The essential elements are of two main types based on the fact that in how much quantity they are needed within the plants. These types are macroelements and microelements.
Macroelements are those elements which are required in higher quantities. Their concentration in the plant tissues is about 1 mg per gram of dry matter. They are nine in number and include carbon, hydrogen, oxygen, nitrogen, phosphorous, potassium, magnesium, sulfur and calcium.
Microelements are those essential elements which are needed in very fewer amounts i.e. less than 1 mg per gram of dry matter. They are also known as trace elements. They are seven in number i.e. iron, copper, zinc, boron, manganese, molybdenum and chlorine.
Sources of Essential Elements in Plants:
The plants obtain all the required essential elements from their environment. Example- they take carbon from the air in the form of CO2,hydrogen is mainly taken from water and oxygen comes from either water or air. Such essential elements which are taken either from water or air are known as non-mineral elements. Example- C, H and O.
Air also contains a lot of Nitrogen, but this free nitrogen is inactive and is not taken up by the plants. Plants take up the fixed nitrogen. The nitrogen is fixed during lightening and the rainfall brings the fixed nitrogen into the soil. Some special microorganisms are known as nitrogen fixers also fix the nitrogen example- rhizobium. They convert the atmospheric free nitrogen either into anionic forms like NO3– (nitrate) or NO2– (nitrite) or into cationic form like ammonium (NH4+). These fixed forms of nitrogen enter into plants through roots and then become the part of organic compounds within the plant body. From the plants, this organic nitrogen is then passed to heterotrophs.
In addition to C, H, O and N all other types of elements are present within the soil where they come from the disintegration and weathering of rocks. All such elements are known as mineral elements. Nitrogen is both mineral as well as the non-mineral element.
General Functions of Mineral Elements in Plants:
The various inorganic elements do a number of functions within the plant body. Some of them are as follows-
The inorganic elements help in the synthesis of various chemicals within the plant body example- nitrogen is required in the synthesis of the chlorophyll molecule, auxin, nucleic acids, proteins and amino acids etc. Calcium is required in the formation of calcium pectate of the middle lamella. Sulfur is needed in the synthesis of some amino acids like methionine and Cysteine and some proteins. Phosphorous is a component of ATP, NADP, DNA and RNA.
Some of these elements help in the activation of enzymes during the biochemical reactions. Example- Mg acts as a cofactor. Mn, Cl, Na and K also work as cofactors. Of all these cofactors‘K‘ is most effective and about 40 different enzymes depend on ‘K‘.
Many of these elements control the membrane permeability of the cell. Example- ‘Na‘ increases the membrane permeability and ‘Ca‘ decreases it.
The osmotic potential of the plant cells depends upon the concentration of mineral salts.
The minerals absorbed from the soil affect the pH of cell sap.
Several mineral elements like copper, iron, zinc, manganese, magnesium and cobalt etc. serve as catalysts.
Calcium, potassium and magnesium neutralize the toxic effects of other minerals by retaining the ionic balance.
Symptoms of Mineral Deficiency in Plants:
Whenever a plant is short of an essential element, it shows the signs of ill health. These signs can be seen in the morphology of the plant in the form of some symptoms known as deficiency symptoms. These symptoms are also known as hunger signs as the plants show poor health. The type of deficiency symptoms is different with the change in the type of deficient element. The deficiency symptoms are seen in the form of morphological abnormalities or deformities.
Some plants show very quick and sharp responses to the deficiency of minerals in soils, by showing very clear deficiency symptoms. Such plants can be used as indicator crops. Some common, most important deficiency symptoms are as follows-
Stunted Growth- The stems remain short.
Chlorosis– It is the loss or non-development of chlorophyll resulting in the yellowing of the leaves.
Necrosis- It is the localised death of the tissues of leaves.
Mottling- Appearance of patches of green and non-green areas on the leaves.
Leaf Curles- Abnormal curling of leaves due to unequal growth.
Abscission- Premature fall of flowers, fruits and leaves.
Wilting- It is the loss of turgor in the cells resulting in the drooping of leaves, young stems and tips.
Dieback of shoots.
Poor reproductive development- The flowers and other reproductive structures show poor development.