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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. 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. Those plants which uses NH4+ as their source of N have high N concentration in their tissues and low mineral cations in their tissues compared to plants which use NO3- as their N source. (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
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.