An element is essential to plants if it is necessary for supporting its normal growth and reproduction. The requirement of this element must be specific and is not replaceable by any another element in the soil. They must be directly involved in the metabolism of the plant.
Criteria for Essentiality
An element can not be considered as essential merely on the basis of its presence in the plant. It is considered essential on the basis of the following criteria
(i) The plant is unable to grow normally and complete its life cycle in the absence of the element.
(ii) The element is specific and can not be replaced by another element.
(iii) The element plays a direct role in the metabolism of the plants.
The essential elements are further classified into two categories
(a) Macroelements These are the elements required by plants in larger quantities. These are $\mathrm{C}, \mathrm{H}, \mathrm{O}, \mathrm{N}, \mathrm{P}, \mathrm{K}, \mathrm{Mg}, \mathrm{Ca}$ and S .
(b) Microelements (Trace elements) These are required by plants in low quantities (often less than 1 ppm). These include B, $\mathrm{Zn}, \mathrm{Mn}, \mathrm{Cu}, \mathrm{Mo}, \mathrm{Cl}, \mathrm{Fe}$ and Ni.
Essential elements are involved in performing variety of functions in plants. Some of the major functions are enlisted below
(i) Frame work elements Essential elements as components of biomolecules and hence are structural elements of the cell. Carbon, hydrogen and oxygen are considered as framework elements because they constitute carbohydrates which form cell wall.
(ii) Protoplasmic elements $\mathrm{N}, \mathrm{P}$ and S are considered as protoplasmic elements as they form protoplasm along with $\mathrm{C}, \mathrm{H}$ and oxygen.
(iii) Catalytic enzyme Essential elements that activates or inhibit enzymes, i.e., without the presence of these elements some enzymes can not function e.g., $\mathrm{Mg}^{2+}$ acts as an activator for both ribulose biphosphate carboxylase oxygenase (Rubisco) and phosphoenol pyruvate carboxylase (PEP carboxylase). Both are the critical enzymes involved in photosynthetic carbon fixation in plants.
(iv) Balancing elements Elements counteract the toxic effect of other minerals by causing ionic balance (e.g., calcium, magnesium and potassium).
(v) Influencing on the osmotic pressure of the cell Some essential elements alters the osmotic potential of the cell. Plant cells contain dissolved mineral elements in the cell sap influencing osmotic pressure of the cell, e.g., K is involved in opening and closing of stomata.
Plants can tolerate a specific amount of micronutrient. A slight lesser amount of it can cause deficiency symptom and a slight higher amount can cause toxicity. The mineral ion concentration which reduces the dry weight of a tissue by $10 \%$ is called toxic concentration.
This concentration is different for different micronutrients as well as for different plant, e.g., $\mathrm{Mn}^{2+}$ is toxic beyond $600{ }^{\circ} \mathrm{gg}^{-1}$ for soyabean and beyond $5300 \mathrm{ogg}^{-1}$ for sunflower.
It is very difficult to identify the toxicity symptoms of mineral ion. It is because excess uptake of one element can reduces the uptake of other element at a time.
e.g., manganese $\left(\mathrm{Mn}^{2+}\right)$ becomes toxic when absorbed by plants in higher amounts. The toxicity is expressed in form of brown spots surrounded by chlorotic vein. It is due to the following
(i) Reduction in uptake of $\mathrm{Fe}^{3+}$ and $\mathrm{Mg}^{2+}$.
(ii) Inhibition of binding of $\mathrm{Mg}^{2+}$ to specific enzymes.
(iii) Inhibition of $\mathrm{Ca}^{2+}$ translocation in shoot apex.
Thus, excess of $\mathrm{Mn}^{2+}$ causes deficiency of iron, magnesium and calcium.
Formation of Root Nodule The coordinated activities of the legume and the Rhizobium bacteria depend on the chemical interaction between the symbiotic partners.
The principle stages in the nodule formation are summerised in the following diagram
Leg haemoglobin is an oxygen scavenger, it protects nitrogenase enzyme from $\mathrm{O}_2$ and also creates anaerobic conditions for the reduction of $\mathrm{N}_2$ to $\mathrm{NH}_3$ by Rhizobium.
Formation of root nodule in pulse plant is the result of infection of roots by Rhizobium. The following figure shows the process of nodule formation
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(a) | Rhizobiun divide near the root hair |
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(b) | Successful infection of the root hair causes it to curl |
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(c) | Infected thread carries the bacteria to enter the cortex. Bacteria cause cortical and pericycle cells to divide, lead to nodule formation. |
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(d) | Mature nodule with vascular tissues continuous with those of the roots. |
The chemical reaction is as follows
$$\mathrm{N}_2+8 \mathrm{e}^{-}+8 \mathrm{H}^{+}+16 \mathrm{ATP} \longrightarrow 2 \mathrm{NH}_3+\mathrm{H}_2+16 \mathrm{ADP}+\mathrm{P}_1 \mathrm{i}$$
The reaction takes place in presence of enzyme nitrogenase which acts in anaerobic conditions created by leghaemoglobin.
Fate of Ammonia
There are two ways by which ammonia is further used
(a) Reductive Amination
$$\alpha \text {-ketoglutaric acid }+\mathrm{NH}_4^{+}+\mathrm{NADPH} \xrightarrow[\text { Dehydrogenase }]{\text { Glutamate }} \text { glutamate }+\mathrm{H}_2 \mathrm{O}+\mathrm{NADP}$$
Ammonia reacts with $\alpha$-ketoglutaric acid to form glutamate.
(b) Transamination
In this process, transfer of $\mathrm{NH}_2$ group take place from one amino acid to other amino acid; enzyme transaminase catalyses this reaction.