The basic needs of all living organisms are essentially the same. They require macromolecules such as carbohydrates, proteins, fats, water, and minerals for their growth and development.
The basic needs of all living organisms are essentially the same. They require macromolecules such as carbohydrates, proteins, fats, water, and minerals for their growth and development. This essay focuses mainly on the mineral nutrition of plants.
ESSENTIAL MINERAL ELEMENTS
Most of the minerals nutrition present in soil can enter plants through the roots. In fact, more than sixty of the 105 elements discovered so far are found in different plants. Some plant species accumulate selenium, while some plants growing near nuclear test sites take up radioactive strontium.
There are techniques that are able to detect the minerals even at very low concentrations (10–8 g/mL). The criteria for the essentiality of an element are given. 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 seeds.
The requirement of the element must be specific and not replaceable by another element. In other words, a 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 on 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. (i) Macronutrients (ii) Micronutrients
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, sulphur, 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 are needed in very small amounts (less than 10 mmol/kg 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. 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 plant cells, such as permeability of the cell membrane, maintenance of the osmotic concentration of cell sap, electron transport systems, buffering action, enzymatic activity, and acting as major constituents of macromolecules and co-enzymes. Various forms and functions of essential nutrient elements are given.
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 plants from the soil in the form of phosphate ions (either as H2 PO4– or HPO2-4). Phosphorus is a constituent of cell membranes, certain proteins, and all nucleic acids and nucleotides and is required for all phosphorylation reactions.
Potassium:
It is absorbed as a potassium ion (K+). 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, the opening and closing of stomata, the activation of enzymes, and the maintenance of the turgidity of cells.
Calcium:
Plants absorb 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 wall, particularly as calcium pectate in the middle lamella.
Magnesium:
It is absorbed by plants in the form of divalent Mg2+. It activates the enzymes of respiration and photosynthesis, which are 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.
Sulphur:
Plants obtain sulphur in the form of sulphate. Sulphur is present in the two amino acids cysteine and methionine and is the main constituent of several coenzymes, vitamins (thiamine, biotin, and 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 oxidised 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 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 of plants. Like iron, it is associated with certain enzymes involved in redox reactions and is reversibly oxidised from Cu+ to Cu2+.
Boron:
It is absorbed as BO33- or B4 O2-7. Boron is required for the 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 (MoO2+2). It is a component of several enzymes, including nitrogenize and nitrate reductase, both of which participate in nitrogen metabolism.
Chlorine:
It is absorbed in the form of a 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 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 the critical concentration.
The element is said to be deficient when it is 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.
The deficiency symptoms vary from element to element, and they disappear when the deficient mineral nutrition is provided to the plant. However, if deprivation continues, it may eventually lead to the death of the plant. The parts of the plant that show deficiency symptoms also depend on the mobility of the element in the plant.
For elements that are actively mobilised 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 mobilisation in the 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 sulphur and calcium are part of the structural component of the cell and hence are not easily released. This aspect of the mineral nutrition of plants is of great significance and importance to agriculture and horticulture.
The kinds of deficiency symptoms shown in plants include chlorosis, necrosis, stunted plant growth, premature fall of leaves and buds, and inhibition of cell division. Chlorosis is the loss of chlorophyll that leads to yellowing of leaves.
This symptom is caused by a 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 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 toxic symptoms are difficult to identify.
Toxicity levels for any element also vary for different plants. Many times, excess of an element may inhibit the uptake of another element.
Mechanism Of Absorption Of Elements
Studies on the mechanism of absorption of elements by plants have been carried out in isolated cells, tissues, or organs. An 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 slowly taken into the ‘inner space, the simplest of the cells.
The passive movement of ions into the apoplast usually occurs through ion channels, the transmembrane proteins that function as selective pores. On the other hand, the entry or exit of ions 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 called influx, and the outward movement is called efflux.
Soil As a Reservior 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 harbours nitrogen-fixing bacteria and other microbes, holds water, supplies air to the roots, and acts as a matrix that stabilises the plant. Since a deficiency of essential minerals affects crop yield, there is often a need to supply them through fertilisers. Both macro-nutrients (N, P, K, S, etc.) and micro-nutrients (Cu, Zn, Fe, Mn, etc.) form components of fertilisers and are applied as per need.
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 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 converting 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).
Summary
Plants obtain their inorganic nutrients from air, water, and soil. Plants absorb a wide variety of mineral elements. Not all the mineral nutrition elements that they absorb are required by plants. Out of the more than 105 elements discovered so far, less than 21 are essential and beneficial for normal plant growth and development. The elements required in large quantities are called macronutrients, while those required in smaller quantities or in trace amounts are termed micronutrients.
These elements are either essential constituents of proteins, carbohydrates, fats, nucleic acids, etc. or take part in various metabolic processes. Deficiency of each of these essential elements may lead to symptoms called deficiency symptoms. Chlorosis, necrosis, stunted growth, impaired cell division, etc. are some prominent deficiency symptoms.
Plants absorb minerals through their roots through either passive or active processes. They are carried to all parts of the organism through the xylem, along with water transport. Nitrogen is very essential for the sustenance of life. Plants cannot use atmospheric nitrogen directly. But some plants, in association with N2-fixing bacteria, especially the roots of legumes, can fix this atmospheric nitrogen into biologically usable forms.
Nitrogen fixation requires a strong reducing agent and energy in the form of ATP. N2 fixation is accomplished with the help of nitrogen-fixing microbes, mainly Rhizobium. The enzyme nitrogenize, which plays an important role in biological N2 fixation, is very sensitive to oxygen. Most of the processes take place in an anaerobic environment. The energy, ATP, required is provided by the respiration of the host cells.
This article is jointly authored by Hafiz Muhammad Bilal from the Department of Horticulture, Auburn University, AL, USA; Amanullah Baloch from the National Key Lab of Crop Genetic Improvement and the College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, China; and Saleem Sajjad from the Department of Soil and Environmental Sciences, College of Agriculture, University of Sargodha, Sargodha, Pakistan.