Chapter 9 — Mineral Nutrition
Key Concepts Quick Recap
| Term | Definition |
|---|---|
| Essential element | Element absolutely necessary for plant growth and completing life cycle |
| Macronutrient | Essential element required in large amounts (>1g/kg dry weight): N, P, K, Ca, Mg, S |
| Micronutrient | Essential element required in trace amounts: Fe, Mn, Cu, Zn, B, Mo, Cl, Ni |
| Deficiency symptom | Visible effect of nutrient shortage (chlorosis, necrosis, stunting) |
| Nitrogen fixation | Conversion of atmospheric N₂ → NH₃ by microorganisms |
| Nitrification | NH₃ → NO₂⁻ → NO₃⁻ by bacteria |
| Denitrification | NO₃⁻ → N₂ by anaerobic bacteria |
| Hydroponics | Growing plants in nutrient solution without soil |
Essential Elements and Their Functions
| Element | Form absorbed | Function | Deficiency |
|---|---|---|---|
| Nitrogen (N) | NO₃⁻, NH₄⁺ | Proteins, nucleic acids, chlorophyll | Chlorosis (old leaves first); stunted growth |
| Phosphorus (P) | H₂PO₄⁻ | ATP, nucleic acids, cell membranes | Purple/red colouration (anthocyanin accumulation); poor root growth |
| Potassium (K) | K⁺ | Enzyme activation, stomatal movement, protein synthesis | Scorched leaf margins (necrosis); weak stems |
| Calcium (Ca) | Ca²⁺ | Cell wall (calcium pectate), cell membrane, enzyme | Tip burn; stunted roots; curling of young leaves |
| Magnesium (Mg) | Mg²⁺ | Chlorophyll (central atom), enzyme activation | Interveinal chlorosis (old leaves) |
| Sulphur (S) | SO₄²⁻ | Amino acids (cysteine, methionine), vitamins | Chlorosis of young leaves |
| Iron (Fe) | Fe²⁺, Fe³⁺ | Chlorophyll synthesis, cytochromes, electron transport | Interveinal chlorosis (young leaves) |
| Manganese (Mn) | Mn²⁺ | Photolysis of water (in PSII), enzyme activation | Interveinal chlorosis, brown spots |
| Zinc (Zn) | Zn²⁺ | Enzyme activation, IAA synthesis | Little leaf disease, interveinal chlorosis |
| Boron (B) | BO₃³⁻ | Cell wall synthesis, pollen germination, sugar transport | Death of shoot tips; poor fruit/seed set |
| Molybdenum (Mo) | MoO₄²⁻ | Nitrogenase (nitrogen fixation), nitrate reductase | Whiptail disease (marginal scorch, narrow leaves) |
I. Choose the Best Answer (MCQs)
1. The element that is a constituent of chlorophyll is: (a) Iron (b) Manganese (c) Magnesium (d) Calcium
2. Nitrogen fixation in leguminous plants is done by: (a) Azotobacter (b) Rhizobium (c) Nitrosomonas (d) Clostridium
3. The enzyme responsible for biological nitrogen fixation is: (a) Nitrate reductase (b) Nitrogenase (c) Urease (d) Nitrite reductase
- 16 ATP per N₂ molecule fixed.
- Anaerobic conditions (O₂ inactivates nitrogenase).
- In root nodules, leghaemoglobin (pink pigment) scavenges O₂ to maintain anaerobic conditions.
4. A plant shows purple coloration of leaves. This indicates deficiency of: (a) Nitrogen (b) Potassium (c) Phosphorus (d) Magnesium
5. Hydroponics is used to determine: (a) Light requirements of plants (b) Essential mineral requirements of plants (c) Water requirements (d) Temperature requirements
6. Which of the following is a carnivorous plant? (a) Mistletoe (b) Cuscuta (c) Nepenthes (d) Ficus
7. Nitrification is carried out by: (a) Rhizobium (b) Nitrosomonas and Nitrobacter (c) Azotobacter (d) Pseudomonas
These are chemoautotrophic bacteria that get energy from these oxidations.
II. Short Answer Questions
Common Mistakes
- Confusing chlorosis (yellowing) and necrosis (death/browning): N deficiency causes chlorosis; K deficiency causes necrosis of leaf margins.
- Saying nitrogen is fixed by nitrification — nitrification is NH₃ → NO₃⁻, not N₂ fixation.
- Forgetting Mo is needed for nitrogenase — Mo deficiency impairs nitrogen fixation.
- Confusing Rhizobium (symbiotic) with Azotobacter (free-living).
Exam Checklist
- Macronutrients: C, H, O, N, P, K, Ca, Mg, S (9 elements).
- Deficiency table: N → chlorosis old leaves; P → purple; K → necrosis margins; Mg → interveinal chlorosis.
- Nitrogen fixation enzymes: nitrogenase; requires Mo; anaerobic conditions; leghaemoglobin provides anaerobic environment.
- Nitrification: Nitrosomonas (NH₄⁺ → NO₂⁻) + Nitrobacter (NO₂⁻ → NO₃⁻).
- Nitrogen cycle: fixation → nitrification → assimilation → ammonification → denitrification.