Describe the symptoms in a plant with nitrate ion deficiency and magnesium ion deficiency.

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Topic 6.2 – Plant Nutrition: Mineral Requirements

Learning objective

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 divisionGeneral chlorosis of older leaves, stunted growth
Phosphorus (H₂PO₄⁻)ATP, nucleic acids, root developmentDark‑green/purplish leaves, poor root growth
Potassium (K⁺)Stomatal regulation, enzyme activationMarginal leaf necrosis, weak stems
Magnesium (Mg²⁺)Core of chlorophyll; enzyme co‑factorInterveinal chlorosis of older leaves, leaf‑margin necrosis
Calcium (Ca²⁺)Cell‑wall stability, membrane integrityDistorted young leaves, tip die‑back
Sulphur (SO₄²⁻)Amino acids & vitaminsUniform yellowing of young leaves
Iron (Fe²⁺/Fe³⁺)Cytochromes, chlorophyll synthesisInterveinal chlorosis of young leaves
Manganese (Mn²⁺)O₂‑evolving complex of photosystem IIYellow speckles, brown necrotic spots
Zinc (Zn²⁺)Auxin synthesis, enzyme activationShort internodes, rosetting
Copper (Cu²⁺)Electron transport, lignin synthesisTwisted new leaves, shoot‑tip die‑back
Boron (B³⁺)Cell‑wall formation, pollen tube growthStunted 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.

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