07 Plant Growth and Mineral Nutrition - part 16 - Mineral nutrition

07 Plant Growth and Mineral Nutrition - part 16 - Mineral nutrition

Mineral nutrition : 

• Minerals constitute most commonly  - 

1. Occuring solid
2. Inorganic materials obtained from the earth's crust. 

• Plant absorbs mineral elements from surroundings (soil, air and water) for its use. 
• About 36 to 40 different elements of periodic table are used as minerals by the plants. 
• These are absorbed in ionic (dissolved) form as PO₄ , CO₃ , SO₄ , etc.,usually through roots (regions of elongation and growth). 

Sources of minerals : 

• Carbon enters the plant as atmospheric carbon-dioxide. 
• Source of hydrogen is water and oxygen comes from air and water. 
• Carbon, Hydrogen and oxygen are not minerals in origin. 
• Source of nitrogen is the soil. 
• Plant derives nitrogen from both mineral and non-mineral origin. 

Classificaion of minerals : 

1. Classificaion of minerals [on the basis of their requirement minerals] :   -

1. Essential and 
2. Non-essential. 

1. Essential minerals : 

• Those that are indispensible without which plants can not complete their life cycle 
• e.g. C, H, O, N, P, etc. 
• These elements play structural and physiological roles. 
• Their absence can produce/ cause major deficiency symptoms. 

2. Non-essential elements :

• Are not indispensable and they do not produce/ cause any deficiency symptoms. 
• This classification is absolete now.

2. Classificaion of minerals [Based on the quantity requirement ]  : -

1. Minor or Microelements and 
2. Major or Macroelements. 

1. Microelements  :

• Required in traces because they function in the catalytic role 
• e.g. Zn, Cu, Al, Si, etc. as co-factors. 

2. Macroelements :

• Required in large quantity. 
• They mainly play the nutritive and structural roles 
• e.g. C, H, O, P, Mg, N, K, S, etc. C, H, O are non mineral major elements. 
• This classification is not accepted now.

Know the Scientist : 
Criteria for Essentiality of elements proposed by Arnon and Stout (1939) : 

• The elements must be absolutely necessary for supporting normal growth and reproduction. 
• These elements must be having specific functions and should not be replaceable by another element. 
• The element must be directly involved in the nutrition of the plant. 
• If an element satisfies all above three criteria, it is considered to be an essential element.

Do you know ? 

• Liebig showed the Essentiality of mineral nutrients and put forth the `Law of minimum'. 
• According to this law, the yield of crop plant is determined by the amount of essential element that is present (available) in the minimum quantity.
• Now a days minerals are classified on the basis of their biochemical functions.

Symptoms of Mineral deficiency in plants : 

• Any visible deviation from the normal structure and function of the plant, is called symptom or hunger sign. 
• The concentration of the essential elements below which plant growth is retarded, is termed as critical concentration. 
• The element is said to be deficient when present below the critical concentration. 
• Certain morphological changes are indicativeof the deficiency of particular element. 
• Deficiency symptoms also depend on the mobility of the elements in the plants. 
• The deficiency symptoms appear first in young tissues when elements are relatively immobile e.g. sulphur, calcium. 
• When the elements are actively mobilized within the plants and exported to young developing tissues, the deficiency symptoms are visible first in the older tissues (senescent leaves). e.g. nitrogen, magnesium, potassium. 

Important deficiency symptoms seen in plants are: 

1. Stunting : The growth is retarded. The stem appears condensed and short. 
2. Chlorosis : It is the loss or non-development of chlorophyll resulting in the yellowing of leaves 
3. Necrosis : It is the localized death of tissue of leaves. 
4. Mottling : Appearance of green and nongreen patches on the leaves. 
5. Abscission : Premature fall of flowers, fruits and leaves.
   
   
   
   

Do you know ? 

1. Constituents of protoplasm and cell walls :C,H,O,N,P,S are important and permanent constituents of the protoplasm and the cell wall. 
2. Osmotic potential : The osmotic pressure of cell is the result of mineral salts and organic compounds dissolved in the cell sap. 
3. Catalytic functions : Many elements like Fe, Cu, Zn, Mg, Mn, Cl, etc. act as catalysts in various enzyme reaction of plant metabolism. 
4. Permeability of cytoplasmic membrane:Permeability of membrane is influenced by the presence of the various `cations' and `anions' of the mineral elements in the external medium like ca+, k+. 
5. Components of energy related chemical compounds : Phosphorus in ATP and Mg in chlorophyll.

Do you know ? 

• The atmospheric CO2 is virtually the only source of carbon, which is the basic constituent of all the organic compounds.

Toxicity of Micronutrients : 

• Mineral ion concentration which reduces the dry weight of tissues by 10%, is considered as toxic. 
• Toxicity of one element may inhibit the uptake of the other 
• e.g. Mn competes with Fe, Mg for uptake, but inhibits Ca translocation to shoot apex. 
• Thus, Mn toxicity develops deficiency symptoms of Ca, Mg and Fe.

Minerals salt absorption : 

• Most minerals in the soil are charged particles hence, they can not pass across cell membrane.
• Hence most of the minerals are absorbed actively with the expenditure energy. 
• Minerals can also be absorbed passively without expenditure of energy. 
• Mineral ion absorption is independent of water absorption. 
• Mineral ion absorption can occur in two ways : 

1. Passive Absorption :

• Movement of mineral ions into the root occurs by diffusion. 
• Molecules or ions diffuse from a region of their higher concentration to a region of their lower concentration. 
• The movement of mineral ions into root cells as a result of diffusion is without expenditure of energy is called passive absorption. 
• Passive absorption can take place by -  

1. Direct ion-exchange, 
2. Indirect ion-exchange mass flow and 
3. Donnan equilibrium. 

 Donnan equilibrium : 

• It is based on the assumption that certain negatively charged ions, after their entry into the cell, become fixed on the inner side of the cell membrane and can not diffuse outside through the cell membrane. 
• Therefore, additional mobile cations are required to balance these fixed anions. 
• Obviously concentration of cations become more due to accumulation. 
• This kind of passive absorption of anions/ cations from cell exterior against their own concentration gradient in order to neutralize the effect of cations/ anions, is called Donnan equilibrium. 

Active Absorption: 

• Uptake of mineral ions against concentration gradient, is called active absorption.
• Such movement requires an expenditure of energy by the absorbing cell. 
• This energy is derived from respiration and is supplied through ATP. 
• When the roots are deprived of oxygen, they show a sudden drop in active absorption of minerals. 
• The mineral ions accumulated in the root hairpass into the cortex and finally reach the xylem.
• The minerals in the xylem are then carried along with water to other parts of the plant along the transpiration stream and are subsequently assimilated into organic molecules and then redistributed to other parts of the plant through the phloem. 

Do you know ? 

• To explain active absorption of minerals Hoagland and Davis (1923) put forth Carrier hypothesis. 
• Specific proteins in the membrane of root cells actively pump ions into the cytoplasm of epidermal cells of root. 
• These proteins are called carriers that pump both cations and anions from the soil. 
• According to Bennet and Clarke (1956) protein conjugated with lecithin acts as carrier.