We find that Rhizobium forms nodules on the roots of leguminous plants. Also Frankia another microbe forms nitrogen fixing nodules on the roots of non-leguminous plant Alnus.
(a) Can we artificially induce the property of nitrogen-fixation in a plant, leguminous or non-leguminous?
(b) What kind of relationship is observed between mycorrhiza and pine trees?
(c) Is it necessary for a microbe to be in close association with a plant to provide mineral nutrition? Explain with the help of one example.
(a) Artificial induction in leguminous and non-leguminous plants have been tried by scientists. It's success rate is very low because gene expression is highly specific phenomenon.
Even if desired gene is introduced, it may not work because conditions for its expressions are very specific.
(b) Symbiotic mutualistic relationship (mutualism) is found between the pine roots and mycorrhiza as both are benefitted mutually.
(c) Yes, microbe has to be in close association, to develop a physical relationship for example Rhizobium gets into the root and involve root tissues, then only helps in nitrogen-fixation.
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.