Mineral Nutrition

Methods to Study the Mineral Requirements of Plants:

• In 1860, ==Julius von Sachs==, a prominent German botanist, demonstrated, for the first time, that plants could be grown to maturity in a ==defined nutrient solution in the complete absence of soil.==
  • This technique of growing plants in a nutrient solution is known as hydroponics.
• Since then, a number of improvised methods have been employed to try and determine the mineral nutrients essential for plants.
• The essence of all these methods involves the culture of plants in a soil-free, defined mineral solution.
  • These methods require purified water and mineral nutrient salts.
• After a series of experiments in which the roots of the plants were immersed in nutrient solutions and wherein an element was added/substituted/removed or given in varying concentrations, a mineral solution.

Essential Mineral Elements:

• Most of the minerals present in soil can enter plants through roots.
• ==In fact, more than sixty elements of the 105 discovered so far are found in different plants.==
  • Some plant species accumulate selenium, some others gold, while some plants growing near nuclear test sites take up radioactive strontium.
• There are techniques that are able to detect the minerals even at a very low concentration (10^-8 g/ mL).

Criteria for Essentiality:

• The criteria for the essentiality of an element are given below:
  • The element must be absolutely necessary for supporting ==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 and not replaceable by another element.
  • In other words, deficiency of any one element cannot be met by supplying some other element.
  • The element must be directly involved in the ==metabolism of the plant.==
• Based upon the above criteria only a few elements have been found to be absolutely essential for plant growth and metabolism.
• These elements are further divided into two broad categories based on their quantitative requirements.
  • Macronutrients are generally present in plant tissues in large amounts (in excess of 10 mmole Kg –1 of dry matter).
  • The macronutrients include ==carbon, hydrogen, oxygen, nitrogen, phosphorous, sulfur, potassium, calcium, and magnesium.==
  • ==Of these, carbon, hydrogen, and oxygen are mainly obtained from CO2 and H2O, while the others are absorbed from the soil as mineral nutrition.==
  • Micronutrients or trace elements are needed in very small amounts (less than 10 mmole Kg –1 of dry matter).
  • ==These include iron, manganese, copper, molybdenum, zinc, boron, chlorine, and nickel.==
  • In addition to the 17 essential elements named above, there are some beneficial elements such as ==sodium, silicon, cobalt, and selenium.==
  • They are required by ==higher plants.==
  • Essential elements can also be grouped into four broad categories on the basis of their diverse functions.
  • These categories are:
  • Essential elements as components of ==biomolecules== and hence structural elements of cells (e.g., carbon, hydrogen, oxygen, and nitrogen).
  • Essential elements that are components of energy-related chemical compounds in plants (e.g., magnesium in chlorophyll and phosphorous in ATP).
  • Essential elements that activate or inhibit enzymes, for example, ==Mg2+ is an activator for both ribulose bisphosphate carboxylase oxygenase and phosphoenol pyruvate carboxylase, both of which are critical enzymes in photosynthetic carbon fixation; Zn2+ is an activator of alcohol dehydrogenase and Mo of nitrogenase during nitrogen metabolism.==
  • Some essential elements can alter the osmotic potential of a cell.
  • Potassium plays an important role in the opening and closing of stomata.

Role of Macro- and Micro-nutrients:

• Essential elements perform several functions.
• They 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, and act as major constituents of macromolecules and co-enzymes.==
• Various forms and functions of essential nutrient elements are as follows.

Nitrogen:

• This is the essential nutrient element required by plants in the greatest amount.
• It is absorbed mainly as NO3 – though some are also taken up as NO2 – or NH4+.
• Nitrogen is required by all parts of a plant, particularly the meristematic tissues and the metabolically active cells.
• Nitrogen is one of the major constituents of proteins, nucleic acids, vitamins, and hormones.

Phosphorus:

• Phosphorus is absorbed by the plants from the soil in the form of phosphate ions (either as HPO2 4 − or HPO4 2−).
• Phosphorus is a constituent of cell membranes, certain proteins, all nucleic acids, and nucleotides, and is required for all phosphorylation reactions.

Potassium:

• It is absorbed as a potassium ion (K+ ).
  • In plants, this is required in more abundant quantities in the meristematic tissues, buds, leaves, and root tips.
  • Potassium helps to maintain an anion-cation balance in cells and is involved in protein synthesis, opening and closing of stomata, activation of enzymes, and the maintenance of the turgidity of cells.

Calcium:

• The plant absorbs calcium from the soil in the form of calcium ions (Ca2+).
  • Calcium is required by meristematic and differentiating tissues.
  • During cell division, it is used in the synthesis of the cell walls, particularly as calcium pectate in the middle lamella.
  • It is also needed during the formation of the 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 the enzymes of respiration, and photosynthesis and is involved in the synthesis of DNA and RNA.
  • ==Magnesium is a constituent of the ring structure of chlorophyll== and helps to ==maintain the ribosome structure.==

Sulfur:

• Plants obtain sulfur in the form of sulfate ( SO4 2−).
  • Sulfur is present in two amino acids – ==cysteine and methionine== and is the main 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 an @@important 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 the ==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 oxygen during photosynthesis.==

Zinc:

• Plants obtain zinc as Zn2+ ions.
  • It activates various 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 of plants.
  • Like iron, it is associated with certain enzymes involved in ==redox reactions and is reversibly oxidized from Cu+ to Cu2+.==

Boron:

• It is absorbed as BO3 3− or B O4 7 2−.

• Boron is required for the ==uptake and utilization of Ca2+, membrane functioning, pollen germination, cell elongation, cell differentiation, and carbohydrate translocation.==

  Molybdenum:

• Plants obtain it in the form of molybdate ions (MoO2 2+).

• It is a component of several enzymes, including ==nitrogenase and nitrate reductase== both of which participate in nitrogen metabolism.

  Chlorine:

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

  • Along with Na+ and 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==, a reaction that leads to oxygen evolution.

Deficiency Symptoms of Essential Elements:

• Whenever 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 ==critical concentration.==
  • The element is said to be deficient when present below the critical concentration.
  • Since each element has one or more specific structural or functional roles in plants, in the absence of any particular element, plants show certain morphological changes.
  • ==These morphological changes are indicative of certain element deficiencies and are called deficiency symptoms.==
    • The deficiency symptoms vary from element to element and they disappear when the deficient mineral nutrient is provided to the plant.
  • However, if deprivation continues, it may eventually lead to the ==death of the plant.==
  • The parts of the plants that show the deficiency symptoms also ==depend on the mobility of the element in the plant.==
  • For elements that are actively mobilized within the plants and ==exported to young developing tissues, the deficiency symptoms== tend to appear first in the older tissues.
  • For example, the ==deficiency symptoms of nitrogen, potassium, and magnesium are visible first in the senescent leaves.==
  • In the older leaves, biomolecules containing these elements are broken down, making these elements available for mobilizing to younger leaves.
  • The deficiency symptoms tend to appear first in the young tissues whenever the elements are relatively immobile and are not transported out of the mature organs, for example, elements like ==sulfur and calcium are a part of the structural component of the cell and hence are not easily released.==
  • This aspect of mineral nutrition of plants is of great significance and importance to agriculture and horticulture.
• The kind of deficiency symptoms shown in plants includes ==chlorosis, necrosis, stunted plant growth, premature fall of leaves and buds, and inhibition of cell division.==
  • ==Chlorosis is the loss of chlorophyll leading to yellowing in leaves.==
  • This symptom is caused by the deficiency of elements ==N, K, Mg, S, Fe, Mn, Zn, and Mo.==
  • Likewise, ==necrosis, or death of tissue,== particularly leaf tissue, is due to the deficiency of ==Ca, Mg, Cu, and K.==
  • ==Lack or low levels of N, K, S, and Mo cause inhibition of cell division==.
  • Some elements like N, S, and Mo ==delay flowering if their concentration in plants is low.==

Toxicity of Micronutrients:

• The requirement of micronutrients is always in low amounts while their moderate decrease causes the deficiency symptoms and a moderate increase causes toxicity.
  • In other words, there is a narrow range of concentration at which the elements are optimum.
  • Any mineral ion concentration in tissues that ==reduces the dry weight of tissues by about 10 percent is considered toxic.==
    • Such critical concentrations vary widely among different micronutrients.
• The toxicity symptoms are difficult to identify.
  • Toxicity levels for any element also vary for different plants.
• Many times, an excess of an element may inhibit the uptake of another element.
  • For example, the ==prominent symptom of manganese toxicity is the appearance of brown spots surrounded by chlorotic veins.==
• It is important to know that ==manganese competes with iron and magnesium== for uptake and with magnesium for binding with enzymes.
  • Manganese also ==inhibits calcium translocation in the shoot apex.==
  • Therefore, excess manganese may, in fact, induce deficiencies in iron, magnesium, and calcium.
  • Thus, what appears as symptoms of manganese toxicity may actually be the deficiency symptoms of iron, magnesium, and calcium.

Mechanism of Absorption of Elements:

• Much of the studies on the mechanism of absorption of elements by plants have been carried out in isolated cells, tissues, or organs.
  • These studies revealed that the process of absorption can be demarcated into two main phases.
  • ==In the first phase, the initial rapid uptake of ions into the ‘free space’ or ‘outer space’ of cells – the apoplast, is passive. In the second phase of uptake, the ions are taken in slowly into the ‘inner space’ – the symplast of the cells.==
  • The passive movement of ions into the apoplast usually occurs through ion channels, the trans-membrane proteins that function as selective pores.
  • On the other hand, the entry or exit of ions to and from the symplast requires the expenditure of metabolic energy, which is an active process.
• The movement of ions is usually called flux; the inward movement into the cells is in flux and the outward movement, is efflux.

Translocation of Solutes:

• Mineral salts are translocated through the xylem along with the ascending stream of water, which is pulled up through the plant by transpirational pull.
  • Analysis of xylem sap shows the presence of mineral salts in it.

Soil as Reservoir of Essential Elements:

• The majority of the nutrients that are essential for the growth and development of plants become available to the roots due to weathering and the breakdown of rocks.
  • These processes enrich the soil with dissolved ions and inorganic salts.
• Since they are derived from rock minerals, their role in plant nutrition is referred to as mineral nutrition.
• Soil consists of a wide variety of substances.
  • Soil not only supplies minerals but also harbors nitrogen-fixing bacteria, and other microbes hold water, supply air to the roots, and act as a matrix that stabilizes the plant.
  • Since deficiency of essential minerals affects crop yield, there is often a need for supplying them through fertilizers.
• Both macro-nutrients (N, P, K, S, etc.) and micro-nutrients (Cu, Zn, Fe, Mn, etc.) form components of fertilizers and are applied as per need.

Metabolism of Nitrogen:

Nitrogen Cycle:

• Apart from carbon, hydrogen, and oxygen, nitrogen is the most prevalent element in living organisms.

  • Nitrogen is a constituent of amino acids, proteins, hormones, chlorophyll, and many vitamins.
  • Plants compete with microbes for the limited nitrogen that is available in the soil.
  • Thus, nitrogen is a limiting nutrient for both natural and agricultural ecosystems.
  • Nitrogen exists as two nitrogen atoms joined by a very strong triple covalent bond (N ≡ N).
    • The process of conversion of nitrogen (N2 ) to ammonia is termed nitrogen fixation.
  • In nature, lightning and ultraviolet radiation provide enough 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.
    • The 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:

  

• Ammonia is first oxidized to nitrite by the bacteria Nitrosomonas and/or Nitrococcus.

  • The nitrite is further oxidized to nitrate with the help of the bacterium Nitrobacter.
  • These steps are called nitrification.
  • These nitrifying bacteria are chemoautotrophs.
  • The nitrate thus formed is absorbed by plants and is transported to the leaves.
  • In leaves, it is reduced to form ammonia which finally forms the amine group of amino acids.
  • Nitrate present in the soil is also reduced to nitrogen by the process of denitrification.
  • Denitrification is carried by the bacteria Pseudomonas and Thiobacillus.

Biological Nitrogen Fixation:

• Very few living organisms can utilize nitrogen in the form of N2, available abundantly in the air.
  • Only certain prokaryotic species are capable of fixing nitrogen.
  • The reduction of nitrogen to ammonia by living organisms is called biological nitrogen fixation.
  • The enzyme, nitrogenase which is capable of nitrogen reduction is present exclusively in prokaryotes.
  • Such microbes are called N2 - fixers.
    • The nitrogen-fixing microbes could be free-living or symbiotic.
    • Examples of free-living nitrogen-fixing aerobic microbes are Azotobacter and Beijerinckia while Rhodospirillum is anaerobic and free-living.
    • In addition, a number of cyanobacteria such as Anabaena and Nostoc are also free-living nitrogen-fixers.

Symbiotic biological nitrogen fixation:

• Several types of symbiotic biological nitrogen-fixing associations are known.
• The most prominent among them is the legume-bacteria relationship.
  • Species of rod-shaped Rhizobium have a relationship with the roots of several legumes such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans, etc.
  • The most common association with roots is nodules.
• These nodules are small outgrowths on the roots.
  • The microbe, Frankia, also produces nitrogen-fixing nodules on the roots of nonleguminous plants (e.g., Alnus).
  • Both Rhizobium and Frankia are free-living in soil, but as symbionts, can fix atmospheric nitrogen.

Nodule Formation:

• Nodule formation involves a sequence of multiple interactions between Rhizobium and the roots of the host plant.
• Principal stages in the nodule formation are summarised as follows:
  • Rhizobia multiply and colonise the surroundings of roots and get attached to epidermal and root hair cells.
  • The root-hairs curl and the bacteria invade the root hair.
  • An infection thread is produced carrying the bacteria into the cortex of the root, where they initiate the nodule formation in the cortex of the root.
  • Then the bacteria are released from the thread into the cells which leads to the differentiation of specialized nitrogen-fixing cells.
  • The nodule thus formed, establishes a direct vascular connection with the host for the exchange of nutrients.
• The nodule contains all the necessary biochemical components, such as the enzyme nitrogenase and leghaemoglobin.
  • The enzyme nitrogenase is a Mo-Fe protein and catalyzes the conversion of atmospheric nitrogen to ammonia, the first stable product of nitrogen fixation.
  • The enzyme nitrogenase is highly sensitive to molecular oxygen; it requires anaerobic conditions.
• The nodules have adaptations that ensure that the enzyme is protected from oxygen.
  • To protect these enzymes, the nodule contains an oxygen scavenger called leghemoglobin.
• It is interesting to note that these microbes live as aerobes under free-living conditions (where nitrogenase is not operational), but during nitrogen-fixing events, they become anaerobic (thus protecting the nitrogenase enzyme).
  • The ammonia synthesis by nitrogenase requires a very high input of energy (8 ATP for each NH3 produced).
• The energy required, thus, is obtained from the respiration of the host cells.

The fate of ammonia:

• At physiological pH, the ammonia is protonated to form NH4 + (ammonium) ion.
• While most plants can assimilate nitrate as well as ammonium ions, the latter is quite toxic to plants and hence cannot accumulate in them
• There are two main ways in which this can take place:
  • Reductive amination: ==In these processes, ammonia reacts with α-ketoglutaric acid and forms glutamic acid.==
  • Transamination: It involves the transfer of amino groups 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,== the amino group takes place and other amino acids are formed through transamination.
  • The enzyme transaminase catalyzes all such reactions.
• The two most important amides – ==asparagine and glutamine – found in plants are a structural part of proteins.==
  • They are formed from two amino acids, namely aspartic acid and glutamic acid, respectively, by the addition of another amino group to each.
• The hydroxyl part of the acid is replaced by another NH2 – radicle.
• Since amides contain more nitrogen than 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., soybean) export the fixed nitrogen as ureides.
• These compounds also have a particularly high nitrogen-to-carbon ratio.

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