Plant Physiology - Musiyenko M.M. 2001

Root Nutrition of Plants
Mineral Nutrition. Classification of Mineral Elements

Mineral Nutrition is The process of absorption and assimilation of chemical elements from the environment that are essential for the vital activity of a plant Organism.

MODERN CONCEPTS OF Plant Mineral Nutrition trace their roots back to Aristotle's doctrine of plant nutrition via earth juices (Aristotle, 384 BC), the Water theory of nutrition by J.B. van Helmont (1629), and the humus theory of plant nutrition proposed by A. Thaer in the late 18th and early 19th centuries. Despite precise experiments by the Swiss researcher N.T. de Saussure (1804) proving that soil is the source of mineral nutrition for plants, the humus theory remained dominant until the 1840s.

The humus theory was superseded by the diametrically opposed doctrine of J. Liebig (1840). He argued that only inorganic soil substances possess nutritional value for plants, completely rejecting the value of humus, and believed that plants obtain all minerals, including nitrogen oxide, from the soil. In his view, all Mineral Substances absorbed by a plant must necessarily be returned to the soil. J. Liebig formulated this principle as the law of restitution, emphasizing that all removed substances must be returned to the soil through fertilizers.

J. Liebig also substantiated the application of fertilizers through his established law of the minimum, according to which crop yield increases when fertilizers supply the element that is present in the soil in a relatively minimal amount.

The humus theory was refuted by the experiments of W. Knop and J. Sachs (1859), whose empirical evidence substantiated The Theory of mineral nutrition. They laid the foundation for the application of the vegetative method, including water and sand cultures.

Nutritive elements are those chemical elements that are required by the plant and cannot be replaced by any others. Any chemical element present in a given soil zone can also be found within the plant. The mineral fraction of plants (ash) ranges from 0.2% to 20% of the dry matter (Table 15). However, The chemical composition of ash does not reflect the plant's nutritional requirements. Plant requirements for mineral substances are determined by growing plants in nutrient solutions of specific composition.

Table 15. Ash composition of various agricultural crops (as a % of total ash)

The methodology for growing plants in water cultures was developed by W. Knop. The nutrient solution he formulated for water cultures is known as Knop's nutrient solution and has the following composition per 1 L of water:

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Alongside water cultures, sand cultures are widely used to study plant requirements for ash elements. Their advantage over water cultures is that the roots develop in an environment closely resembling natural conditions. The nutrient solution for sand cultures was created by D.M. Pryanishnikov:

D.M. Pryanishnikov's mixture is also used for water cultures. He substantiated the combined application of organic and mineral fertilizers, established the features of Nitrogen METABOLISM, and developed various Methods for Fertilization and studying plant nutrition (such as sterile culture, flowing solution, and Physicochemical methods of soil and plant analysis).

Based on the quantitative composition of mineral elements in plant Tissues, they are divided into the following groups:

Macronutrients — their quantitative content ranges from tenths to hundredths of a percentage point. In addition to organogens (C, O, H, N), this group includes silicon, potassium, magnesium, sodium, phosphorus, sulfur, and aluminum. Iron lies on the boundary between macro- and micronutrients.

Micronutrients — their content ranges from thousandths to hundred-thousandths of a percentage point: manganese, boron, copper, zinc, barium, nickel, molybdenum, cobalt, etc. (Table 16).

Table 16. Micronutrient content in plants (per 100 g of absolutely dry matter, mg)

Ultramicronutrients — with a content on the order of parts per million of a percentage point: cesium, cadmium, silver, radium, etc.

Significant amounts of mineral nutrient ions are incorporated as Structural components of plant organic substances: C, H, and O are universal components of Organic compounds, while nitrogen and sulfur are components of Proteins, Nucleic Acids, and Porphyrins.

Iron, manganese, zinc, molybdenum, and cobalt are constituents of Enzymes or their Cofactors. Molybdenum and cobalt are involved in Nitrogen Fixation, Mo in nitrate reduction, and Mn in water photolysis. Iron is essential for chlorophyll synthesis, and chloride ions participate in photosynthetic oxygen evolution. Elements such as Fe, Mn, Cu, Mo, and Co are part of the active sites or prosthetic group components of enzymes—especially oxidoreductases—that drive Photosynthesis and Respiration (Flavoproteins, ferredoxins, Cytochromes, plastocyanin, phenoloxidases, etc.). As cofactors, they act as auxiliary elements in chelate formation and ensure the binding of enzymes or Coenzymes with substrates (manganese, magnesium, zinc) (Table 17).

Table 17. Content of essential inorganic nutrients in plants

Phosphorus and boron occur in the form of phosphoric and boric acids (ATP, sugar phosphates, nucleic acids).

Potassium, magnesium and calcium primarily affect the Hydration of protoplasmic colloids. Potassium influences The activity of nearly 60 enzymes. Salts of Ca²⁺ and Mg²⁺ are Components of the middle lamellae (pectates, phytin). Magnesium often acts as a structural stabilizer in Ribosomes, while calcium plays the same role in Chromosomes and membranes.

Thus, The primary function of ions in metabolism is structural and catalytic. Overall, mineral elements in plants can perform both specific and non-specific Functions. For instance, all ions that regulate water balance are interchangeable, and therefore perform a non-specific function in osmotic pressure regulation.

Many mineral elements are essential parts of biological molecules or components of enzyme systems, which is a highly specific function.

The form in which the major nutrients are utilized by plants:

·carbon, hydrogen, and oxygen — in the form of CO₂, H₂O, O₂.

·non-metals — in the form of anions: nitrates, sulfates, phosphates (NO₃^-, SO₄^2-, H₂PO₄^-, PO₄^3-); nitrogen also in the form of the ammonium cation NH₄⁺.

·metals — in the form of cations; additionally, molybdenum — in the form of MoO₄^2-, boron — in the form of BO₃^2-, chlorine — Cl^-.