Question

Who gave the criteria for essentiality
(a) R. Hill
(b) F. F. Blackman
(c) Thimann
(d) D. I. Arnon

Answer 1

According to the criteria of essentiality, an essential element is one which is indispensable for the completion of the life cycle in a deficiency of which plants may not set seeds, the element cannot be substituted by any other element and the element must be directly involved in the plant metabolism.

Complete answer:
The criteria of essentiality were stated by Arnon and Stout. The essentiality of an element for plant life is determined not by considering its biological role alone. Some of the criteria adopted to determine the essentiality of an element are;

1. The elements are inevitable or indispensable for normal growth, development, and reproduction, and in its absence, the plant is unable to complete its normal life cycle.
2. The biological role of the element must be specific and not replaceable by other elements.
3. The elements must have a direct nutritional role.
4. The elements must be able to serve either a structural component or as a functional constituent; more specifically it must be an integral part of an essential plant constituent or a metabolite. The essentiality of Mg can be taken as an example. Mg is an essential constituent of chlorophyll. So, in its absence no chlorophyll would be synthesized, no photosynthesis would take place, and no organic matter would be produced.

Additional Information:
- Plants absorb essential elements from the soil, water, and atmosphere. Of all essential elements, carbon, Hydrogen, and oxygen are absorbed from soil and atmosphere. So, they are called non- mineral elements.
- Most of the others are released to the soil by weathering of rocks and are absorbed mainly as inorganic ions. So, they are called mineral elements.
- Nitrogen is available from both mineral and non- mineral sources. So, plants absorb it from atmospheric air and soil water.
So, the correct answer is 'D. I. Arnon'.

Note:
- The essentiality of elements can be determined by water culture experiments or hydroponics.
- Many non-essential elements are useful to plants in different ways. Some of them promote growth and some others functionally supplement some essential elements.
- The useful elements which promote growth include sodium, cobalt, silicon, aluminum, etc.