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Class 10 Class 12
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What are the conditions necessary for fixation of atmospheric N2 by Rhizobium ? What is their role in N2 fixation.

Conditions for N2 fixation : There are following conditions necessary for N2 fixation :

(1) A strong reducing agent like NADPH : It supplies hydrogen to N2.

(2) ATP as source of energy : It provides energy. For fixation of NH3 molecules, 8 ATP molecules are needed.

(3) The enzyme system : Nitrogenase is necessary for N2 fixation.

(4) Compound for trapping ammonia : Ammonia is very toxic and needs to be trapped in the form of amino acids.

(5) Leghaemoglobin : It is O2 scavenger and maintains anaerobic environment otherwise in the presence of O2 the enzyme nitrogenase is toxicated.


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?

The appearance of the deficiency depends on the mobility of the element in plants. Therfore in certain plants the deficiency appears in the younger parts while in other in the mature plants.
If the element is mobile, then the deficiency appears in the older tissues first as the biomolecules in the older tissue are broken down to release the element which is then transported to the young tissues. For example, nitrogen being a mobile, its deficiency appear in senescent or older leaves. 
If the element is immobile then the deficiency first appears in the young parts as the these are not transported from the older tissues. Deficiency of the immmobile element calcium is first seen in young parts. 


All elements that are present in plant need not be essential to its survival. Comment.

All elements present in the plant need not be essential to its survival. For a element to be essential it has to meet all the criteria for essentiality. The criteria for essentiality are :

i. The element must be absolutely esential for supporting the normal growth and reproduction. The plant should be unable to complete its lifecycle in the absene of the element.

ii. The element requirement should be specific and not replaceble by another element. 

iii. The element must be directly involved in the metabolism of the plant. 

Since, all the elements do not meet all the criteria, they are not considered to be essential to the plant. 

Name at least five different deficiency symptoms in plants. Describe them and correlate them with concerned mineral deficiency.

1. Chlorosis : It is the yellowing of leaves due to the loss of chlorophyll. It is due to the deficiency of Mn, N, K, Mg, S, Fe, Zn, and Mo.

2. Die back: It is the withering of stem from apex downward. It is due to the deficiency of copper.

3. Necrosis : It is the death of the tissues.  The leaf tips develop dead areas or necrotic areas. It may be also due to Ca, Mg, Ni, Cu and K.

4. Whiptail disease : It occurs in cauliflower. It is due to the deficiency of Mo.

5. Little leaf disease: It is due to the deficiency of Zn. It also causes rosette of apple and peaches.



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

The purification of water and nutrient salts in hydroponics is very important because of the following reasons:

1. Impure water contains impurities, which will interfere in the growth of plant and make the study for the essntiality difficult.

2. Impurities may be toxic to the plant.

3. Impurities may compete with other minerals.

4. Impurities may inhibit some enzymes.

Therefore, only pure water and nutrients should be taken for hydroponics.