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Which of the following is a micronutrient?
A) Mg
B) Zn/Cu
C) Ca
D) P

Answer
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Hint: Elements which are necessary only in small quantities are known as micronutrients.

Complete Answer:
Let us first understand what the term micronutrient means. In plants, nutrients are divided into two types on the basis of the quantity of the nutrient required by the plant. The two types are,
A) Macronutrient: The nutrients which are required in large quantities are known as macronutrients.
B) Micronutrient: The nutrients which are required in smaller quantities are known as micronutrients.

1) Option a: Mg or Magnesium is required for maintenance of plant health and growth. It is required in larger quantities, therefore it is an example of macronutrient. Therefore, this is the incorrect option.

2) Option b: Zn or Zinc is required by the plants in small compounds, thus it is an example of micronutrients.
Zinc plays a very important role in plant development. The various processes in which zinc is required are,
- It is present as a key constituent in proteins and enzymes.
- It has a role in growth hormone production in the plant.
- It helps internode elongation.
- It helps in withstanding cold temperatures in some plants.

Cu or copper is required by the plants in small compounds, thus it is an example of micronutrients. The role of copper are as follows,
- It is required for both seed and chlorophyll production.
- It is required for enzymatic activities associated with lignin synthesis.
- It is important for respiration of plants.
- It has a role in plant metabolism of proteins and carbohydrates.
- Deficiency of copper can lead to diseases like ergot.
Therefore, this is the correct option.

3) Option c: Ca or Calcium has a role in plant nutrition, cell deposition and plant growth. It is required in larger quantities, therefore it is an example of macronutrient. Therefore, this is the incorrect option.

4) Option d: P or Phosphorus plays an important role in plants as it is a component of ATP. It is required in larger quantities, therefore it is an example of macronutrient. Therefore, this is the incorrect option.

Thus, the correct option is b) Zn/Cu.

Note: There are 7 micronutrients required by the plants, they are- Boron, Chlorine, Copper, Iron, Manganese, Molybdenum, and Zinc.