Describe the symptoms shown by a plant that is deficient in nitrate ion (NO₃⁻) and magnesium ion (Mg²⁺), and explain why those symptoms occur.
Core focus (required for the IGCSE exam)
NO₃⁻ (nitrate) – main source of nitrogen; needed for amino‑acid, protein and chlorophyll synthesis; highly mobile, so deficiency first appears on older leaves.
Mg²⁺ (magnesium) – central atom of the chlorophyll molecule and co‑factor for many enzymes; also mobile, but older leaves are sacrificed first.
1. Quick overview of mineral nutrients (extension)
Mineral (ion)
Primary function(s)
Typical deficiency symptom(s)
Nitrogen (NO₃⁻, NH₄⁺)
Protein & chlorophyll synthesis; cell division
General chlorosis of older leaves, stunted growth
Phosphorus (H₂PO₄⁻)
ATP, nucleic acids, root development
Dark‑green/purplish leaves, poor root growth
Potassium (K⁺)
Stomatal regulation, enzyme activation
Marginal leaf necrosis, weak stems
Magnesium (Mg²⁺)
Core of chlorophyll; enzyme co‑factor
Interveinal chlorosis of older leaves, leaf‑margin necrosis
Calcium (Ca²⁺)
Cell‑wall stability, membrane integrity
Distorted young leaves, tip die‑back
Sulphur (SO₄²⁻)
Amino acids & vitamins
Uniform yellowing of young leaves
Iron (Fe²⁺/Fe³⁺)
Cytochromes, chlorophyll synthesis
Interveinal chlorosis of young leaves
Manganese (Mn²⁺)
O₂‑evolving complex of photosystem II
Yellow speckles, brown necrotic spots
Zinc (Zn²⁺)
Auxin synthesis, enzyme activation
Short internodes, rosetting
Copper (Cu²⁺)
Electron transport, lignin synthesis
Twisted new leaves, shoot‑tip die‑back
Boron (B³⁺)
Cell‑wall formation, pollen tube growth
Stunted growth, poor fruit set
2. Nitrate ion (NO₃⁻) deficiency
2.1 Role in the plant
Primary source of nitrogen for most crops.
Reduced in roots to ammonium, then incorporated into amino acids → proteins, nucleic acids and chlorophyll.
Supports cell division, leaf expansion and overall vegetative growth.
2.2 Uptake & transport (simplified)
Absorbed by active transport into root‑hair cells (energy‑dependent H⁺ pump creates the gradient).
Moved upward in the xylem as an anion; highly mobile because nitrogen can be re‑allocated from older to younger tissues.
2.3 Why the symptoms appear
When nitrate is scarce the plant breaks down proteins in older leaves to recycle nitrogen for new growth → loss of chlorophyll in those leaves.
Limited amino‑acid synthesis reduces production of enzymes and structural proteins, slowing cell division and elongation.
2.4 Deficiency symptoms
General chlorosis – uniform yellowing of older leaves while veins stay green.
Stunted growth – short internodes, thin weak stems, fewer leaves.
Pale, slow‑growing new shoots – because nitrogen is being diverted to them.
Reduced flowering and fruit set – insufficient protein for reproductive development.
Lower protein content in edible parts, affecting nutritional quality.
3. Magnesium ion (Mg²⁺) deficiency
3.1 Role in the plant
Central atom of the chlorophyll molecule – essential for light capture.
Co‑factor for >300 enzymes (e.g., ATP‑dependent reactions, Rubisco activation).
Involved in carbohydrate metabolism and nucleic‑acid synthesis.
3.2 Uptake & transport (simplified)
Taken up by active transport into root hairs (Mg²⁺/H⁺ antiporter).
Mobile within the plant, but redistribution from old to new tissue is slower than for nitrate.
3.3 Why the symptoms appear
Insufficient Mg²⁺ limits chlorophyll synthesis → reduced green pigment in mesophyll cells.
Mg‑dependent enzymes become less efficient, lowering photosynthetic output.
Because Mg²⁺ is mobile, the plant sacrifices older leaves first, producing a characteristic pattern.
3.4 Deficiency symptoms
Interveinal chlorosis – yellowing between veins while veins remain green, most obvious on older leaves.
Leaf‑margin necrosis – brown or dead tissue at leaf edges when the deficiency is severe.
Premature leaf drop of the affected older foliage.
Reduced growth rate due to lower photosynthetic efficiency.
Reddish‑purple coloration on leaf undersides (anthocyanin accumulation) in very severe cases.
4. Interactions and practical notes
Excess potassium (K⁺) can mask magnesium deficiency because K⁺ competes for the same uptake sites.
High soil pH reduces the availability of Mg²⁺ (and Fe), often leading to simultaneous deficiencies.
Fertiliser recommendation: a balanced N‑PK‑Mg formulation (e.g., 20‑20‑20 + MgSO₄) helps prevent the two most common deficiencies in many crops.
5. Comparison of nitrate and magnesium deficiency symptoms
Feature
Nitrate deficiency (NO₃⁻)
Magnesium deficiency (Mg²⁺)
First leaves affected
Older leaves (N is highly mobile)
Older leaves (Mg is mobile, but redistribution is slower)
Pattern of chlorosis
Uniform yellowing of the whole leaf
Interveinal yellowing; veins stay green
Additional visual signs
Thin stems, reduced leaf number, pale new growth, delayed flowering/fruiting
Leaf‑margin necrosis, possible reddish‑purple undersides, premature leaf drop
Underlying physiological cause
Insufficient N for amino‑acid & chlorophyll synthesis → protein breakdown in old tissue
Lack of Mg²⁺ for chlorophyll and Mg‑dependent enzymes → impaired photosynthesis
Suggested diagram: side‑by‑side illustration of a leaf showing (a) uniform chlorosis typical of nitrate deficiency and (b) interveinal chlorosis typical of magnesium deficiency.