Chapter 3: Nutrition in Plants

3.1 Main Inorganic Mineral Nutrients

Plants require inorganic elements absorbed from the soil solution to sustain growth, metabolic functions, and structural integrity.

1. Classification of Nutrients

• Macronutrients: Mineral elements required in large quantities (greater than 10 mg/g of dry matter). Includes Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), and Sulfur (S).
• Micronutrients: Mineral elements required in small trace amounts (less than 0.1 mg/g of dry matter). Includes Chlorine (Cl), Iron (Fe), Manganese (Mn), Boron (B), Zinc (Zn), Copper (Cu), Nickel (Ni), and Molybdenum (Mo).

2. Functions and Deficiency Symptoms of Key Macronutrients

• Nitrogen (N):
  • Functions: Synthesis of proteins, nucleic acids (DNA/RNA), enzymes, and chlorophyll molecules.
  • Deficiency: Chlorosis (yellowing of leaves, particularly older leaves), stunted growth, premature leaf fall.
• Phosphorus (P):
  • Functions: Synthesis of nucleic acids, ATP, and cell membrane phospholipids; promotes root development and flowering.
  • Deficiency: Poor root growth, dull green leaves with red or purple spots, delayed maturation.
• Potassium (K):
  • Functions: Regulates stomatal opening and closing (via $K^+$ ion transport), protein synthesis, carbohydrate metabolism, and osmotic pressure regulation.
  • Deficiency: Premature death of leaves, yellow margins/browning (leaf scorch) with curled tips.
• Calcium (Ca):
  • Functions: Component of the middle lamella (calcium pectate) holding cell walls together; regulates cell elongation and spindle fiber formation.
  • Deficiency: Stunted growth of young leaves and apical shoot tips; distorted growth and cell necrosis.
• Magnesium (Mg):
  • Functions: Central structural atom of the chlorophyll molecule; activates enzymes involved in carbohydrate metabolism.
  • Deficiency: Interveinal chlorosis (yellowing between leaf veins while veins remain green), starting on older leaves.
• Sulfur (S):
  • Functions: Constituent of key amino acids (methionine and cysteine) and coenzymes.
  • Deficiency: General chlorosis of whole leaves, starting primarily in younger leaves.

3.2 Organ for Water and Mineral Salt Absorption

1. Root Adaptation for Absorption

• Root Cap: Protects the delicate apical meristem from abrasion as the root penetrates soil particles.
• Root Hair Cells: Long, narrow tubular outgrowths of epidermal cells located in the differentiation zone.
  • Large surface area-to-volume ratio speeds up water and mineral absorption.
  • Large central vacuole maintains low water potential to promote continuous osmosis.
  • Lacks a thick waxy cuticle layer, reducing barrier resistance to fluid entry.

2. Mechanism of Water and Mineral Intake

• Water Uptake: Soil solution is hypotonic relative to the root hair cell sap. Water moves into root hairs down a water potential gradient by osmosis.
• Mineral Ion Uptake:
  • When soil concentration is high: Mineral ions diffuse into root cells via facilitated diffusion.
  • When soil concentration is low: Mineral ions are absorbed against their concentration gradient via active transport, requiring ATP and carrier proteins.

3.3 Diversity in Plant Nutrition

While most plants are autotrophs performing photosynthesis, specialized ecological adaptations exist for acquiring nutrients under challenging environmental conditions.

1. Carnivorous Plants

• Examples: Pitcher plant (Nepenthes sp.), Venus flytrap (Dionaea muscipula).
• Adaptations: Possess photosynthetic leaves modified into traps to capture and digest insects/small invertebrates using secreted digestive enzymes.
• Nutritional Role: Grow in nitrogen-deficient, acidic soils (e.g., bogs). Digestion of prey supplies essential nitrogen and phosphorus for survival.

2. Parasitic Plants

• Examples: Rafflesia (Rafflesia sp.), Dodder (Cuscuta sp.).
• Adaptations: Develop modified parasitic roots called haustoria that penetrate host stems/roots directly into vascular tissues.
• Nutritional Role: Tap into host xylem (for water/minerals) and phloem (for sucrose/organic nutrients), causing reduced vigor or death of the host plant. Parasitic plants like Dodder lack chlorophyll entirely.

3. Epiphytic Plants

• Examples: Bird's nest fern (Asplenium nidus), Staghorn fern (Platycerium sp.), Wild orchids.
• Adaptations: Grow attached to high tree branches of host trees without taking nutrients from them. Possess spongy aerial roots adapted to absorb rain moisture and humidity quickly.
• Nutritional Role: Non-parasitic; use host trees solely for structural support to gain better access to sunlight for photosynthesis. Obtain minerals from falling organic debris/decaying leaf litter.