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

	(a)	Rhizobiun divide near the root hair
	(b)	Successful infection of the root hair causes it to curl
	(c)	Infected thread carries the bacteria to enter the cortex. Bacteria cause cortical and pericycle cells to divide, lead to nodule formation.
	(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.

Although, hydrophonics is a successful technique for plants still many crops are grown on land because

(i) The cost is the major concern. The setting and handling of hydrophonics requires much more investment than that of the soil based production.

(ii) Sanitization is extremely important especially with indoor hydroponic environments. Water borne disease can spread quickly through some methods of hydroponic production.

(iii) Hydroponics is relatively a new technique and not used by the traditional farmers due to lack of knowledge.

(iv) Plants are less adaptable to the surrounding atmosphere. Hot weather and narrow oxygenation may minimise the production and quality of plant produce/yield.