Question

Which of the following is not a criterion for essentiality of an element?
A. Requirement of the element is specific
B. Necessary for normal growth and reproduction
C. Not replaceable by another element
D. Indirectly involved in plant metabolism

Answer 1

Hint: The elements like hydrogen, carbon, oxygen, phosphorus, nitrogen, sulphur, potassium, magnesium, and calcium are directly involved in plant metabolism.

Complete answer:
- Any of the minerals found in the soil can reach the plant via the roots. In fact, more than 60 of the 105 elements discovered so far have been found in different plants. Some plant species are collecting selenium, several are gold, and certain plants developing near nuclear test sites are picking up radioactive strontium. There are techniques that can detect minerals even at a very low concentration.
- Only a few elements have been found to be completely necessary for plant growth and metabolism on the basis of the essentiality requirements of the material. These components are further classified into two large groups on the basis of their quantitative criteria.

There are: macronutrients and micronutrients.
(i) In general, macronutrients are found in vast quantities in plant tissues. Macronutrients include hydrogen, carbon, oxygen, phosphorus, nitrogen, sulphur, potassium, magnesium, and calcium. Of these, carbon, hydrogen and oxygen are primarily derived from carbon dioxide and water, while the others are consumed from the soil as mineral nutrients.

(ii) Micronutrients or trace elements are required in very small quantities. These contain iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel. In addition to the 17 basic elements, there are other useful elements such as sodium, silicon, cobalt and selenium.

The criteria for the essentiality of an element are given below:
(1) The element must be strictly required to sustain natural growth and reproduction. In the absence of an element, the plants do not finish the life cycle or set the seeds.
(2) The necessity for an element must be particular and not replaceable by another element. In other words, the lack of any single element cannot be met by the availability of any other element.
(3) The element must be directly engaged in the metabolism of the plant.

Thus, the correct answer is option D. i.e., Indirectly involved in plant metabolism.

Note: Important elements serve a variety of functions. They engage in various metabolic processes in plant cells, like cell membrane permeability, buffering, cell sap osmotic concentration management, electron transport systems, enzymatic activity and function as main constituents of co-enzymes and macromolecules.