symbiotic Nitrogen Fixation in Legumes (Root Nodule Formation):

The association between Rhizobium bacteria and legume roots leads to the formation of root nodules, where nitrogen fixation occurs. The process involves several steps:

1. Rhizobium bacteria are attracted to the root hairs of legumes.
2. Bacteria proliferate near root hairs and infect them, causing root hair curling.
3. An infection thread is formed, which carries the bacteria into the cortex of the root.
4. Bacteria are released into the cortical cells, where they induce cell division and differentiation, leading to the formation of a root nodule.
5. Within the nodule cells, bacteria differentiate into nitrogen-fixing forms called bacteroids.
6. The nodule establishes a vascular connection with the root for nutrient exchange.

Inside the nodule, the bacteroids contain the nitrogenase enzyme. To protect nitrogenase from oxygen (as it is oxygen-sensitive), the nodule contains an oxygen scavenger protein called leg-haemoglobin. This protein is pink/red in colour (similar to haemoglobin in blood) and is produced by the combined effort of the plant cell and the bacterium.

Leg-haemoglobin maintains a low oxygen concentration around the nitrogenase, allowing it to function and convert $N_2$ into ammonia. The ammonia produced is then assimilated by the plant into amino acids and other nitrogenous compounds.

*(Image shows a legume plant with root nodules, and a cross-section diagram of a nodule showing infected cells with bacteroids, vascular tissue, and indicating the presence of leg-haemoglobin)*

Biological nitrogen fixation is a vital process that contributes significantly to the availability of usable nitrogen in the biosphere, supporting global plant productivity.