Nitrogenase is the most crucial enzyme found in the root nodules for $\mathrm{N}_2$-fixation. It is a Mo- Fe protein that catalyses the conversion of atomospheric nitrogen to ammonia. Pink coloured pigment called leghaemoglobin creates anaerobic conditions for the functioning of nitrogenase enzyme.

Critical Concentration	Deficient
A concentration of a nutrient measured in tissue, just below the level that gives maximum growth is defined as critical concentration. e.g., N, P and K are known as critical elements.	The concentration that is below the critical concentration and the deficiency of nutrient element can lead to a gradual fall in the plant growth is called deficient.

	Critical concentration	Deficient
Zn	0.5-1%	less than 0.5%
Fe	3.5-5%	less than 3.5%

Carnivorous (insectivorous) plants are mainly found in nitrogen deficient soil. To make up the nitrogen deficiency, they have developed insect trapping mechanism in which leaves have taken the shape of a pitcher containing insect digesting proteolytic enzymes. They trap insect and absorb the nitrogen derived from them.

Plant can tolerate a specific amount of micronutrients. A slight lesser amount of it can cause deficiency symptoms and a slight higher amount can cause toxicity. The mineral ion concentration which reduces the dry weight of the tissues by $10 \%$ is called toxic concentration.

This concentration is different for different micronutrients as well as for different plants, e.g., $\mathrm{Mn}^{2+}$ is toxic beyond $600 \mathrm{ogg}^{-1}$ for soyabean and beyond $5300 \mathrm{ogg}^{-1}$ for sunflower.

It has also been observed that the toxicity of one micronutrient causes the deficiency of other nutrients. To overcome such problems, farmers should use these nutrients in prescribed concentration so that the excess uptake of one element do not reduce the uptake of other element.

(a) The plants try to supply more nutrients to its younger leaves than the older leaves. When nutrients are mobile, the deficiency symptoms are shown by the older leaves first because that particular nutrient reaches the top first and lower leaves does not get that nutrient.

(b) Highly mobile elements are P, K and Mn. Less mobile elements are $\mathrm{Ca}^{+}$and $\mathrm{K}^{+}$.

(c) The aspect of mobility of essential elements is important in horticulture and agriculture in the following ways

(i) A crop in which older leaves are harvested if show deficiency symptoms, will decrease its economic value.

(ii) The crops in which flowers, fruits and inflorescence are harvested, the immobile nutrients will not reach to the apex/tip because of immobility, so this will reduce the yield.