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.
- 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.
- 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.
- Absorbed as phosphate ions (H2PO4− or HPO42−).
- Constituent of cell membranes, certain proteins, nucleic acids, and nucleotides.
- Required for all phosphorylation reactions.
- 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.
- 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.
- 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.
- Obtained as sulfate (SO42−).
- Present in amino acids cysteine and methionine.
- Constituent of coenzymes, vitamins (thiamine, biotin, Coenzyme A), and ferredoxin.
- 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.
- 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.
- Obtained as Zn2+ ions.
- Activates various enzymes, especially carboxylases.
- Needed in the synthesis of auxin.
- Absorbed as cupric ions (Cu2+).
- Essential for overall plant metabolism.
- Associated with enzymes in redox reactions, reversibly oxidized from Cu+ to Cu2+.
- Absorbed as BO33− or B4O72−.
- Required for Ca2+ uptake and utilization, membrane functioning, pollen germination, cell elongation, cell differentiation, and carbohydrate translocation.
- Obtained as molybdate ions (MoO22+).
- Component of enzymes like nitrogenase and nitrate reductase, which participate in nitrogen metabolism.
- 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 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.
- 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.