Mineral Nutrition (Legacy/Detailed)
A comprehensive guide to essential mineral elements, their roles, deficiency symptoms, and the mechanism of absorption in plants.
Mineral Nutrition in Plants
Plants, like all living organisms, require various inorganic and organic substances for their growth and development. Mineral nutrition focuses on the inorganic elements that plants absorb from the soil.
Essential Mineral Elements
A total of about 105 elements have been discovered so far, and more than 60 are found in different plants. However, only a few are considered essential.
Criteria for Essentiality
- The element must be absolutely necessary for supporting normal growth and reproduction.
- The requirement of the element must be specific and not replaceable by another element.
- The element must be directly involved in the metabolism of the plant.
Classification of Essential Elements
Essential elements are divided into two broad categories based on their quantitative requirements:
1. Macronutrients
Elements required in large amounts (excess of 10 mmole of dry matter).
- Examples: Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulfur (S).
- C, H, O: These are mainly obtained from and .
2. Micronutrients (Trace Elements)
Elements needed in very small amounts (less than 10 mmole of dry matter).
- Examples: Iron (Fe), Manganese (Mn), Copper (Cu), Molybdenum (Mo), Zinc (Zn), Boron (B), Chlorine (Cl), Nickel (Ni).
Roles of Macro and Micronutrients
| Element | Key Functions | Deficiency Symptoms |
|---|---|---|
| Nitrogen (N) | Constituent of proteins, nucleic acids, vitamins, and hormones. | Chlorosis (yellowing of leaves), stunted growth. |
| Phosphorus (P) | Component of ATP, cell membranes, and certain proteins. | Purple spots on leaves, poor root growth. |
| Magnesium (Mg) | Core of the chlorophyll molecule; activates enzymes for respiration. | Interveinal chlorosis. |
| Iron (Fe) | Essential for chlorophyll synthesis; part of cytochromes. | Chlorosis in young leaves. |
Deficiency Symptoms
When the supply of an essential element becomes limited, plant growth is retarded. The concentration of an essential element below which plant growth is retarded is termed as critical concentration.
Key Symptoms
- Chlorosis: Loss of chlorophyll leading to yellowing in leaves ().
- Necrosis: Death of tissue, particularly leaf tissue ().
- Stunted Growth: Inhibition of cell division ().
Mechanism of Absorption of Elements
The process of absorption can be demarcated into two main phases:
First Phase (Apoplast)
An initial rapid uptake of ions into the 'free space' or 'outer space' of cells. It is a passive process (occurs through ion channels).
Second Phase (Symplast)
Ions are taken in slowly into the 'inner space' or 'cytoplasm' of the cells. It is an active process, requiring expenditure of metabolic energy (ATP).
Nitrogen Metabolism
Nitrogen is the most prevalent element in living organisms. Plants compete with microbes for the limited nitrogen available in the soil.
Nitrogen Fixation
The process of conversion of atmospheric nitrogen () to ammonia ().
- Biological Nitrogen Fixation: Carried out by prokaryotes using the enzyme Nitrogenase.
- Symbiotic: Rhizobium in the root nodules of legumes.
- Free-living: Azotobacter, Anabaena.
The Nitrogenase Enzyme
Nitrogenase is highly sensitive to molecular oxygen. Root nodules contain leghaemoglobin (an oxygen scavenger) to protect the enzyme and maintain anaerobic conditions.
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