Plant Physiology - Musiienko M.M. 2001

Plant root nutrition
Nitrogen metabolism

Nitrogen, along with carbon, oxygen, and hydrogen, forms a group of so-called organogenic elements. Although the atmosphere is nearly 78% nitrogen, it is unavailable to most plants. Nitrogen does not support Respiration or combustion, which is why D. Rutherford called it lifeless, despite the fact that it is a constituent of Nucleic Acids, Proteins, and many other Organic compounds. Nearly 8 tons of nitrogen are concentrated above every square meter of the Earth's surface. The amount of nitrogen in nature is not limited to this. It is estimated that in the plowed soil layer to a depth of 30 cm per 1 ha, the amount of organic and mineral nitrogen in chernozem is 18 tons. However, the Main sources of Plant nitrogen Nutrition—nitrates and ammonium salts—do not exceed 1%. Therefore, no other element limits nutrient resources in ecosystems as much as nitrogen.

Academician D.M. Pryanishnikov dedicated almost his entire career to Structure/149.html">The problem of nitrogen in plant life and agriculture. Plant nitrogen nutrition remains a central issue in nutritional physiology, agrochemistry, microbiology, and agriculture.

Indeed, one of the most pressing challenges facing humanity today is the food shortage, primarily of feed and food proteins. Their main source is plant organisms, with their unique ability to synthesize proteins from CO₂, H₂O, and Inorganic Compounds, but for this, they desperately need nitrogen. Therefore, modern agriculture is unimaginable without The Use of nitrogen fertilizers. While in the early 1980s, global nitrogen consumption reached 60 million tons,

by 1985 it was 70 million tons, and in 2000 it reached 110-140 million tons. However, producing 1 ton of nitrogen fertilizer requires up to 4 tons of oil, meaning that nitrogen fertilizer production accounts for about 30-50% of agricultural energy consumption and about 1% of all energy consumed by developed nations.

Nowadays, industry produces approximately as much nitrogen compounds as are formed in nature. We do not yet know what the accumulation of nitrogen in the biosphere might lead to. Massive contamination of groundwater with nitrates is already occurring. The acceptable limit of nitrates is 300 mg/kg of human body weight.

That is why issues related to nitrogen assimilation by agricultural crops are of paramount theoretical and practical importance.

The sources of nitrogen for plants can be:

·molecular atmospheric nitrogen, which is assimilated only by certain species of microorganisms;

·nitrogen from ammonium salts and nitric acid salts, i.e., inorganic compounds (NH₄⁺, NO₃ );

·nitrogen from various organic compounds (urea, manure, PLANT AND ANIMAL residues).

Atmospheric nitrogen is fixed in the form of ammonium (NH₄⁺), which is then absorbed by the ROOT and assimilated inside Cells as nitrogenous organic compounds.

The process of decomposition of proteins, Amino Acids, and other organic substances in the soil is called ammonification, and the soil organisms that carry out this process are called ammonifiers. They are characterized by active Enzymes that intensively break down proteins into amino acids. The latter are deaminated to form ammonia.

The Mineralization of organic nitrogen, initiated by ammonification, is continued by the process of nitrification, during which aerobic nitrifying Bacteria (Nitrosomonas, Nitrobacter) convert ammonium into nitrates:

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and In the second stage, Nitrobacter oxidizes nitrite to nitrate, as nitrites are toxic to plants:

The intensity of nitrification is an indicator of soil fertility. Nitrates are the most optimal form of nitrogen for plant nutrition.

An important role in completing The Nitrogen Cycle is played by the processes of denitrification, during which denitrifying bacteria reduce nitrates and nitrites to molecular nitrogen. The reduction is catalyzed by nitrate reductase, a molybdenum-containing enzyme closely associated with the bacterial membrane. There are several denitrifying bacteria capable of reducing nitrates to molecular nitrogen (Pseudomonas, Micrococcus, Thiobacillus). In eukaryotes, nitrate reductase is an enzyme complex with a Molecular Weight of 200-300 kDa, which includes flavin (FAD), heme (cytochrome b₅₅₇), and molybdenum. Electrons coming from NADPH (due to respiration or Photosynthesis) are transferred to nitrate via the Electron Transport Chain of the enzyme complex:

Nitrate reductase is possibly localized in the Cytoplasm or loosely bound to the outer chloroplast membrane.

In connection with the problem of nitrate transport, the discovery of a membrane-bound form of nitrate reductase is important. In particular, Plasmalemma-bound nitrate reductase activity has been identified in Chlorella cells. Enzyme synthesis in The Cell occurs in response to the influx of NO₃ (Fig. 125).

Fig. 125. Proposed scheme of nitrate reductase synthesis

Induction of nitrate reductase synthesis is stimulated by nitrate, as well as by CARBOHYDRATES and Phytochrome (Fig. 126). During the enzymatic conversion of nitrate to nitrite, the substrate directly binds to the Mo²⁺-containing center of the enzyme before reduction. It is believed that the Active Site of nitrate reductase ensures the release of only one O₂ atom from nitrates, which prevents the denitrification process. The resulting nitrite binds to nitrite reductase via a nitrogen atom, and the entire process is completed without unnecessary loss of nitrogen. Nitrate reductase is the key enzyme in regulating nitrate reduction during nitrogen assimilation in higher plants (Figs. 127, 128).

Fig. 126. Regulation of nitrate reductase. The synthesis of nitrate reductase is stimulated by carbohydrates (such as glucose) and light (+) and inhibited by glutamine or Other Amino Acids (-)

Fig. 127. Interrelationship of metabolic processes in cell structures during nitrate assimilation in a Higher Plant Cell

Fig. 128. Reduction of nitrates to ammonia

Nitrite reductase ensures the reduction of nitrites to ammonia by transferring six electrons to NO₂:

Nitrate assimilation takes place in three stages: — uptake of nitrates into The plant cell;

Initial nitrate uptake is a process associated with the appearance of nitrate-specific permeases in the membrane. The reduction stages require an influx of electrons and protons, donated by NADH and reduced ferredoxin. The reduction process takes place in the roots and leaves of plants.

Nitrites formed During the first stage of nitrate reduction hardly accumulate; instead, they are rapidly reduced to ammonia by the enzyme nitrite reductase. The activity of this enzyme is always significantly higher (5–20 times) than that of nitrate reductase. Nitrite reductase utilizes reduced ferredoxin as an electron donor:

Nitrite reductase is a relatively low-molecular-weight protein comprising about 600 amino acid residues. This enzyme contains an unusual heme—siroheme tetrahydroporphyrin—which simultaneously accepts electrons and serves as the binding site for the substrate, and possibly for other intermediate reduction products on the pathway to ammonium (NH₄) formation. It has been established that The transfer of six electrons is catalyzed by a single enzyme, with nitrites being reduced to ammonia without the accumulation of free intermediates such as hyponitrite (HNO)₂ and hydroxylamine (NH₂OH). The process of nitrite reduction mediated by nitrite reductase, like The First stage of nitrate reduction, occurs in both leaves and roots. In leaves, nitrite reductase is localized in the METABOLISM/14.html">Chloroplasts (Fig. 129), where the electron donor ferredoxin is reduced during cyclic

Fig. 129. Interaction between the chloroplast and cytoplasm during nitrogen assimilation

Fig. 130. Probable scheme of interaction between metabolic processes in the root cortex and central cylinder during nitrogen assimilation and transport

transport within The electron transport chain of the thylakoid membrane during the light-dependent stage of photosynthesis. Ammonium ions inhibit nitrate assimilation by repressing the synthesis of nitrate and nitrite reductase enzymes.

Nitrites are also readily metabolized in the roots (Fig. 130), where nitrate reductase is associated with proplastids (the electron donor in this case remains unknown). It is hypothesized that the source of electrons in root Tissues may be NADPH₂, which is generated in the Pentose Phosphate Pathway during respiration.

There is a close relationship between nitrate reduction and photosynthesis:

Photosynthesis also acts as a source of ATP for the synthesis of nitrate and nitrite reductases and for The transport of nitrates into the cell (Fig. 131).

Fig. 131. Sequence of electron transfer in Photosystems and the possibility of its utilization in the reduction of nitrates to nitrites

It is interesting to note that in higher plants, alongside the reduction of nitrates to ammonia, the reverse process—The oxidation of the ammonium form of nitrogen to the nitrate form—also occurs (B.A. Yagodin). This refutes the widely held belief regarding the exclusively exogenous origin of plant nitrates.