In remote pasts, it was realized that the land plant obtains nourishments from soils through their roots. This idea was supported by the common practice of additions of animal and plant manures to the soils for increasing the productivity of the crops.
Only during the first half of the 19th century did botanists begin to understand that plants grow and develop only when they are supplied with certain chemical elements. These elements were termed essential elements as without these the plant was unable to live and grow.
These elements are absorbed by roots mainly as inorganic ions from the soils. The source of these inorganic ions in the soils was mineral constituents of the soil. Thus term mineral nutrition was suggested to refer to an inorganic ion obtained from the soil and required for plant growth.
Essential Mineral Elements
Of many elements detected in plant tissue, only 16 are essential to all higher plants. Experimental evidence by growth experiments in nutrient solutions for each of these elements suggests that in the absence of each essential element, plants develop deficiency symptoms due to nutrition disorders.
Criteria for an Element to be Essential
There are two principal criteria by which an element can be judged essential or non-essential to any plant. Either criterion is sufficient to demonstrate essentiality or most elements in our list 16 have met both.
- An element is essential if the plant cannot complete its life cycle i.e., form viable seeds, in the absence of that element; and
- An element is essential if it forms part of any molecules or constituent of the plant that is itself essential in the plant, e. g., N in proteins and Mg in chlorophyll.
The elements that are required in relatively larger amounts by the plants are called macroelements. These include carbon, hydrogen, oxygen, nitrogen, potassium, phosphorus, sulfur, potassium, calcium, and magnesium. The macroelements are absorbed by the plants in different chemicals such as K+, Ca2+, Mg2+, NO3-, H2P04– etc.
The elements like boron (B), chlorine (Cl), copper (Cu), and iron (Fe). Manganese (Mn), molybdenum (Mo), and zinc (Zn) are required in relatively smaller amounts, therefore, called microelements. The microelements are available to plants either as inorganic ions or undissociated molecules or organic complexes (e.g. chelates).
In Inorganic form, the microelements are supplied as borate (B033), chloride (Cl), cupric (Cu2+), ferrous or ferric (Fe2+ or Fe+), manganous (Mn2+), molybdate (Mo) and zinc (Zn2+). Sometimes microelements are required in extremely small amounts, these microelements are called minor or trace elements.
In addition to above mentioned 16 elements, some other elements have also been found to be essential for plant growth. For example:
- Sodium is an essential microelement to certain salt-marsh plant species in which CO2 assimilation in photosynthesis takes place by the C-4 dicarboxylic acid pathway.
- Cobalt is an essential microelement to wheat and one legume species.
- Silicon is essential to certain grasses and horsetails with the naturally high silicon content.
- Selenium is essential for the growth of certain species of Astragulus, a plant growing in saline soils.
- Nickle has been shown to be an essential component of enzymes.
SOURCES OF NUTRIENTS
The autotrophic higher plants obtain nutrients necessary for their growth and development from three sources in the environment, namely: atmosphere, water, and soil.
The atmosphere provides carbon dioxide and oxygen. All the carbon atoms, plus most oxygen atoms (approximately two-thirds), in the dry matter of higher plants, are derived from CO2. The other source of oxygen is water. The atmospheric oxygen is used for oxidation in respiration.
During the process, it is reduced to water (respiratory water) which mixes with water already present in the plant cells. A small amount of atmospheric 02 is incorporated into certain organic constituents in the process called oxygen fixation.
Some higher plants (legumes) fix atmospheric nitrogen into nitrogenous compounds through microorganisms living in their roots usually (root nodules). Finally, trace amounts of ammonia and sulfur dioxide gases sometimes present in the atmosphere are also assimilated by some plants.
Water is the second source of nutrition for plants. The source of water for the plants is the soil water absorbed through roots.
Water is added to the plant material mainly via hydration and hydrolytic reactions, For example, when fumaric acid is converted to malic acid in the TCA cycle, water is added to the fumaric acid (hydration), and when the starch is degraded (hydrolyzed), there is a small gain in the dry weight of the products because water is chemically added.
Moreover, water is a reactant in the light reaction in photosynthesis. The hydrogen of water is incorporated into the organic matter of the plant and oxygen is released during the process.
iii. Soil (Source of Nutrients)
Soils are a heterogeneous and variable mixture of inorganic mineral particles (sand, silt, and clay), decaying organic matter (humus), and living microorganisms, along with air and various inorganic molecules dissolved in water.
The mineral particles are present as sand, smaller silt, and still smaller clay, all composed mainly of silicon, oxygen, and aluminum. The clay mineral fraction is more abundant in many soils. The clay minerals consist of aluminum (Al3+) and silicon (Si4+) largely and their surfaces become negatively charged because some atoms of AI3 + and so are replaced by Mg2+ or Fe2+.
Such replacement results in the availability of negative sites on the clay particles so that dissolved cations are adsorbed on their surfaces. Similarly, organic matter in the soil also has negatively charged surfaces. The phenolic compounds in the humus ionize. Their carboxyl groups (-COOH) ionize into —C00 and H ions, and hydroxyl groups (-OH) into 0 and H ions.
These negative surface charges of soil particles are important in the adsorption of mineral cations to the surface of particles. Because the adsorbed cations are not easily lost by leaching when water moves down through the soil, they provide a nutrient reserve available to plant roots for absorption.
Mineral nutrients adsorbed on the surfaces of soil particles can be replaced by other cations in a process known as cation exchange. For example, the replacement of K with W is an important cation exchange. The organic acids such as malic acid, produced during respiration in the roots, ionize and release H into the soil which replaces K adsorbed on the surfaces of mineral particles of the soil.
Anion Adsorption and Exchange
Mineral anions are usually repelled by the negative charges of the soil particles. Thus, the anion exchange capacity of most soils is small as compared to the cation exchange capacity. Among the most commonly required anions, phosphate (P04), nitrate (N07), sulfate (5042), and chloride (Cl–) are more soluble and all are repelled by the negatively charged so particles.
Because of this repulsion, these are leached from soils as water passes through them. Phosphate (PO4) and sulfate (S04) ions may bind to soil particles containing aluminum or iron. The positively charged iron and aluminum ions have hydroxyl (0H) groups that can be exchanged with sulfate, phosphate, and other anions.
The soil pH (hydrogen ion concentration) affects the absorption of mineral ions. The soil pH can affect the growth of plant roots and soil microorganisms. Root growth is generally favored by slightly acidic pH values Soil pH also determines the availability of plants’ nutrients. A low pH favors the weathering of rocks and the release of ions such as K+, Mg2+, Ca2, and Mn2+. At low pH values, the salts present in the soil as carbonates, sulfates, and phosphates are more soluble. Increasing solubility facilitates absorption by roots.
Factors Necessary for Mineral Absorption
Following are plant and environmental factors that contribute to effective mineral absorption:
i. Plant Root System
Water and mineral elements are present in the soils. To absorb both water and mineral elements from the soil depends upon their capacity to develop an extensive root system. Plant roots grow continuously and root growth and development depend upon the soil environment.
In moist and fertile soils, the roots proliferate extensively. If water is available deep in the ground, roots grow deep as well. The root hairs also contribute to the absorption of eons and water, e.g. in the lye plant the root hairs contribute about 67% of the total root surface area.
ii. Mycorrhizae (myco=fungi + rhiza=root)
The fungi receive organic nutrients from the plant and in turn improve the mineral salts and water-absorbing properties of the roots. The mycorrhizae help increase the capacity of the root system to absorb nutrients such as phosphorus and trace elements like zinc and copper.
iii. Movement of Nutrients within Soil
Within the soil, nutrient movement to the root surface can occur both by bulk flow and by diffusion. Bulk flow occurs when nutrients are carried in the flow of water moving through the soil towards the root. The bulk flow is effective when the concentration of nutrients in the soil solution is high and the rate of water flow through the plant is greater. Diffusion occurs when mineral nutrients move from a region of higher concentration to a region of lower concentration.
iv. Nature of the Membranes
Plasmalemma and tonoplast are the biological membranes involved in ion transport. These membranes consist largely of proteins and lipids.
The proteins are usually about one-half to two-thirds of the membrane’s dry weight. Some of the hydrophobic proteins called integral proteins penetrate deeply into the lipid-rich interior whereas some extend all the way through the bilayer. The proteins in membranes are of three types:
- Catalytic Proteins (Enzymes): These usually catalyze the hydrolysis of Nu to ADP and l-12P01. These are called ATPases. All membranes of all organisms have at least one kind of ATPases
- Transporters: Several kinds of proteins are called carrier or transporters, each of which combines with and transport a different ion or molecule across the membrane.
- Structural Proteins: These proteins contribute to structure only but it is difficult to prove that these contribute to structure only.