MODE OF NUTRITION
Plants nutrition:-Like other living organisms, plants also require raw materials for building their structure and maintaining body functions. It is called nutrition. The various inorganic and organic chemical substances (such as Carbon dioxide, minerals, carbohydrates, proteins, fats, etc.) required by plants for their life activities are called nutrients.
TYPES OF NUTRITION
Basically, there are two modes of nutrition, autotrophic and heterotrophic.
(A) Autotrophic Nutrition:
It is a type of nutrition in which living organisms manufacture their own organic food from simple inorganic raw materials. The KingdomPlantae is characterised by autotrophic mode of nutrition. The autotrophs require external energy source for the manufacture of organic substances. Green plants obtain the energy from solar radiation and, therefore, they are called photoautotrophs.
The process is called photosynthesis and occurs in presence of chlorophyll. Certain bacteria obtain energy from exergonic chemical reactions for the synthesis of organic substances. These organisms are called chemoautotrophs. The process is called chemosynthesis.
(B) Heterotrophic Nutrition:
Certain non-green organisms (such as animals, fungi, many bacteria and certain an- giosperms) fail to synthesize their own organic nutrients from inorganic substances. These “or- ganisms are thus, dependent on some other external sources for their organic nutrition. Such plants which are dependent on some other sources for their organic nutritional requirements are called heterotrophic plants and the mode of nutrition is called heterotrophic nutrition.
heterotrophic plants are broadly categorized into three main groups, depending upon the source from which they get their nourishment-saprophytes, parasites and insectivorous (or carnivorous) plants.
INORGANIC NUTRITION IN PLANTS
The plants require inorganic elements which they chiefly obtain from the soil where these elements occur in the form of minerals. The absorption, distribution and metabolism of various mineral elements by plants is called mineral nutrition, the
mineral elements play a paramount role in the performance and maintenance of life activities. Some of the important roles are –
(i) Structural constituents of the plant body;
(ii) Influence in the osmotic pressure of cells;
(iii) Permeability of cytoplasmic membranes;
(iv) Toxic effects; (P) Catalytic effects; and
(vi) Antagonistic effects.
Classification of Inorganic Nutrients
In the middle of nineteenth century, the mineral elements were grouped under two broad categories, essential and non-essential elements. Carbon, oxygen, hydrogen, nitrogen, sulphur, phosphorus, potassium, magnesium, calcium and iron were known as essential elements while others were grouped as non-essential ones. Further studies with refined techniques and careful purification of salts used in mineral nutrition solution, confirmed that boron, manganese, zinc, copper, molybdenum, sodium and chlorine are also essential for normal plant growth and development.
The terms essential and non-essential elements are, therefore mislead- ing. Various authors then used the terms macro-nutrients (required in large amount) and micronutrients or trace elements (required in smaller amounts). They kept carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, magnesium and calcium under the group major elements or macronutrients. The rest of the elements such as, iron boron, manganese, zinc, copper, molybdenum, chlorine and nickel were regarded as micronutrients or trace elements However, the world authorities disregard the distinction between ‘macro-nutrients, and ‘micronutrients’ because such a distinction between the two is very arbitrary.
Some of the authors in the past have used these terms only on some convenient grounds but of little use in the present context. For instance, calcium is regarded as a macronutrient but it may be a micronutrient for some plants (e.g. Algae). Even in a given plant the quantitative requirement for a given element may vary greatly, depending upon the conditions that are prevailing either because of the presence of certain other elements in the medium or due to some other factor(s).The average concentration of various elements as calculated on the basis of dry matter by Strout (1961) are given in
Shkolnik (1950) proposed a convenient classification of minerals required by plants, which is given below:
1. Structural elements.
Carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus and magnesium.
2. Bio-catalytical elements.
The elements forming part of bio- catalysts such as iron, copper, manganese and cobalt.
3. Indispensable elements.
Potassium, calcium, boron and molybdenum.
4. Radioactive elements. Uranium, thorium, radium, actinium and rhodium.
5. Stimulating elements. Sodium, chlorine, arsenic, tanium, cobalt, nickel, iodine, bromine and rubidium.
Differences between Macroelements and Microelements (or trace elements)
| Macroelements | Microelements (or trace elements) |
|---|---|
| 1. These are nutrient elements present in plants in easily detectable quan- tities. | 1. These elements are present in plants in very minute amounts or in traces. |
| 1. These are nutrient elements present in plants in easily detectable quantities. | 2. The average amount of microelement present in 1 gm of dry matter is usually less than 1 mg. |
| .3. Slight excess of these elements are generally not toxic to plants. | .3. These elements be- come toxic if increase in concentration. |
| .2. Theaverage amount of macro-elements present in 1 gm of dry matter is usually more than 1 mg. | 4. Microelements are generally required in the functioning of enzymes, as cofactors or metal activators |
Balanced Nutrient Solution
Balanced nutrient solutions (also known as balanced salt solutions) are prepared by dissolving definite proportions of salts in distilled water re- quired for normal plant growth. Plants growing in such solutions survive much longer because they get all the essential and trace elements from the medium. Several workers proposed various combinations and concentrations of salts, one of them is listed
Solution Culture (Hydroponics)
If we want to study mineral requirement of the plants, the use of soil in the experiment will be irronious because we cannot control the quantity of mineral elements in it. A better technique of solution culture has been developed to avoid contamination. In the solution culture, plants are directly grown in pure salt solutions. The solution is taken in suitable vessels preferably made of borosillicate glass or natural polyethylene (Hewitt, 1963) to minimise contamination.
Aeration of solution culture is necessary, therefore, air is forced through the solution by means of an inlet tube . This practice is very useful in experiments related with mineral nutrition. It is also being used commercially in countries like United States, Abu Dhabi, etc. Since the plants are grown in water containing required minerals, the soil being not required at all, the process is also called soil-less growth or hydroponics
Sand culture
In this process the plants are grown in sand supplemented with nutrient solution. Suitable vessels are filled with pure quartz sand thoroughly washed with acid and then with distilled water. Artificial nutrient solution is added into the sand from time to time. The sand culture is preferred over solution culture for many types of investigations because it provides solid media and natural aerated condition for root growth.
Detection of Essentiality of Mineral Elements
Different criteria for knowing the essentiality of elements are as follows-
(1) The element must be absolutely essential for supporting the normal growth and reproduction of plants.
(ii) Withholding of the element must cause the deficiency symptoms. These disorders are removed by supplying the particular element.
(iii) The element must be directly involved in the nutrition of plants. It must be either a component of an organic molecule or participates in a biochemical reaction.
(iv) Requirement of the particular element must be specific. It cannot be replaced by any other element.
The essentiality of a given mineral element can be detected by using sand culture or solution culture method. A plant is grown in acid-washed sand (pure quartz sand) kept in glass container and supplied with the nutritional solution made stand- ard for the control. Another similar plant is grown in a second container filled with acid-washed sand.. The second container was supplied with the normal nutritional solution minus the mineral element to be tested.
That means the second plant is not getting one mineral element, otherwise all the conditions and nutrition is common in both. Now if the mineral element to be tested is essential for plant, it will show some deficiency symptoms or affect the normal growth of plant and if it is not essential, the plant will grow normally which can be compared with the control.
Occurrence and Availability of Essential Elements for Plants
All essential elements required by the plants are ultimately derived from the atmosphere, water and soil. Carbon occurs in the form of carbon dioxide which is fixed to carbohydrates by the process of photosynthesis or chemosynthesis. Hydrogen is obtained mainly from water. Oxygen comes from the air or from water. The ultimate source of Nitrogen is atmospheric nitrogen present in the form of gas. It is fixed in usable forms by several biological and non-biological agencies.
Nitrogen is also present in the soil in the form of nitrates and ammonical salts. The fixed form of nitrogen (nitrate, nitrite or ammonium) enters the plants and assimilated into organic forms. Inorganic fraction of soluble sulphur is present in the form of sulphate ions (SO). The usable forms of phosphorus present in the soil are dihydrogen phasphate (H₂POT) and HPO ions. Calcium is made available for the plants by weathering of anorthite (CaAl2Si2O8) in the soil and Potas- sium is made available to plants by weathering of biotite, muscovite and illite.
Sometimes it occurs as soluble organic salts. Magnesium is present in the form of magnesium silicates which on weathering release the available form of magnesium to plants. Iron is mainly absorbed in ferrous form. Boron is abundant in rocks and marine sediments. All other microelements are absorbed by plants in traces and -derived from the parent rocks by the process of weathering . read more
