The criteria for essentiality of an element are given below:
(a) The element must be absolutely necessary for supporting normal growth and reproduction. In the absence of the element the plants do not complete their life cycle or set the seeds.
(b) The requirement of the element must be specific and not replaceable by another element. In other words, deficiency of any one element cannot be met by supplying some other element.
(c) The element must be directly involved in the metabolism of the plant.
Based upon the above criteria only a few elements have been found to be absolutely essential for plant growth and metabolism.
These elements are further divided into two broad categories based on their quantitative requirements.
- Macronutrients, and
- Micronutrients Macronutrients are generally present in plant tissues in large amounts (in excess of 10 mmole Kg –1 of dry matter).
The macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium and magnesium. Of these, carbon, hydrogen and oxygen are mainly obtained from CO2 and H2O, while the others are absorbed from the soil as mineral nutrition. Micronutrients or trace elements, are needed in very small amounts (less than 10 mmole Kg –1 of dry matter).
These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel. In addition to the 17 essential elements named above, there are some beneficial elements such as sodium, silicon, cobalt and selenium.
They are required by higher plants. Essential elements can also be grouped into four broad categories on the basis of their diverse functions.
These categories are:
- Essential elements as components of biomolecules and hence structural elements of cells (e.g., carbon, hydrogen, oxygen and nitrogen).
- Essential elements that are components of energy-related chemical compounds in plants (e.g., magnesium in chlorophyll and phosphorous in ATP).
- Essential elements that activate or inhibit enzymes, for example Mg2+ is an activator for both ribulose bisphosphate carboxylaseoxygenase and phosphoenol pyruvate carboxylase, both of which are critical enzymes in photosynthetic carbon fixation; Zn2+ is an activator of alcohol dehydrogenase and Mo of nitrogenase during nitrogen metabolism. Can you name a few more elements that fall in this category? For this, you will need to recollect some of the biochemical pathways you have studied earlier.
- Some essential elements can alter the osmotic potential of a cell. Potassium plays an important role in the opening and closing of stomata. You may recall the role of minerals as solutes in determining the water potential of a cell.