Question and Answer:

Question 1.
All elements that are present in a plant need not be essential to its survival’. Comment.
Answer:
All elements that are present in a plant need not be essential to its survival.
More than sixty elements have been found in different plants, but not all of them are necessary for the plant’s growth and development. Some plants may even accumulate elements like selenium, gold, or radioactive strontium without any known essential role.

An element is considered essential only if it satisfies the following criteria:

1. The element must be absolutely necessary for supporting normal growth and reproduction.
2. Its requirement must be specific and not replaceable by another element.
3. It must be directly involved in the metabolism of the plant.

Thus, only those elements that meet these criteria are termed essential elements, while the others, though present, are not essential for the plant’s survival.

Question 2.
Why is purification of water and nutrient salts so important in studies involving mineral nutrition using hydroponics?

Answer:
In 1860, Julius von Sachs, a German botanist, demonstrated for the first time that plants could be grown to maturity in a defined nutrient solution in the complete absence of soil. The essence of these methods lies in growing plants in a soil-free, defined mineral solution. These methods require purified water and pure mineral nutrient salts because any impurities may introduce unknown elements that could affect the results. The presence of pure nutrients ensures clear, accurate, and scientific results in determining the essential elements required for plant growth.

Question 3.
Explain with examples: macronutrients, micronutrients, beneficial nutrients, toxic elements and essential elements.
Answer:

1. Macronutrients:
   These are elements required by plants in large quantities (more than 10 mmol/kg of dry matter).
   Examples: Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulphur (S), Carbon (C), Hydrogen (H), and Oxygen (O).
2. Micronutrients (Trace elements):
   These are required in very small amounts (less than 10 mmol/kg of dry matter).
   Examples: Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Molybdenum (Mo), Boron (B), Chlorine (Cl), and Nickel (Ni).
3. Beneficial Nutrients:
   These are elements that are not essential for all plants but may be required by certain higher plants for better growth and yield.
   Examples: Sodium (Na), Silicon (Si), Cobalt (Co), and Selenium (Se).
4. Toxic Elements:
   When the concentration of a micronutrient increases beyond the optimum range, it becomes toxic and reduces plant growth (about 10% reduction in dry weight).
   Example: Excess Manganese (Mn) causes brown spots surrounded by chlorotic veins and also induces deficiencies of Fe, Mg, and Ca.
5. Essential Elements:
   These are elements absolutely necessary for normal growth and reproduction of plants. Their deficiency prevents plants from completing their life cycle.
   Examples: Nitrogen, Phosphorus, Potassium, Iron, Magnesium, etc.

Question 4.
Name at least five different deficiency symptoms in plants. Describe them and correlate them with the concerned mineral deficiency.
Answer:
Plants show characteristic deficiency symptoms when essential mineral elements are lacking. These symptoms depend on the function of the element and its mobility within the plant.

(i) Iron Deficiency:

• Cause: Iron becomes unavailable in alkaline or waterlogged soils.
• Symptoms: Yellowing between leaf veins (interveinal chlorosis), especially in young leaves, while veins remain green. Fruits are of poor quality.
• Treatment: Use slightly acidic soil (pH 5–6.5) and apply well-rotted manure or compost to improve iron availability.

(ii) Potassium Deficiency:

• Cause: Lack of potassium affects protein synthesis and regulation of stomatal opening.
• Symptoms: Brown scorching and curling of leaf tips, yellowing of veins, and sometimes purple spots on the underside of leaves.
• Treatment: Apply organic potassium-rich fertilizers such as seaweed meal or composted bracken.

(iii) Calcium Deficiency:

• Cause: Insufficient calcium or poor nutrient mobility.
• Symptoms: Stunted growth, necrosis on leaf margins of young leaves, curling of leaves, and death of root tips.
• Treatment: Add agricultural lime to acid soils and improve organic matter to enhance calcium absorption.

(iv) Nitrogen Deficiency:

• Cause: Nitrogen gets used by microorganisms when woody material (like sawdust) is added to soil.
• Symptoms: Stunted growth, pale green or yellow leaves, delayed flowering and fruiting.
• Treatment: Apply organic manure or grow leguminous crops to fix nitrogen.

(v) Manganese Deficiency:

• Cause: Common in poorly drained or highly organic soils.
• Symptoms: Interveinal chlorosis with small veins remaining green, brown spots on leaves, and withering of severely affected leaves.
• Treatment: Improve drainage and apply manganese-rich fertilizers.

Question 5.
If a plant shows a symptom which could develop due to deficiency of more than one nutrient, how would you find out experimentally, the real deficient mineral element?
Answer:
The deficiency of any element may produce multiple symptoms, and the same symptom can result from the deficiency of different elements. To identify the real deficient element, one must carefully study all the symptoms developed in different parts of the plant and compare them with standard deficiency symptom tables. It is also important to note that different plants may respond differently to the deficiency of the same element.

Question 6.
Why is that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs?

Answer:
Deficiency symptoms depend on the mobility of the element in plants.

• Mobile elements (like Nitrogen, Potassium, and Magnesium) are actively transported from older tissues to young developing tissues. Therefore, their deficiency symptoms first appear in older leaves.
• Immobile elements (like Sulphur and Calcium) cannot be easily transported from mature organs. Hence, their deficiency symptoms first appear in young tissues or growing regions.

Question 7.
How are the minerals absorbed by the plants?
Answer:
Studies show that mineral absorption occurs in two main phases:

1. Apoplastic (Passive) Uptake:

• Rapid entry of ions into the ‘free space’ or apoplast of cells.
• Occurs through ion channels in cell membranes and does not require energy.

2. Symplastic (Active) Uptake:

• Slow entry of ions into the symplast of cells.
• Requires metabolic energy (ATP) and involves active transport.
• Influx is inward movement, and efflux is outward movement of ions.

Translocation:
Minerals are transported through xylem along with water pulled by transpirational pull. Xylem sap contains mineral salts, and radioisotope studies confirm their movement.

Question 8.
What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium. What is their role in N₂-fixation?
Answer:

Conditions for N₂ Fixation:

1. Presence of the enzyme nitrogenase (dinitrogenase).
2. Anaerobic conditions around nitrogenase to protect it from oxygen.
3. Energy source (ATP) to drive the reaction.
4. Electron donors such as NADPH, FMNH₂, or ferredoxin.
5. Deficiency of nitrate in the soil to induce nodule formation.
6. Organic carbon sources (like keto acids) for bacterial metabolism.

Role in Nitrogen Fixation:

1. ATP provides energy for nitrogenase, and ferredoxin supplies electrons.
2. Atmospheric N₂ binds to the Fe-Mo component of nitrogenase, weakening the triple bond between the nitrogen atoms.
3. In the presence of energy and electrons, hydrogen combines with nitrogen atoms to form N₂H₂ (diamide), N₂H₄ (hydrazine), and NH₃ (ammonia).
4. A reducing environment is essential and is maintained by leg-haemoglobin, which scavenges oxygen and releases it in small amounts.
5. The produced ammonia is immediately assimilated with the help of organic acids to form amino acids, which are then used by the plant.