Responses of Crop Plants to Ammonium and Nitrate N.

Abstract: Nitrogen (N) is the most important, essential nutrient for all living organisms on earth; it is present in a number of complex organic molecules and plays extremely important roles in their activities. Ammonium N (NH4 +–N) and nitrate N (NO3 −–N) are the main forms taken up by plants in addition to some organic N compounds. More than 90% soil N is in organic form. The intermediate products of complicated organic N substances can be absorbed by plants. Organic N nutrition affects plant product quality and plant metabolism. Organic N passes through the cell wall and arrives at the plasma membrane through the apoplast and cytoplast systems and, in addition to endocytosis, may get transported across the plasma membrane by an active (sugar/proton cotransport) or passive process. After uptake by plants, simple organic N compounds such as amino acids can be rapidly assimilated and transformed into other amino acids by transamination and deamination. The uptake of NH4 +–N and NO3 −–N can be described by the Michaelis–Menten equation, and two parameters, the maximum absorption velocity (Vmax) and affinity constant or Michaelis constant (Km), have been used to measure the ability and efficiency of roots absorbing the two ions of crop plants. The uptake amounts of both NH4 +–N and NO3 −–N at the seedling stage are well in agreement with their absorption kinetic parameters, particularly at low concentrations, but are not fully in agreement with the entire growing periods of crops. In addition to root interception, NH4 + and NO3 − can move from bulk soil to the root surface by mass flow and diffusion. Diffusion is more important to NH4 +–N than NO3 −–N, while NO3 −–N movement mainly depends on mass flow. Roots are the major organs for the uptake of NH4 + and NO3 − ions. On arriving at the root surface, the two N forms can passively enter the root epidermis cell wall through the symplast and apoplast and then radically and vertically move across the cortex where the two ions in the apoplast enter the cortex symplast for passing through the endodermal Casparian trip to the endodermis. From the endodermis, the two ions go to the stele and empty into the xylem, or flow to the apoplast or get stored in vacuoles in addition to reduction or direct assimilation. The movement occurs from cell to cell. On emptying into the xylem, NH4 + and NO3 − are transported to the shoots via the transpiration stream. NO3 −–N is the dominant form of the mineral N with high concentrations in soil solutions, and is usually taken up in great amounts by crops and is readily mobile in the xylem. Transport of NO3 − across the plasma membrane along the electrochemical gradient is thought to be by H+/NO3 − cotransport, or by transport proteins or carriers or by specific ion channels. NH4 + is in equilibrium with NH3, and in most soils, the pH is considerably low and NH3 concentrations are usually very low. NH4 + uptake through the plasma membrane has been assumed to occur in three ways: either active or passive, or both. Passive uptake may occur at the initial stage of uptake, while at the second stage, active uptake may be predominating. For passive uptake, NH4 + ions passing through the membrane are thought to be present in either NH4 + or NH3 form, and in this way may be related to the facilitated diffusion through channels. The NH4 + ion resembles the K+ ion in terms of the ionic radius and size of the hydration shell, and therefore, it may be able to permeate the plasma membrane through K+ channels. The active uptake includes the H+/NH4 + cotransport, and specific transporters. Nitrate N cannot be directly used by plants until it is reduced to ammonia. The reduction is catalyzed by enzymes in two steps: the first step takes place in the cytoplasm by nitrate reductase (NR) transforming NO3 − into nitrous acid (HNO2), and the second occurs in chloroplasts (shoots) or proplastids (roots) by nitrite reductase (NiR) converting HNO2 to NH3. The NO3 − reduction to NO2 − is the rate-limiting step for NO3 − assimilation. Both roots and shoots are capable of reducing NO3 −. The uptake and reduction of NO3 − and the reductive ratios in roots and shoots depend on plant species, carbohydrates in plants, and nitrate reductase activity (NRA) as well as environmental conditions such as NO3 − concentration, medium pH, complementary ions, light, and ambient CO2 concentration. Due mainly to NO3 − uptake exceeding its assimilation by plants, a large amount of NO3 − is accumulated in vacuoles of roots and shoots at vegetative stages. With the exception of vegetables, the storage of NO3 − has no harmful effects on plant product quality and benefits the supply of N nutrient for later growing stages of plants. Ammonia is a central intermediate in plant N metabolism. NH3 is assimilated by plants by the mediation of glutamine synthetase–glutamine (z-) oxoglutarate aminotransferase enzyme systems in two steps: the first step requires adenosine triphosphate to add NH3 to glutamate to form glutamine (Gln), and the second step transfers the NH3 from Gln to α-ketoglutarate to form two glutamates. Once NH3 has been incorporated into glutamate, it can be transferred to other carbon skeletons by various transminases to form additional amino acids. The glutamate and Gln formed can rapidly be used for the synthesis of low-molecular-weight organic N compounds (LMWONCs) (such as other amides, amino acids, ureides, amines, and peptides) that will be further synthesized into high-molecular-weight organic N compounds (HMWONCs) such as proteins and nucleic acids. NH4 +–N and NO3 −–N may exhibit different effects on plant nutrition, growth, and crop production of different plant species. The preferences are the demand nature of plant species together with environmental conditions. The preferences have been evaluated mainly by plant biomass, yield production, or N uptake amount by the application of NH4 +–N or NO3 −–N alone. Based on current results, crop plants may be classified into four types: preference to NH4 +–N; preferences to NO3 −–N; equal effect of NH4 +–N and NO3 −–N; combinative use of the two N sources being superior to either NH4 + or NO3 − alone. Based on the fact that when NH4 +–N and NO3 −–N coexist in media, crops have a higher dry matter production, N uptake, and N utilization efficiency compared to NH4 + or NO3 − alone, the concepts, enhancement of NH4 +–N nutrition to the majority of crops and enhancement of NO3 −–N nutrition to rice, have been proposed. Both measures have significantly improved plant physiological properties, N use efficiency, and crop yields. Plant preferences to NH4 +–N and NO3 −–N are determined by many plant internal factors such as plant species, cultivars, and growing stages. For example, because of a lack of nitrate reductive ability, some plant species cannot efficiently use NO3 − as their major N source. The preferences are also determined by the chemical properties of the two ions, NH4 + and NO3 −, which affect N loss and N efficiency in plants, the uptake effects on absorption of other anions and cations, the balance between cations and anions in plants by uptake of NO3 − or NH4 + alone, and the influence of Fe and Mn uptake or their bioavailability as well as plant tolerances to Mn and Al toxicities. External or environmental factors, such as media pH, cations and anions, NH4 + concentration in solution, temperature, illumination, and aeration, also have great effects on their preferences. The toxicity produced by NH4 +/NH3 nutrition is an important reason for plant preferences for the two N forms. By using NH4 +–N as the sole N source, the toxicity is often characterized by an immediate restriction in plant growth, stem lesions, leaf area decline, total biomass reduction, and finally plant death. Influence of root growth and production of short, thick, less branched and darkly colored roots is the particular feature of the toxicity. The toxicity to plants is the combinative result of internal and external factors. Different species may have different tolerances to NH4 + or NH3, and thus toxicity, can appear to be different among species and even among cultivars in the same species. Carbohydrate amounts in plants substantially affect toxicity. Strong acidification of rhizosphere soil is an important environmental factor and medium acidification associated with NH4 + absorption has been shown to be toxic to many crop plants. The degree of the toxicity is associated with crops’ sensitivity to pH. Depending on the plant species and particular growth conditions, each of these factors may contribute to toxicity. Two hypotheses have been put forward to explain the physiological source or the cause of the toxicity. Some scientists consider that toxicity is due mainly to free ammonia (NH3) that affects plant growth and metabolism at low concentration levels at which NH4 + is found to be not harmful. The NH3 molecules can directly penetrate the membrane and enter cells through diffusion resulting in an increase in cytoplasmic pH that inhibits Glu synthase activity as the primary cause. However, based on the fact that under conditions without the possibility to produce NH3, the toxicity effect still exists, and it is often enhanced by water stress, NH4 + toxicity has been proposed. Injury of membrane and cell wall structure as well as changes of membrane enzyme activities and metabolism have been regarded as the primary causes of NH4 + toxicity, and the retardation of nutrient absorption, decrease of the uptake of essential cations and plant growth, disorders of NH4 +-induced pH changes, excessive consumption of sugars as the secondary causes of the NH4 + toxicity. Crops growing in solution cultures with equal amounts of NH4 +–N and NO3 −–N in their entire life cycle show that they did not absorb equal amounts of each N form, varying with plant species and growing stages of each crop and solution pH. The media pH affects the uptake ratio of NH4 +–N to NO3 −–N of wheat, and the unequal uptake of the two N forms results in significant changes in the pH in the solution. The amounts and ratios of NH4 +–N to NO3 −–N taken up by rice vary with growing stages. Application of NH4 +–N at early stages and alternatively applying NH4 +–N and NO3 −–N at late stages may be a way for the promotion of rice growth and increasing its yield. NH4 +–N and NO3 −–N forms affect some morphological and physiological characteristics of crop plants. Supply of NO3 −–N can increase lateral root growth, but supply of NH4 +–N inhibits root growth and produces abnormal growth of lateral roots. In coexistence of NH4 +–N and NO3 −–N, chlorophyll contents and the net photosynthetic rate are the highest, followed by NO3 −–N, while NH4 +–N is the lowest. NH4 +–N, NO3 −–N, and their mixed N source exerted a greater effect on mesophyll conductance: the highest occurs for the mixed N source, followed by NO3 −–N and the smallest for NH4 +–N. Some results show that on supplying NH4 +–N, the total N, free amino acids, and amides are higher than NO3 −–N, but others show that the two N forms have no great influence on crop N accumulation and N-containing components. Application of NO3 −–N benefits the accumulation of sucrose and increases plant organic acids, total soluble sugar amounts, reductive sugar accumulation, structured polysaccharides contents (cellulose) in roots, stems and leaves of maize seedlings, while NH4 +–N benefits starch accumulation and consumes large amounts of organic acids and carbohydrates and reduces sugar content. The two N forms also affect the mineral ion uptake and nutrient accumulation in plants. The RN amount and activity are closely linked with NO3 −–N. To a certain extent, the NRA is positively correlated to the NO3 −–N accumulation in plants. NH4 +–N as the sole N source inhibits NRA. Nitrate increases or maintains, whereas NH4 +–N decreases the leaf water potential and turgor pressure. Different N forms may produce different effects on the reactive (free) oxygen species (ROS) levels, antioxidant enzyme, and protective enzyme activities. When supplied with NH4 +–N and NO3 −–N together, the activities of the superoxide dismutase (SOD) and catalase (CAT) are the highest in leaves. Nitrate addition can promote the survival and restoration of plants damaged by waterlogging; and can produce NO, a suitable amount of which increases SOD and CAT activities under a salt stress, decreases the forming-velocity and cumulative amounts of H2O2 and O2 − as well as malondialdehyde, and increases antioxidant substances. The two N forms may affect the formation and activity of hormones and thereby may inhibit or promote cell elongation and plant growth. Compared to NH4 +, application of NO3 − significantly and rapidly promotes the synthesis and transport of root-originated hormones, such as cytokinin (CTK) and auxin, especially indole-3-acetic acid (IAA), and increases their activities. The IAA content of tobacco seedlings grown in 100% NO3 −–N solution was higher than those grown in 50% NH4 +–N+50% NO3 −–N solution, and the auxin concentration in the aboveground part of plants cultured by NO3 − was higher than that for NH4 +. [Copyright &y& Elsevier]