12. MINERAL NUTRITION

All organisms require macromolecules (carbohydrates, proteins, fats etc), water & minerals for growth and development.

METHODS TO STUDY THE MINERAL REQUIREMENTS OF PLANTS

-    The technique of growing plants in a defined nutrient solution (without soil) is known as hydroponics.

-    It is demonstrated by Julius von Sachs (German botanist, 1860).

-    The nutrient solutions must be adequately aerated to obtain the optimum growth.

-    Hydroponics was used to identify the essential elements required for plants and their deficiency symptoms. This method requires purified water and mineral nutrient salts.

-    After a series of experiments, a mineral solution suitable for the plant growth was obtained. In this, the roots of the plants were immersed in nutrient solutions and wherein an element was added / removed or given in varied concentration.

-    Hydroponics has been employed in the commercial production of vegetables such as tomato, seedless cucumber and lettuce.

ESSENTIAL MINERAL ELEMENTS

-    More than 60 elements are found in different plants. Some plant species accumulate selenium, gold etc.

-    Some plants growing near nuclear test sites take up radioactive strontium.

-    There are techniques to detect the minerals even at a very low concentration (10^-8 g/mL).

Criteria for Essentiality of an element

·    The element must be necessary for normal growth and reproduction. In the absence of the element the plants do not complete their life cycle or set the seeds.

·    The requirement of the element must be specific. i.e., Deficiency of an element cannot be met by supplying another element.

·    It must be directly involved in the plant metabolism.

Only 17 elements are absolutely essential for plant growth and metabolism.

Based on quantitative requirements these elements are divided into two types: Macronutrients & Micronutrients.

             

              i.  Macronutrients

-    They are present in plant tissues in large amounts (more than 10 mmole Kg^–1 of dry matter).

-    They include carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium & magnesium.

-    Of these, carbon, hydrogen & oxygen are mainly obtained from CO2 and H2O. Others are absorbed from the soil as mineral nutrition.

ii.  Micronutrients (trace elements)

-    They are needed in very small amounts (less than 10 mmole Kg^–1 of dry matter).

-    They include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

For higher plants, there are some other elements required such as sodium, silicon, cobalt & selenium.

Essential elements are also grouped into four categories on the basis of their functions. They are:

(i)       Components of biomolecules and structural elements of cells. E.g. carbon, hydrogen, oxygen and nitrogen.

(ii)     Components of energy-related chemical compounds. E.g. Mg in chlorophyll and phosphorous in ATP.

(iii)    Essential elements that activate or inhibit enzymes. E.g. Mg²⁺ is an activator for RUBISCO and phosphoenol pyruvate carboxylase. They are critical enzymes in photosynthetic carbon fixation. Zn²⁺ is an activator of alcohol dehydrogenase and Mo of nitrogenase during nitrogen metabolism.

(iv)    Essential elements that alter the osmotic potential of a cell. E.g. Potassium functions in the opening and closing of stomata.

Role of Macro- and Micro-nutrients

Essential elements participate in various metabolic processes in the plant cells such as

·       Permeability of cell membrane

·       Maintenance of osmotic concentration of cell sap

·       Electron transport systems

·       Buffering action

·       Enzymatic activity

·       Major constituents of macromolecules and co-enzymes.

Functions of various mineral elements are given below.

Nitrogen:

·       This is required by plants in the greatest amount.

·       It is required by all plant parts, particularly the meristematic tissues and the metabolically active cells.

·       It is absorbed mainly as NO₃^– though some are also taken up as NO₂^– or NH₄⁺.

·       It is the major constituents of amino acids, proteins, nucleic acids, chlorophyll, vitamins and hormones.

Phosphorus:

·       It is absorbed by the plants from soil in the form of phosphate ions (either as H₂PO₄⁻ or HPO₄²⁻).

·       It is a constituent of cell membranes, certain proteins, all nucleic acids and nucleotides.

·       It is required for all phosphorylation reactions.

Potassium:

·       It is absorbed as potassium ion (K⁺).

·       It is required in abundant quantities in the meristematic tissues, buds, leaves and root tips.

·       It maintains an anion-cation balance in cells.

·       It is involved in protein synthesis, opening and closing of stomata, activation of enzymes and in the maintenance of the turgidity of cells.

Calcium:

·       It is absorbed from the soil as calcium ions (Ca²⁺).

·       It is required by meristematic and differentiating tissues.

·       During cell division, it is used in the synthesis of cell wall, particularly as calcium pectate in middle lamella. It is also needed during the formation of mitotic spindle.

·       It accumulates in older leaves.

·       It is involved in the normal functioning of the cell membranes.

·       It activates certain enzymes and plays an important role in regulating metabolic activities.

Magnesium:

·       It is absorbed by plants in the form of divalent Mg²⁺.

·       It activates enzymes of respiration & photosynthesis.

·       It is involved in the synthesis of DNA and RNA.

·       It is a constituent of the ring structure of chlorophyll.

·       It helps to maintain the ribosome structure.

Sulphur:

·       Plants obtain sulphur in the form of sulphate (SO)₄²⁻.

·       It is present in 2 amino acids (cysteine & methionine).

·       It is the constituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme A) and ferredoxin.

Iron:

·       Plants obtain iron in the form of ferric ions (Fe³⁺).

·       It is required in larger amounts in comparison to other micronutrients.

·       It is a main constituent of proteins involved in the transfer of electrons like ferredoxin and cytochromes.

·       It is reversibly oxidized from Fe²⁺ to Fe³⁺ during electron transfer.

·       It activates catalase enzyme, and is essential for the formation of chlorophyll.

Manganese:

·       It is absorbed in the form of manganous ions (Mn²⁺).

·       It activates many enzymes involved in photosynthesis, respiration and nitrogen metabolism.

·       The best defined function of manganese is in the splitting of water to liberate O₂ during photosynthesis.

Zinc:

·       Plants obtain zinc as Zn²⁺ ions.

·       It activates various enzymes, especially carboxylases.

·       It is also needed in the synthesis of auxin.

Copper:

·       It is absorbed as cupric ions (Cu²⁺).

·       It is essential for the overall metabolism in plants.

·       Like iron, it is associated with some enzymes in redox reactions and is reversibly oxidised from Cu⁺ to Cu²⁺.

Boron:

·    It is absorbed as BO₃ ³⁻ or B₄O₇²⁻.

·       It is required for uptake and utilisation of Ca²⁺, membrane functioning, pollen germination, cell elongation, cell differentiation and carbohydrate translocation.

Molybdenum:

·       Plants obtain it in the form of molybdate ions (MoO)₂²⁺.

·       It is a component of several enzymes, including nitrogenase and nitrate reductase. These enzymes participate in nitrogen metabolism.

Chlorine:

·       It is absorbed in the form of chloride anion (Cl^–).

·       Along with Na⁺ & K⁺, it helps in determining the solute concentration and the anion-cation balance in cells.

·       It is essential for the water-splitting reaction in photosynthesis that leads to oxygen evolution.

Deficiency Symptoms of Essential Elements

-   When the supply of an essential element becomes limited, plant growth is retarded.

-   The concentration of the essential element below which plant growth is retarded is termed as critical concentration. The element is said to be deficient when present below the critical concentration.

-   Due to the deficiency or absence of particular element, the plant shows some morphological changes. It is called deficiency symptoms.

-   The deficiency symptoms vary from element to element and they disappear when the deficient mineral nutrient is provided to the plant. If deprivation continues, it may lead to the death of the plant.

-   The parts of the plants that show the deficiency symptoms depend on the mobility of the element. For elements that are actively mobilized and exported to young developing tissues, the deficiency symptoms appear first in the older tissues. E.g. deficiency symptoms of nitrogen, potassium and magnesium are visible first in the senescent leaves.

-   In older leaves, biomolecules containing these elements are broken down. It makes these elements available for mobilizing to younger leaves.

-   If the elements are relatively immobile and are not transported out of the mature organs, the deficiency symptoms appear first in the young tissues. E.g. elements like S and Ca are a part of the structural component of the cell and hence are not easily released.

-   This aspect of mineral nutrition is of a great significance and importance to agriculture and horticulture.

-   The deficiency symptoms include chlorosis, necrosis, stunted growth, premature fall of leaves & buds and inhibition of cell division.

-   Chlorosis is the loss of chlorophyll leading to yellowing in leaves. It is due to the deficiency of elements N, K, Mg, S, Fe, Mn, Zn and Mo.

-   Necrosis is the death of tissue, particularly leaf tissue. It is due to the deficiency of Ca, Mg, Cu, K.

-  Lack or low level of N, K, S, Mo causes an inhibition of cell division. Low concentration of some elements like N, S, Mo delay flowering.

-  Deficiency of an element causes multiple symptoms. The same symptoms may be caused by the deficiency of other elements. Hence, to identify the deficient element, it has to study all the symptoms developed in all parts of the plant. We must also be aware that different plants respond differently to the deficiency of the same element.

Toxicity of Micronutrients

-  A moderate increase in micronutrients causes toxicity.

-  Any mineral ion concentration in tissues that reduces the dry weight of tissues by about 10% is considered toxic.

Such critical concentrations vary widely among different micronutrients.

-   The toxicity symptoms are difficult to identify. Toxicity levels for an element also vary for different plants.

-   Excess of an element may inhibit the uptake of another element. E.g. Excess of Mn induces deficiencies of Fe, Mg & Ca because it competes with Fe & Mg for uptake and with Mg for binding with enzymes. Mn also inhibits Ca translocation in shoot apex. Thus symptoms of Mn toxicity may actually be the deficiency symptoms of Fe, Mg & Ca. Main symptom of manganese toxicity is the appearance of brown spots surrounded by chlorotic veins.

MECHANISM OF ABSORPTION OF ELEMENTS

-  The process of absorption includes 2 main phases.

o First phase: Initial rapid and passive uptake of ions into the ‘free space’ or ‘outer space’ of cells (apoplast). It usually occurs through ion-channels, the trans-membrane proteins that function as selective pores.

o Second phase: The ions are taken in slowly into the ‘inner space’ of the cells (symplast). It is an active process (requires metabolic energy).

-   The inward movement of ions into the cells is called influx and the outward movement is efflux.

Translocation of solutes

-   Mineral salts are translocated through xylem along with the ascending stream of water.

-   Analysis of xylem sap shows the presence of mineral salts in it. Use of radioisotopes of mineral elements also proved that they are transported through the xylem.

SOIL AS RESERVOIR OF ESSENTIAL ELEMENTS

-  Weathering and breakdown of rocks enrich the soil with dissolved ions and inorganic salts.

-  Roles of soil:

o It supplies minerals and holds water.

o It harbours nitrogen-fixing bacteria and other microbes. o It supplies air to the roots.

o It acts as a matrix that stabilizes the plant.

-   Deficiency of essential minerals affects the crop-yield. So fertilizers should be supplied. Both macro-nutrients and micro-nutrients form components of fertilizers.

METABOLISM OF NITROGEN

Nitrogen Cycle

-   Nitrogen is the most prevalent element in living organisms.

-  Plants compete with microbes for the limited nitrogen in soil. Thus, nitrogen is a limiting nutrient for both natural and agricultural eco-systems.

-  The process of conversion of nitrogen (N₂ or NºN) to ammonia is termed as nitrogen fixation.

-  In nature, lightning and UV radiation provide energy to convert nitrogen to nitrogen oxides (NO, NO₂, N₂O). Industrial combustions, forest fires, automobile exhausts and power-generating stations are also sources of atmospheric nitrogen oxides.

-  Decomposition of organic nitrogen of dead plants and animals into ammonia is called ammonification.

-  Some of this ammonia volatilizes and re-enters the atmosphere but most of it is converted into nitrate by soil

bacteria in the following steps:

2NH₃ + 3O₂ → 2NO₂^- + 2H⁺ + 2H₂O

2NO₂^- + O₂ → 2NO₃^-

-  NH₃ is oxidized to nitrite by the bacteria Nitrosomonas & Nitrococcus. Nitrite is oxidized to nitrate by a bacterium

Nitrobacter. These steps are called nitrification. These nitrifying bacteria are chemo-autotrophs.

-   The nitrate is absorbed by plants and is transported to the leaves. In leaves, it is reduced to form ammonia that finally forms the amine group of amino acids.

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Nitrate present in the soil is also reduced to nitrogen by the process of denitrification. It is carried by bacteria Pseudomonas and Thiobacillus. 

Biological Nitrogen Fixation

-  It is the reduction of N₂ to NH₃ by living organisms in presence of nitrogenase enzyme.

-  Very few living organisms can utilize the nitrogen in the form N₂, available in the air.

-  Only certain prokaryotic species are capable of fixing N₂.

-  Nitrogenase is present exclusively in prokaryotes. Such microbes are called N2- fixers.

-  Nitrogen-fixing microbes are 2 types:

o Free-living: E.g. Azotobacter & Beijernickia (aerobic microbes), Rhodospirillum & Bacillus (anaerobic), cyanobacteria such as Anabaena & Nostoc.

o Symbiotic: E.g. Rhizobium.

Symbiotic Biological Nitrogen Fixation

-  The most prominent symbiotic biological nitrogen fixing associations is the legume-bacteria relationship. E.g. Rhizobium species (rod-shaped) seen in the roots of legumes such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans etc.

-  The most common association on roots is as nodules.

-  The microbe, Frankia also produces N₂ fixing nodules on the roots of non-leguminous plants (e.g. Alnus).

-  Rhizobium & Frankia are free-living in soil, but as symbionts, can fix atmospheric nitrogen.

-  The section of a nodule shows red or pink colour in the central portion. This is due to the presence of leguminous haemoglobin or leg-haemoglobin.

Principal stages in the Nodule formation:

o  Rhizobia multiply and colonise the surroundings of the

roots and get attached to epidermal and root hair cells.

o  Root-hairs curl and the bacteria invade the root-hair.

o  An infection thread is produced carrying the bacteria into root cortex, where they initiate nodule formation.

o  The bacteria are released from thread into cells. It leads to differentiation of specialized nitrogen fixing cells.

o The nodule thus formed, establishes a direct vascular connection with the host for exchange of nutrients.

-   The nodule contains all essential biochemical components, such as nitrogenase enzyme & leg-haemoglobin.

Nitrogenase (a Mo-Fe protein) catalyzes the conversion of atmospheric nitrogen to NH₃, the first stable product of N₂ fixation (Fig.12.5 in T.B). The reaction is as follows: 

Nitrogenase is highly sensitive to the molecular oxygen. So it requires anaerobic conditions to protect from oxygen. For this, the nodule contains an oxygen scavenger called leg-haemoglobin.

-   These microbes live as aerobes under free-living conditions (where nitrogenase is not operational), but during nitrogen-fixing events, they become anaerobic (to protect nitrogenase enzyme).

-   The ammonia synthesis by nitrogenase requires a very high input energy (8 ATP for each NH₃ produced). It is obtained from the respiration of the host cells.

Fate of ammonia:

-   At physiological pH, the NH₃ is protonated to form NH₄⁺ (ammonium) ion. While most of the plants can assimilate nitrate as well as ammonium ions, the latter is quite toxic to plants and hence cannot accumulate in them.

-   In plants, NH₄⁺ is used to synthesize amino acids by two ways:

a.  Reductive amination: In these processes, ammonia reacts with a-ketoglutaric acid to form glutamic acid.

a- ketoglutaric acid + NH₄⁺ + NADPH^{Glutamate dehydrogenase} Glutamate + H₂O + NADP

b. Transamination: It involves the transfer of amino group from one amino acid to the keto group of a keto acid. Glutamic acid is the main amino acid from which the transfer of NH₂, (amino group) takes place and other amino acids are formed through transamination. The enzyme transaminase catalyses all such reactions. For example:

-   Asparagine & glutamine are two most important amides found in plants. They are structural part of proteins. They are formed from 2 amino acids- aspartic acid & glutamic

acid- by addition of another amino group to each. The hydroxyl part of the acid is replaced by another NH₂^- radical.

-   Since amides contain more nitrogen than the amino acids, they are transported to other parts of the plant via xylem vessels. In addition, along with the transpiration stream the nodules of some plants (e.g. soyabean) export the fixed nitrogen as ureides. These compounds also have particularly high nitrogen to carbon ratio.

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