Question

Which is not a microelement?
A. Cu
B. Mn
C. Bo
D. Ca

Answer 1

Micronutrients are those nutrients that are required in a very small amount. They are needed for several functions of the plants. To get to the answer we need to know about the requirements of these elements.

Complete answer:
Many elements are needed by our body for many metabolic processes and for plant growth. They are broadly categorised into micronutrients and macronutrients.
Macronutrients are those that are generally present in plant tissues in large amounts (in excess of 10 mmol kg of dry matter) and micronutrients are trace elements that are needed in very small amounts (less than 10 mmol kg of dry matter). These elements participate in various metabolic processes in plant cells such as permeability of cell membrane, maintenance of osmotic concentration, electron - transport systems, buffering action, enzymatic activity, and act as major constituents of macromolecules and co - enzymes.
Let us first infer about their various forms and function-
1. Copper- It is absorbed as cupric ion and is essential for metabolism in plants. like iron, it is associated with certain enzymes involved in redox reaction and is reversibly oxidised from $Cu^+$ to $Cu^{2^+}$.
2. Manganese- It is absorbed in the form of manganese ion that activates many enzymes involved in photosynthesis, respiration and nitrogen metabolism. It is most importantly used in splitting of water to liberate oxygen during photosynthesis
3. Boron- It is absorbed as BO3-3 OR B4O2-7. It is required for uptake and utilisation of calcium ion, membrane functioning, pollen germination, cell elongation, cell differentiation and carbohydrates translocation.
4. Calcium- Plants absorb calcium from the soil in the form of calcium ions. They are required for meristematic and differentiating tissues. During cell division it is used in the synthesis of cell walls, particularly of calcium pectate in the middle lamella. It activates certain enzymes and plays an important role in regulating metabolic activities.

So, the correct answer is “Option D”.

Note:
The requirement of micronutrients is always in low amounts while their moderate decrease causes deficiency symptoms and moderate increase in them cause toxicity. Any mineral ion concentration in tissues that reduces the dry weight of tissues by 10 per cent is considered toxic. Such critical concentration varies widely among different micronutrients.