Zinc
Explanation
Essential elements for plant growth are classified into two categories based on the quantitative requirement:
- Macronutrients: Required in relatively large amounts (in excess of 10 mmole kg-1 of dry matter). These include Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, and Sulphur.
- Micronutrients (Trace Elements): Required in very small amounts (less than 10 mmole kg-1 of dry matter). These include Iron, Manganese, Copper, Molybdenum, Zinc, Boron, Chlorine, and Nickel.
Option Analysis:
- Calcium: Incorrect. Calcium is a macronutrient essential for the formation of the middle lamella in cell walls and for mitotic spindle formation.
- Magnesium: Incorrect. Magnesium is a macronutrient that serves as the central atom in the chlorophyll molecule and activates enzymes in respiration and photosynthesis.
- Zinc: Correct. Zinc is a micronutrient. It activates various enzymes, such as carboxylases, and is essential for the synthesis of the plant hormone auxin (Indole Acetic Acid).
- Sulphur: Incorrect. Sulphur is a macronutrient present in two amino acids (cysteine and methionine) and is a constituent of several coenzymes and vitamins.
Key Takeaway:
Zinc is a micronutrient, whereas Calcium, Magnesium, and Sulphur are macronutrients. The classification depends on the quantity of the element required by the plant for normal growth and reproduction.