Class 11-science NCERT Solutions Biology Chapter 12 - Mineral Nutrition

There are some elements like Zn, Mo, Bo etc. which are non-essential for plants. They take part in some metabolic reactions but are not essential for their survival. About 60 out of 105 discovered elements are absorbed by the roots of the plants, but only 17 elements are found essential for plant growth and development. 

Impure water contains a large number of impurities in the form of soluble minerals dissolved in it. Salts also contain impurities. If such impure water and mineral salts are used as solution culture for growing plants in hydroponics, then these impurities interfere with the experiments used for detection of essentiality of an element. Therefore, purified water and purified nutrient salts are used in studies involving mineral nutrition using hydroponics. 

1. Macronutrients: Macronutrients are the elements which are required in large amounts for plant growth. They are present in plant tissues in concentration of 1 to 10 mg/L of dry matter. The macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium and magnesium. Of these, carbon, hydrogen and oxygen are mainly obtained from CO₂ and H₂O, while the others are absorbed from the soil as mineral nutrition.
2. Micronutrients: Micronutrients or trace elements are the elements which are needed in very small amounts for plant growth (less than 0.1 mg/L of dry matter). These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.
3. Beneficial elements: In addition to the 17 essential elements, there are some beneficial elements such as sodium, silicon, cobalt and selenium. These are the elements required by higher plants in addition to macro and micronutrients.
4. Toxic elements: The requirement of micronutrients is always in low amounts; a moderate decrease causes deficiency symptoms and a moderate increase causes toxicity. In other words, there is a narrow range of concentration at which the elements are optimum. Any mineral ion concentration in tissues which reduces the dry weight of tissues by about 10% is called a toxic element.
5. Essential elements: An essential element is an element which is directly involved in the metabolism of the plant and causes a specific deficiency symptom if not supplied to the plant from an external medium.  

1. Chlorosis: It is the loss of chlorophyll leading to the yellowing of leaves. It is caused by the deficiency of N, K, Mg, S, Fe, Mn, Zn and Mo.
2. Necrosis: It is the killing of cells and tissues and is usually expressed in the form of leaf spots, blights and rots. These are caused due to the deficiency of Ca, Mg, Cu and K.
3. Inhibition of cell division: Inhibition of cell division is expressed in stunted growth. This is caused by lack or low levels of N, K, S and Mo.
4. Delay of flowering: Low concentration of N, S and Mo causes delay in flowering in certain plants.
5. Deformation: Deficiency of boron causes deformation, discolouration and disorganisation of meristematic tissue and finally death of a growing plant. 

The plants which show symptoms due to the deficiency of more than one nutrient are grown in different sets in water culture. Set 1 is given one of the several nutrients showing deficiency symptoms, set 2 is given another nutrient and set 3 is given yet another nutrient. In this way, the different sets are grown in hydroponics by supplementing one of the different nutrients showing deficiency. The real deficient mineral element, if supplemented in the medium, causes plants to grow under normal conditions reverting the deficiency symptom.

Mobility of an element determines whether the deficiency symptom appears in younger parts or in the older parts of the plant. If the elements are relatively immobile, then they cause deficiency symptoms in younger parts, whereas if the elements are relatively mobile, then they cause deficiency symptoms in older parts. For example, the deficiency of nitrogen, potassium and magnesium are visible first in senescent leaves and the deficiency of sulphur and calcium appear in young leaves.

Plants uptake essential elements from the soil through their roots and from the air through their leaves. Nutrient uptake in the soil is achieved by active absorption. It has two phases - initial phase and metabolic phase. The initial phase is a passive process wherein the ions absorbed from the outside pass in the outer and free space of the cells. It comprises intercellular spaces and cell walls. The metabolic phase is an active process wherein ions enter the cytoplasm and cell vacuole. Entry of ions into the cells is called influx.

The following conditions are necessary for fixation of atmospheric nitrogen by Rhizobium: 

 i. Reducing environment

 ii. Presence of enzyme nitrogenase

 iii. Source of energy as ATP

 iv. Source of reducing power, NAD(P) H₂ or FMNH₂

 v. Ferredoxin as electron donor

 vi. Keto acids for picking up the amino group

 vii. Reduced availability of nitrate in the substrate

Role of Rhizobium in N₂ fixation:

Rhizobium fixes nitrogen from the atmosphere into a plant usable form, ammonium, utilising the enzyme nitrogenase. In return, the plant supplies the bacteria with carbohydrates, proteins and sufficient oxygen so as not to interfere with the fixation process. Leghaemoglobin, a plant protein, helps to provide oxygen for respiration while keeping the free oxygen concentration low enough not to inhibit the nitrogenase activity.

Steps in the development of root nodules:

1. Rhizobium bacteria contact a susceptible root hair and divide near it.
2. The bacteria cause root hair to curl on successful infection.
3. An infected thread carries the bacteria to the inner cortex. The bacteria get modified into rod-shaped bacteroids and cause inner cortical and pericycle cells to divide. Division and growth of cortical and pericycle cells lead to nodule formation.
4. A mature nodule is complete with vascular tissues continuous with those of the root.

a)     True

b)     False. Every mineral element which is present in a cell is not needed by the cell. 

c)      False. Nitrogen as a nutrient element is highly mobile in plants.

d)   False. It is very difficult to establish the essentiality of micronutrients because they are required only in trace quantities.