Nitrogen in Relation to Physiology, Growth and Response of Plants

After reading this article you will learn about nitrogen in relation to physiology, growth and response of crops.

Nitrogen in Relation to Physiology of Plants

Nitrate (NO₃—N) and ammonium (NH₄—N) are two major forms of inorganic nitrogen taken up by the plants. Most of the ammonium has to be incorporated into organic compounds in the plant roots, whereas nitrate is readily mobile in the xylem and can also be stored in the vacuoles of roots, shoots and different storage organs.

Nitrate accumulation in vacuoles is believed to be of considerable importance for cation-anion balance for osmoregulation, especially in nitrophilic plant species (e.g. Chenopodium album) and also for the quality of vegetables and forage crops. However, for the incorporation of N into organic structures of the plants as well as fulfilling its essential functions, nitrate form has to be reduced to ammonia.

Nitrate reduction in plants occurs by the following reactions:

NO₃^– + 8H⁺ + 8e^– → NH₃ + 2H₂O + OH^–

The reduction of nitrate to ammonia is carried out by two enzymes:

(i) Nitrate reductase (NR) which involves the two-electron reduction of nitrate to nitrite, and

(ii) Nitrite reductase (NiR) which involves for the conversion of nitrite to ammonia in a six-electron reduction.

Nitrite reductase is a monomeric polypeptide of about 60-63 kDa (Killo-Dalton) containing a siroheme prosthetic group. In contrast to nitrate reductase which is localised in the cytoplasm, nitrite reductase is localised in the chloroplasts in leaves and in the proplastids of roots and other non-green plant tissues.

In green leaves the electron donor is reduced forredoxin generated in the height by photosystem I. In the dark and particularly in the roots and other non-green plant tissues a protein similar to ferredoxin may serve in this function. Nitrite reductase rarely accumulates in intact plants under normal conditions.

In different plants, both roots and shoots are able to reduce nitrate and the rate of such reduction in shoots and roots depends on various factors namely level of nitrate supply, plant species, age of the plant etc.

Nitrate reduction has some important consequences for the mineral nutrition as N is an indispensable constituent of various organic N compounds of general importance (amino acids, proteins, and nucleic acids) and it also helps in carbon economy of plants.

When the external nitrate supply (soil solution) is low, a higher amount of nitrate is reduced in the roots. With an increasing supply of nitrate, the ability of roots for the reduction of nitrate becomes a limiting factor and an increasing amount of the total nitrogen is trans located to the shoots in the form of nitrate.

Reduction and assimilation of nitrate have a high energy requirement and are costly processes when carried out in roots. However, in relation to ATP equivalents, 15 mol of ATP requires for the reduction of 1 (one) mol of NO₃^– and an additional 5 mol of ATP requires for ammonia assimilation.

In roots, nitrate reductase activity is high in expanding cells of the apical zones and declines rapidly towards the basal root zones. Because of low mobility of nitrate in the phloem, in fully expanded leaves with low nitrate reductase activity, high nitrate contents are of limited use for the nitrogen metabolism of plants.

The rate of release of nitrate from the vacuoles in leaf cells is increased by decreasing nitrate import into the leaf, the release from the vacuole into the cytoplasm can become a rate limiting step for nitrate reduction and thus for the utilisation of stored nitrate nitrogen in growth processes of plants. Interruption of nitrate supply to the roots may, therefore, lead to a drop in both nitrate reductase activity in leaves.

Excessive nitrate assimilation in shoot might cause osmotic problems if nitrate-reduction continues after the termination of leaf cell expansion.

However, various mechanisms exist for the removal of excess osmotic solutes from the short tissue:

(i) Precipitation of excess solutes in an osmotically inactive form. Synthesis of oxalic acid for charge compensation in nitrate reduction and precipitation as calcium oxalate are common in plants.

(ii) Re-translocation of reduced nitrogen (amino-acids and amides) together with phloem- mobile cations, viz. K and Mg to areas of new growth.

(iii) Re-translocation of organic acid anions namely, malate together with potassium into the roots and release of an anion (OH^– or HCO₃^–) after decarboxylation.

Ammonium and in particular its equilibrium partner ammonia,

NH₃(dissolved in water) DNH₄⁺ + OH^–

Ammonium Assimilation:

Ammonium Assimilation is toxic at low concentrations. The formation of amino-acids, amides and related compounds is the main pathway of detoxification of either ammonium ions (NH₄⁺) taken up by the roots or ammonia derived from the reduction of nitrate or N₂ fixation.

The main steps in the assimilation of ammonium ions supplied to the roots are uptake into the root cells and incorporation into amino acids and amides with simultaneous release of protons (H⁺) for charge compensation.

As shoots have limited capacity for disposal of protons, all of the ammonium taken up are assimilated in the roots and transported in the xylem as amino acids and amides to the shoots. In rice growing under waterlogged conditions, a substantial proportion of the ammonium (NH₄⁺) may be transported to, and assimilated in the shoots.

Ammonium assimilation in roots has a large requirement for carbon skeletons for amino acid synthesis. These carbon skeletons are provided by the tri-carboxylic acid cycle (TCA), and the removed intermediates have to be replenished by increased activity of phosphoenol pyruvate (PEP) carboxylase.

In order to minimise the carbon costs for root to shoot transport, the bulk of the nitrogen assimilated in the roots is transported as nitrogen-rich compounds with N: C ratios > 0.4. Although ammonia assimilation occurs in different sites like roots, root nodules and leaves, the key enzymes involved in each case are glutamine synthetase and glutamate synthase.

Both enzymes are found in roots in chloroplasts and in N₂-fixing micro-organisms and assimilation of ammonia derived from various processes like uptake of ammonium, N₂-fixation, nitrate reduction and photorespiration etc. is mediated by the glutamine synthetase glutamate synthase pathway.

Amino Acid and Protein Biosynthesis:

The organically bound nitrogen of glutamate and glutamine are used for the synthesis of their amides as well as ureides, amino acids, amines, peptides, and high molecular weight peptides such as proteins. Although plants may contain about 200 different amino acids, only about 20 of them are required for protein synthesis.

In protein synthesis, the individual amino acids are coupled by peptide bonds (R₁—CO—NH—R₂) as the following condensation reaction:

Proteins are polypeptides formed from more than 100 individual amino acids, and their sequence is determined by the genetic information carried by double stranded molecules. Expression of the genetic information starts in the nucleus of the cell with the synthesis of messenger ribonucleic acid (mRNA) which is a single stranded copy of an activated DNA fragment (transcription).

The mRNA diffuses into the cytoplasm and becomes attached by ribosomes, the “factories” of protein biosynthesis. Ribosomes allow two molecules of aminoacyl tRNA to bind to the mRNA according to their codon at the top. Now the two amino acid residues are in position to be enzymatically linked by the formation of a peptide bonding.

Subsequently, the ribosome moves on along the mRNA, releases the first tRNA and allows the next aminoacyl tRNA to bind to the mRNA-resulting in an elongation of the peptide. This translocation process is terminated when the ribosome reaches the stop signal demanding its breakdown and the release of the peptide chain.

Low Molecular Weight Organic N Compounds:

In higher plants low molecular weight organic nitrogen compounds listed below not only act as intermediates between the assimilation of inorganic nitrogen and the synthesis— or degradation of high molecular weight N compounds. Of the low molecular weight nitrogen compounds mainly amino acids and amides act as buffer and transient storage besides functioning in long distance transport of reduced nitrogen.

Recently it has been reported that polyamines act as secondary messengers and protect membranes. In addition, polyamines are involved in cell division, embryogenesis, floral initiation and development. Polyamines are also effective in delaying senescence of leaves by inhibiting the activity of acid proteniases.

Polypeptide synthesis in plants is frequently induced when plants are subjected to high concentrations of heavy metals, particularly of Cd and due to such polypeptide synthesis containing sulphur containing amino acid cysteine, large amounts of heavy metals line Cd are bound in it forming “phytochelatins” which may play an important role in heavy metal detoxification.

Nitrogen Uptake and Responses:

Both nitrate (NO₃^–) and ammonium (NH₄⁺) forms of N are taken up and metabolised by plants. Nitrate is often a preferential source for crop growth depending on plant species and other environmental conditions. Upland arable crops mainly take up NO₃ —N even when NH₄⁺ fertilizers are applied, because of microbial oxidation of the NH₄⁺ in the soil.

The rate uptake of NO₃—N is generally very high as plants require large amounts of N. It is now established that in this uptake process there is both an influx and an efflux component.

Influx of NO₃^– is an active process with NO₃^– moving against an electro-chemical gradient and dependent on the NO₃^– concentration in the soil solution, Nitrate (NO₃^–) efflux was thought to be passive, a leakage of NO₃^– resulting from moving down its electro-chemical gradient.

Nitrate efflux is a carrier mediated process, dependent on the internal NO₃^– concentration and hence on NO₃^–reduction and translocation. However, NO₃^– efflux can define the overall rate of net uptake. It is evident that ammonium (NH₄⁺) may depress NO₃^–uptake by stimulation of NO₃^– efflux.

The plant growth and yield with the supply of either ammonium or nitrate depends on various factors. An important factor is the plant species, calcifuges-plants adapted to acid soils and low soil redox potential (e.g. wetland rice) have a preference for ammonium whereas, calcicoles—plants with a preference for calcareous, high pH soils-utilise nitrate preferentially.

However highest growth rates and yield of crops are obtained by combined supply of both NH₄⁺ and NO₃^–.

A most important difference between the uptake of NO₃— N and NH₄—N is in their sensitivity to pH. The uptake of NH₃—N takes place best in a neutral soil pH and it is depressed as pH falls, whereas more rapid uptake of NO₄—N takes place in soils having low pH values and such lower uptake of NO₄—N at high pH values may be due to the competitive effect of OH^– ions suppressing the NO₃ uptake transport system.

The form of N supply has a strong but adverse impact on the uptake of other cations and anions, on cellular pH regulation and on rhizosphere pH. The ammonium (NH₄⁺) assimilation in roots produces about one proton (H⁺) per molecule ammonium taken up which has to be excreted into the soil solution. Ammonium nitrogen (NH₄—N) uptake is also influenced by the carbohydrate content of the plants.

The high carbohydrate content in the plant favours the uptake of NH₄—N possibly by enhancing NH₃-assimilation by the provision of carbon skeletons and energy. Nitrate uptake has been found to be completely depressed by the NH₄—N whereas NH₄—N uptake is not affected by NO₃—N.

In ammonium fed plants, the growth retardation and fall in pH in the soil solution may be due to the impairment of net excretion of protons as well as decline in the cytosolic pH. In ammonium fed plants, not only low but also high substrate pH greater than 7 can become critical because of an increase in free ammonia concentrations in the substrate resulting toxicity of ammonia.

Nitrate has the advantage of being a storage form in plants with no necessity to be assimilated in the roots. In addition, nitrate nutrition induces an increase in rhizosphere pH and there is no risk of toxicity at alkaline pH. But at high pH induced by nitrate there might have negative side effects on iron availability within the plants.

However, on supplying both forms of N, it is easier for the plant to regulate intracellular pH and to store some of the nitrogen at low energy costs. In higher plants a major factor contributing to the higher rates of vegetative and particularly reproductive growth are certainly related to the effects of ammonium (NH₄⁺) on the phytohormone balance in plants.

Nitrogen in Relation to Growth of Plants:

Depending on the plant species, development stage, and organ, the nitrogen content required for optimum growth varies from 2 to 5% of the plant dry weight. When the N supply is sub-optimal, growth is retarded, nitrogen is mobilised in mature leaves and re-trans-located to regions of new growth.

Typical nitrogen deficiency symptoms, such as enhanced senescence of older leaves, can be observed. An increase in the N supply not only delays senescence and stimulates growth but also changes morphology of plant especially when N availability is high in the rooting medium (soil) during the early growth.

In cereals the enhancement of stem elongation due to excess N application increases the susceptibility to lodging and this becomes the dominant yield limiting factor.

Short stem length which is obtained by breeding in most high-yielding cultivars, can also be induced by the application of growth retardants such as chlorocholine chloride (CCC, trade names, chloromequat or cycocel), counteracting the negative side effects of an excess N supply.

Nitrogen modifies plant composition to a greater extent than any other mineral nutrient. Carbohydrate content decreases and the N and lignin content increases due to application of higher amount of N.

Total nitrogen and crude protein (total N content multiplied by a factor between 5.7 to 6.25 depending on plant species and source of crude protein) are the sum of both protein and soluble nitrogen (amino acids, amides and nitrates). In general the ratio of soluble nitrogen increases with an increasing supply of N and is higher in leaves and storage organs with high water content but low in grains and seeds.

However, such shifting in plant composition due to increased N supply reflects a competition for photosynthetic among the various metabolic pathways and this competition is modulated by internal and external factors. When the nitrogen supply is sub-optimal, ammonia assimilation increases both the protein content and growth of leaves with simultaneous increase in leaf area index (LAI).

When the increase in LAI is correlated with an increase in net photosynthesis, the requirement of carbon skeletons for ammonia assimilation does not substantially depress other biosynthetic pathways related to carbohydrates (sugars, starch, cellulose, etc.), storage lipids or oils and at this level of N supply, the composition of the plant does not change substantially, but the total production of plant constituents per unit surface area or per hectare increases.

As the nitrogen supply is further increased, a higher proportion of the assimilated nitrogen is sequestered in storage pools as amides.

Besides the amount of lipids in green leaves is closely related to the nitrogen supply. In cereals high grain protein content is necessary for processing and for nutrition. Increasing N supply to the roots however affects the nitrogen content of grains mainly indirectly via re-translocation from vegetative growth and might lead to mutual shading and lodging.

These complication may be overcome by late application of nitrogen until anthesis either as foliar or soil application. It is also evident that due to soil application of N, most of it by passes the leaves and is directly translocated as amides and amino acids from the roots to the developing grains and such late application of N has very little effects on lysine content and quality of proteins.

The uptake of nitrogen by the plants also takes place as gaseous ammonia form. This net uptake depends on the partial pressure of NH₃ in the atmosphere. Increasing partial pressure of NH₃ increased the net uptake and lowering the partial pressure (pNH₃) resulted in a loss of NH₃ from the plant. Such release of NH₃ may be from the decomposition of protein of senescent leaves and also from the photorespiration.

Response of Plants to Nitrogen:

It is evident that the fertilizer N has the most important effects in terms of increasing crop production. Response to N depends on soil conditions, crop species and nutrient supply in general. Usually, the response of N is poor in soils containing higher amount of N.

In the absence of response, residual N and/or the rate of release of N by microbial decomposition of soil organic matter is possibly sufficient to meet the demands of the crop. Water is the most important growth factor which can limit the response to N application in different crops.

An optimum water regime gives rise to the highest N response for rice. However, the response to N also depends on how best the crop is supplied with other plant nutrients.

It has been observed that without P and K applications, the yield response to increasing N levels was smaller than when sufficient amounts of P and K were applied. Besides, the response to P and K application has been found to be more in presence of adequate amount of N supply.

The efficiency of nitrogenous fertilizers is dependent on various factors like water supply and the presence of other plant nutrients in the soil. There are different terms which are useful for evaluating the efficiency of different N fertilizers in relation to crop yields.

where, F and C denotes fertilized and unfertilized, control respectively.

High agronomic efficiency may be obtained if the yield increment per unit N application is high and this may happen when the soil contains low amount of available N as well as the rate of N application is also low. Higher recovery may be obtained if no loss or fixation of applied N occurs.

However, in practice, recovery levels of applied N are about 50% with optimum soil and fertilizer management practices. Highest physiological efficiency occurs when 33 kg grain are produced for every kg of N taken up by the crop.

In order to obtain higher response to N application, suitable rate of N fertilizer is to be used and such application rate depends on crop species and soil types.

An application of excess N leads to depression of yields of crops through lodging, disease incidences, interference with the availability of other plant nutrients etc. In addition, the nature and type of fertilizer N also influence the crop yields.