Plants are multicellular organisms that, except water and light, require some essential elements to grow, develop, produce, and complete their life cycle. Essential nutrients are the definition of these elements. According to scientists, 16 essential nutrients are necessary. Οther essentials like Sodium (Na), Silicon (Si), Cobalt (Co), Nickel (Ni), Iodine (I), and Selenium (Se) are not always necessary for a plant to survive. Still, they should be good for humans and animal’s health.
Macronutrients and micronutrients
The essentials are classified into macronutrients and micronutrients, depending on the quantity that the plant needs to accumulate. Nutrients that plants require in more massive amounts are called macronutrients, such as Carbon (C), Hydrogen (H), Oxygen, (O) Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg) and Sulfur (S). Besides, the elements that are required in smaller amounts by the plants are called micronutrients, such as Boron (B), Zinc (Zn), Manganese (Mn), Iron (Fe), Copper (Cu), Molybdenum (Mo), and Chloride (Cl).
Plant uptake of nutrients can only proceed when they are present in a plant-available form. The nutrient must be in the form of either a positively charged ion (cation) or a negatively charged ion (anion). A plant cannot use organic compounds, such as those in manure or dead leaves until they are broken down into their elemental or ionic forms.
The basic role
Plants may need higher amounts of some nutrients than others, but that doesn’t mean that the nutrients in small quantity need are insignificant. So, let’s talk about the potential of each essential nutrient.
It constitutes 45% of the dry weight of the plants. All the substances that plants produce (carbohydrates, fats, proteins) are small or large chains of carbon atoms. With hydrogen and oxygen, it constitutes ≈ 94% of living organisms. It is recruited as CO² by the leaves. Plants take in carbon dioxide and convert it into energy for growth. When the plant dies, carbon dioxide is given off from the decomposition of the plant.
Oxygen (O) is responsible for cellular respiration in plants. The energy required for the various parts results from the oxidation of the carbon chains within the cell. It constitutes 43% of the plants’ dry weight, and it is absorbed mainly as O2 from the leaves and furthermore, as CO2, SO4, CO3, and OH‾. This element has a critical role in photosynthesis, as well as its storage for energy. It is also released as a by-product.
It constitutes 6% of the dry weight of the plants. Its absorption is as an ion (H +) and water molecules (H2O). Its main contribution consists of building the carbon atom chains, following the carbon in the plant’s metabolic pathways. This process happens during the photosynthesis process and releases oxygen into the atmosphere used by all living beings.
It is the fourth most frequently encountered element. Proteins contain 18% N. It is a component of amino acids, proteins, coenzymes, nucleic acids, pyrimidines, and chlorophyll. Each chlorophyll molecule has a central Mg atom that places four Pyrrole rings, carrying an N atom and 4 C atoms. Around 70% of Ν is present in chloroplasts, the compound by which plants use sunlight energy to produce sugars from water and carbon dioxide. This element is responsible for the vegetation and fruit set.
The root system absorbs it as NO−3 or a NO4- ion. Absorption of one form or another depends on the type of plant, soil type, temperature, and other factors. In very acidic soils, the plants prefer NO−3, while in alkaline, they prefer NO4-. Its uptake is also carried out as urea, amino acids, and nucleic acids.
It is a component of high energy compounds (ATP, ADP, AMP) and Nucleic acids, Phytic acids, coenzymes, and phospholipids. P has an essential role in the deposit of reproductive organs. The newly developing fruits will store large quantities of P, especially in their seeds, while much smaller amounts will be in the ripe fruits. It also affects root growth and achieves plant maturity, and that’s why its presence in the early stages of plant development is significant. Compounds of P in the cell also act as a pH regulator. Moreover, It participates in the metabolism of carbohydrates, fats, and proteins, and it plays a vital and essential role in the energy metabolism of the plant cell.
P’s absorption can be as an H2PO4‾ in case that PH is less than 7, or as an HPO4(2-) in case that PH is more than 7. In case that PH is equal to 7, the absorption can be with both formulations. Remarkably, in low temperatures, the absorption of P is too challenging to be achieved.
Its role could be said to be multifaceted. It participates in the metabolism of carbohydrates, especially in the synthesis and decomposition of starch, N’s metabolism, and protein synthesis. Also, it neutralizes organic acids and regulates the action of various elements.
Potassium regulates the opening and closing of the stomata as well as the water economy of the plant. Moreover, it affects the quality of the fruit and the plant’s resistance to diseases, while its lack reduces photosynthesis and increases respiration and protein nitrogen concentration. More than 50 enzymes are either utterly dependent on or activated by potassium ions. It is absorbed by the root system, like potassium ion (Κ+).
Magnesium is part of the chlorophyll molecule (2.7%); it is essential for synthesizing oils. Simultaneously, as a structural component, it is crucial for maintaining the organization and activity of the following organisms: chloroplasts, mitochondria, ribosomes, and nuclei.
Mg is also present in storage tissues, especially in seeds, while significant amounts are also present in phytin. It enters the processes of active transport, during the absorption mainly of monovalent cations (K +, Na +, etc.). It is also necessary to activate almost all enzymes that take part in the transport of phosphate radicals.
The presence of Mg is requirable for the synthesis of ATP and ADP. It is taken as a cation (Mg ++).
Calcium is responsible for elongation and cell division, forming the mitotic spindle and meristems’ development. It’s also responsible for the germination of pollen and the extension of the pollen tube. Generally plays a vital role in all meristematic zones with increasing phenomena.
Calcium’s other potential is to participate in cell walls formation, where it is located as pectin. It is acting as a fixative while determining tissues’ susceptibility to fungal infections and fruit ripening. It increases the activity of enzymes, affects the transport of carbohydrates, and neutralizes the action of high concentrations of other elements. Moreover, it regulates the PH and helps activate the ATPases of cell membranes associated with ions’ active transport. Simultaneously, it is considered that it takes part in the movement of plant hormones through cell membranes.
Ca regulates the uptake of K, Na, and Mg and combines with acids where it forms insoluble salts, protecting the cell from toxic effects. It is also responsible for the respiration and production of ethylene. The root system of plants absorbs it as a cation (Ca++)
Sulfur is a constituent of three S- containing amino acids (cysteine, cystine, and methionine), the protein building blocks. That means that its presence is necessary for protein synthesis. Other functions of S in plants are oil synthesis, enzyme activation, and chlorophyll formation. It also plays an important role in the defense of plants against stresses and pests. Plants can absorb it from the root system as sulfate anion (SO₄²-) and foliar from the atmosphere as SO2.
It participates in sugar transportation along cell membranes. The free molecules of sugars, due to their polarity, can not penetrate the cell membranes, and in this, it serves the formation of chemical compounds with boron. It has a vital role in cell division and pectin synthesis, while its role in RNA, DNA synthesis, and glycolysis is also essential. Boron is needed to synthesize nitrogenous bases, including uracil, which is a critical component of RNA.
It affects the water economy, sugar metabolism, flowering, and fruiting. Also, it increases carbohydrate content, promotes growth, and increases pollen viability. It is taken in the following forms by plants: BO33−, B4O7-2, H2BO3 ‾, HBO3–, with the most common being the first 2.
Participates in the formation of the chlorophyll molecule, without being part of it. Also, it is necessary to maintain the structure and function of the chloroplast. Its role is essential in the process of DNA, respiration, photosynthesis, and redox systems. It activates various enzymes and plays some role in the binding of atmospheric N.
The absorption of Fe can be as divalent or trivalent cations (Fe++, Fe+++) but more commonly as divalent.
70% of the total Copper of the leaves exists in chloroplasts. It is a component of redox enzymes and proteins while acting as an activator in several enzymes. It acts as a catalyst in the synthesis of chlorophyll and the metabolism of carbohydrates and proteins. With iron and boron, it participates in the biosynthesis of phenols.
It is a key component of plastocyanin. Plastocyanin is a component of the electron transport chain in photosystem I (PSI). Thus, there is a decrease in the plastocyanin concentration and a photosynthetic activity reduction in Cu deficiency conditions.
The plants absorb this essential as a cation (Cu++).
Molybdenum is a component of the enzymes’ nitrogenase (binding of atmospheric nitrogen) and nitrate reductase (reducing nitrates to nitrites). In vascular plants, only the above two enzymes carry an active Mo group.
Mo plays an essential role in those enzyme systems involved in the binding and transport of N. It also stimulates the biosynthesis of nucleic acids and proteins. It increases the content of chlorophyll and vitamins while participating in phosphorus metabolism. The plants absorb it as a molybdenum anion (MoO4–).
It is a micronutrient with an essential role in the plants. It activates carbohydrate metabolism enzymes, redox enzymes while playing an integral role in photosystem II (PSII), and oxygen-releasing reactions. Mn participates as an electron carrier and catalyzing water-splitting reaction in photosystem II.
It is also found in chloroplasts in the form of an enzyme carried by an Mn atom. This enzyme participates in the photolysis of water and has its primary function to protect the photosynthetic mechanism from the toxic effects of oxygen. Finally, it regulates the available amount of iron in leaves, while it is necessary for nitrates reduction. The root system takes it up as a cation (Mn ++).
Zinc is essential in the formation of auxins, which help with growth regulation and stem elongation. It is also crucial in the formation of some carbohydrates and the formation of chlorophyll. Moreover, it plays an important role in redox reactions while participating in protein and RNA metabolism.
The plants can absorb it as a cation (Zn++).
Chloride involves in photosynthesis reactions that lead to the photolysis of water and the release of oxygen. It is also related to the root system’s development, and the plants take it as an anion (Cl-). Moreover, Chloride determines greater leaf expansion, increased fresh and dry biomass, increased elongation of leaf and root cells, improved water relations, and better water- and nitrogen-use efficiency.
Knowing its exact concentration in water and soil is of great importance because it contributes to salinity formation. In high concentrations, it is toxic to plants, and it reduces the yield.