Nitrogen is one of the major nutrients that is required by plant for its growth and development of plants. The nitrogen limitation in the soil restricts the productivity of cereals. Rice, Wheat, maize and to a lesser extent barley are the most important cereals cultivated in the world and account for majority of end products used for food and it is mostly likely that they will contribute to the majority of human diets in the next century. The major cereal crops grown in Australia are wheat, barley, sorghum and maize. Cereals are fast growing crops and they demand high levels of nitrogen, which are supplied in the form of nitrogenous fertilizers. In his article (Cassman, 1999) reviewed that since 1967, high yield in rice, wheat and maize largely contributed to the high increase in global food production. The use of these fertilizers in the soil has increased considerably; it has been observed that the nitrogen recoveries by plants are limited (Raun and Johnson, 1999). In earlier studies by Cassman et al., 1998 and Raunn and Johnson (1999) it was observed that only 33 % of the applied nitrogen is actually being taken by the cereals and 67% loss is accountable to about $16 million annual loss of nitrogenous fertilizers. In addition to the effective management of nitrogen fertilizer, better soil management and adopting proper irrigation (Alva et al., 2005, Atkinson et al., 2005) and an improvement in the hybrids or cultivars developed from the ancient germplasms of cereals will be an effective method in improving the nitrogen use efficiency (NUE) of cereals.
Sources of nitrogen in the Environment
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Nitrogen is present in the atmosphere, soils, and biological material of the environment. IN the air nitrogen is present in the gaseous form and is about 78%, which is the largest nitrogen reserve. The largest quantity of N is in the atmosphere which is about 78 percent N2 gas. Nitrogen is this form is unavailable to most plants directly. Nitrogen is deposited in the soil through precipitation, lightning, plant residues, nitrogen fixation by soil organisms, animal manures and commercial fertilizers. Animal manures are important sources of N in the environment. Nitrogen is in organic and inorganic forms in soils. Over 90 percent of soil N is associated with soil organic matter. Nitrogen can be permanently removed from soil by erosion, leaching, denitrification, or volatilization. Global estimates indicate that less than 50% of the applied fertilizer N are used by the crop, while 2-5% are stored in the soil, 25% are emitted to the atmosphere and 20% are discharged to aquatic systems (Galloway et al., 2002). A number of studies have focused on ways to improve fertilizer use efficiency, largely in cereal grain production, and on reducing adverse effects, namely losses of fertilizer N to the environment (Tilman et al., 2002; Mosier et al., 2004). Studies have showed that the contribution N fertilizer to total N uptake ranges from 10-50% and the rest comes from the N present in the soil (Omay et al., 1998; Stevens et al., 2005). Net mineralization of soil N is influenced by chemical properties like total N and C ratio and pH(Breland and Hansen, 1996; Scott et al., 1996) and physical properties such as soil moisture, pore size, pore volume, microbial biomass of the soil along with the climatic factors ie. Temperature and precipitation regimes (McDonald et al., 1995).
Ammonium Vs Nitrate for plants
Nitrogen (N) nutrition of plants mainly depends on the availability of the different inorganic N forms in the environment, but other factors such as pH and concentrations of mineral ions also affect nutrition (Dyhr-Jensen and Brix, 1996; Romero et al., 1999; Brix et al., 2002; Guo et al., 2007).In agricultural soils plant available N is present in soluble inorganic (NO3-, NH4+, NO2-) and organic N forms. Ammonium (NH4+) and nitrate (NO3-) behave quite differently in soils. Positively charged NH4+ is attracted to negatively charged sites on soil particles as are other cations. It is available to plants, but the electrostatic attraction protects it from leaching. Negatively charged NO3- does not react with the predominately negatively charged soil particles, so it remains in the soil solution and moves with soil water. When rainfall is excessive this NO3- may leach out or it is deposited in the top soil when dry conditions prevail. In well-aerated soils of agricultural and natural ecosystems, most of the nitrogen is present as nitrate (NO3-) (Crawford and Glass, 1998). In colder climates, and the irrigated rice fields of the world where the soil pH is low, nitrogen is found in the form of NH4+ (Kronzucker et al., 1997). Terrestrial plants often take up most of their N as nitrate (NO3-) as concentrations of ammonium (NH4+) are generally very low in well-drained soils because of the microbial nitrification processes occurring under oxic conditions (Mengel and Kirkby, 2001). The adaptability of plants to the different environment conditions plays major role in their affinity to NH4+ and NO3- uptake. NH4+ fed plants had higher concentrations of N in the tissues, lower concentrations of mineral cations and higher contents of chlorophylls in the leaves compared to NO3- fed plants suggesting a slight advantage of NH4+ nutrition. (Konnerup and Brix, 2010). Despite these instances of NH4+ tolerance and even preference by plants, however, toxicity symptoms emerge for most species at increased levels of NH4+ supply (Pearson and Stewart, 1993; Kronzuckeret al., 1997, 2003a; Bijlsma et al., 2000; Britto and Kronzucker, 2002). NH4+ over-supply can result in a decrease in plant yield of 15 to 60% in crops such as tomato and bean (Woolhouse and Hardwick, 1966; Chaillou et al., 1986), and results in plant mortality and even species extirpation in some cases (Gigon and Rorison, 1972; Magalhaes and Wilcox, 1983, 1984; Pearson and Stewart, 1993; de Graaf et al., 1998). Another inhibitory effect of NH4+ is on seed germination and seedling establishment (Cooke et al., 1962; Hunter and Rosenau, 1966; Megie et al., 1967; Barker et al., 1970; Westwood and Foy, 1999), and past studies by many researchers concluded that both NH4+ and NO3 oversupply can significantly reduce the extent of mycorrhizal associations (Boxman et al., 1991; Lambert and Weidensaul, 1991; van Breemen and van Dijk, 1988; van der Eerden, 1988; Boukcim et al., 2001; Hawkins and George, 2001). In general, plant responses to excessive inorganic nitrogen depend strongly on nitrogen source, dose, and genetic predisposition (Givan, 1979; Magalhaes and Huber, 1991; Britto and Kronzucker, 2002). Some evidence indicates that the soil [NH4+] at which toxicity occurs is lower for slower growing species such as poplar and Douglas-fir trees, relative to faster-growing species such as grasses (Britto et al., 2001; Kronzucker et al., 2003a). For most plants, a mixed NO3- and NH4+ nutrition is superior over sole NO3- or NH4+ nutrition, but the optimal proportions of NO3- to NH4+ depend on plant species, environmental conditions, developmental stage and the concentration of supplied N (Chaillou et al., 1991; Claussen, 2002; Zou et al., 2005; Tylova-Munzarova et al., 2005).
Nitrate & Ammonium Uptake by Plants
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The nitrate uptake system of higher plants consists of a constitutive, low affinity transport system (LATS) (possibly a carrier system or an anion channel), and an inducible, high affinity transport system (HATS) regulated by cellular energy supply, and by intracellular nitrate consumption, and whose activity depends on the proton electrochemical gradient. The latter system is regarded as an H+/anion co-transport carrier mechanism that produces transient plasma membrane depolarization upon addition of nitrate. The depolarization is counteracted by the plasmamembrane H+-ATPase (Ullrich, 1992). The plasma membrane proton ATP-ase is induced by nitrate (Santi et al, 1995). High and low affinity transport systems are biochemically distinct modes of transport system (Glass and Siddiqi, 1995). There is distinct difference between ihats and lats.
NH4+ Inhibition on NO3- uptake
Net NO3- uptake (which is defined as the difference between influx and efflux) is influenced by NH4+ in higher plants. There has been extensive study on the inhibitory effect of ammonium upon nitrate uptake by roots in the plants. The results varied considerably, with reports ranging from little or no effect (Smith and Thompson, 1971; Schrader et al., 1972; Oaks et al., 1979) to strong inhibition (Weissman, 1950; Lycklama, 1963; Fried et al., 1965; Minotti et al., 1969; Jackson et al., 1976; Rao and Rains, 1976; Doddema and Telkamp, 1979; MacKown et al., 1982a; Deane-Drummond and Glass, 1983; Rufty et al., 1983; Breteler and Siegerist, 1984; Glass et al., 1985; Ingemarsson et al., 1987; Oscarson et al., 1987; Lee and Drew, 1989; Wamer and Huffaker, 1989; de Ia Haba et al., 1990; Chaillou et al., 1994). ~ The short term effects of NH4 on NO3 uptake is directly the NH4 present in the plasma membrane. The earlier studies conducted to identify the mechanisms responsible for short term inhibition was unable to clarify whether it is due to the direct effect on nitrate influx or by stimulating the efflux. Past studies suggested that the inhibition of ammonium on nitrate uptake was because of its direct influence on the nitrate influx (Jackson et al., 1976; Doddema and Telkamp, 1979; MacKown et al., 1982a; Deane-Drummond and Glass, 1983; Deane-Drummond, 1985, 1986) and later studies implicated the stimulation of efflux is the mechanism responsible for the inhibition of nitrate uptake by ammonim( Glass et al., 1985; Lee and Clarkson, 1986; Ingemarsson et al., 1987; Oscarson et al., 1987; Lee and Drew, 1989; Ayling, 1993; King et al., 1993). The work by Aslam and his coworkers (1994, 1997) concluded that the inhibition of net nitrate uptake by ammonium is by the mechanism of stimulating nitrate efflux. The long term effect of NH4 upon NO3 uptake is thought to effect at the transcriptional level. (Glass and Siddiqi, 1995; Krapp et al., 1998; Zhuo et al., 1999). In one recent report of longer-tem studies with soybeans, Chaillou et al. (1994) showed increased periods of net NO3- efflux in the presence of NH4+. The extent of ammonium inhibition on nitrate uptake declines with the increase in nitrate. Judith Nayiraneza et al., 2009. In accordance with the findings that NH4+ increased NO3- efflux, severa1 reports also showed that NH4+ had no effect on 36ClO3 influx when the latter was used as an analog for NO3- (Deane-Drummond and Glass, 1983; Deane-Drummond, 1985, 1986). Decreased influx of l3NO3- was correlated with depolarization of membrane potentials of barley and tomato (Lycopersicon esculentum) roots (Ayling, 1993) and of Lemna (Ullrich et al., 1984). Investigators using "NO3- and longer term studies also reported that NH4+ inhibited NO3- influx and had no effect on efflux in wheat (Triticum aestivum; Jackson et al., 1976) and corm (Zea mays; MacKown et al., 1982a). In addition, the finding of increased efflux by NH4+ has been attributed to using plants that have been removed from a high concentration of NO3- and placed in a low concentration (Glass et al., 1985; Ingemarsson et al., 1987).The results presented here suggest that the enhancement effect of NH4+ on NO- efflux occurs externally to the plasma membrane. However, it is also possible that the stimulation of NO3- efflux could occur by the NH4+ in the cytoplasm. Roberts and Pang (1992) and (Wieneke, 1995)Wang et al. (1993) reported that NH4+ is rapidly sequestered in the vacuole.
Nitrogen is one of the major nutrient required by plants for their growth and development. The atmosphere constitutes 78 % for elemental nitrogen in the gaseous form. This N is absorbed in the soil by rain fall and lightning. Besides this the N reservoir of the soil is by N fixation of leguminous plants, decomposition of dead and decayed plant debris and animals etc. Plants cannot take up N in the elemental form. N is absorbed by plants in two major inorganic forms namely NH4+ and NO3-. Nitrate is the major form of n uptake by plants.
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Studies have suggested that NH4+ has inhibitory effect on NO3 uptake but a meager quantity of NH4+ in the soil or growing media always increased the plant growth. What is the reason for this? The inhibitory effect of NH4+ is less when NO3- is present in excess? is it because the presence of NH4+ increases the soil pH and uptake of NO3- in creases at low pH levels or is it because NH4+ accelerates total nutritional uptake of the plants? In this study I will be focusing on the effect of ammonium in the NO3- uptake and its effect on the total nutrition of Maize(Gaspe). Flux will be measured to quantify the NO3- uptake. Total nutritional effect of NH4+ will be measured by conducting experiments in the glasshouse hydroponics.
In the flux experiment 15N will be used as an analog for measuring nitrate uptake.
The experiment on total nutrition of plants ammonium will be applied at different levels and no ammonium will used as a control.
Plants fed with NH4
NH4NO3 generally had higher concentrations ofNcomparedtoNO3
fed plants. (Konnerup and Brix, 2010)
percentage of N (%N) and N from fertilizer (NFF) in leaf tissue were highly affected by the rootstock and the season of N application.(Aguirre et al., 2001)
In rice cultivated under flooded conditions, the anaerobic condition favors the formation of NH4+ in the soil, and is therefore considered the main available N source for this crop. (Holzschuh et al., 2009)
The large NH3-sorption capacity of acid humic
peats (Abbes et al., 1993) reduces the risk of NH|
toxicity for onions (Abbes et al., 1995). After incorporation
into the soil, urea in AP is hydrolyzed to NHa,
which reacts with peat acids to produce NHI humates
and other products affecting soil and plant processes.(Abbes et al., 1996)
The model also supports the view that NHJ ââ‚¬"bearing fertilizers are most effective in the spring, when the highsoil moisture content promotes NHJ diffusion in the soil solution and the root system has highest affinity toward the NHJ ion.(Abbes et al., 1996)
Soil NH: and NO; levels were low without N addition.
Nitrate level increased slightly with rate of applied
NH4 (Anghinoni and Barber,. 1988)
Stimulation of root growth rate, and changes in root
radius due to NH: placement were also observed for
corn plants (Anghinoni and Barber,. 1988)
Uptake of NH4+ was faster than NO3- under all conditions of the medium whereas uptake of NH4+ and NO3- both were affected by the pH of the medium. Maximum uptake of NH4+ was observed on pH 4.5. Uptake of NO3- was faster in the culture having pH 7.0. It is therefore concluded that pH of the medium influence the uptake of both NH4+ and NO3- in the cell suspension culture of Vigna radiata.(Afzal et al., 2006)
Suggested that iron may be an important unmeasured covariate in studies of ammonium inhibition on nitrate uptake (Armstrong, 1999)
Response of ammonia to nitrate inhibition depends on the root concentration of NO3(Aslam et al., 2001). In some plants like Pima and Acla cotton when the nitrate concentration in the roots were high the presence of NH4+ inhibition on the net uptake was greater compared to those with low NO3- root concentration. (Aslam et al., 2001).
These losses have been attributed to the combined effects
of denitrification, volatilization, and/or leaching
(Francis et al., 1993; Olson and Swallow, 1984; Karlen et
al., 1996; Wienhold et al., 1995; Sanchez and Blackmer,
1988) when these factors were not measured separately.
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