**1.1 Nitrogen uptake and assimilation**

Among the mineral nutrient elements, nitrogen is a kind of macronutrient. Most plant species are able to absorb and assimilate nitrate (NO3–), ammonium (NH4+), urea and amino acids as nitrogen sources. Most soils do not have sufficient N in available form to support desired production levels. Therefore, addition of N from fertilizer is typically needed to maximize crop yields. Many kinds of N fertilizers are used which contain varying forms of N such as NO3 ––N, NH4+–N and urea. However, NO3– form of nitrogen is the predominant form of N absorbed by plants, regardless of the source of applied N (Breteler and Luczak, 1982). This preference is due to several autotrophic soil bacteria, which rapidly oxidize NH4+ to NO2 –, and then to NO3– in warm, well–aerated soils. Even though NO3– is the most available form of N to plants, it can be more readily lost from the root zone because it is very mobile and easy to leach. This economically and environmentally undesirable process perpetuates a large amount of the uncertainty associated with N fertilizer management (Pessarakli, 2002).

In the soil solution, nitrate is carried towards the root by bulk flow and is absorbed into the epidermal and cortical symplasm. Within the root symplasm, nitrate has four fates: (1) reduced to nitrite by the cytoplasmic enzyme nitrate reductase; (2) efflux back across the plasma membrane to the apoplasm; (3) influx and stored in the vacuole; or (4) transported to the xylem for long−distance translocation to the leaves (Andrews, 1986; Ashley et al., 1975; Black et al., 2002; Cooper and Charkson, 1989). Translocated from the xylem, nitrate enters the leaf apoplasm to reach leaf mesophyll cells, where nitrate is again absorbed and either reduced to nitrite or stored in the vacuole.

Nitrate translocated from the roots through the xylem is absorbed by a mesophyll cell via one of the nitrate−proton symporters into the cytoplasm, reduced to nitrite by nitrate reductase (NR) in the cytoplasm, and then reduced to ammonium by nitrite reductase (NiR)

<sup>\*</sup> Corresponding author

Effect of Mixed Amino Acids on Crop Growth 121

consume plant material with rich nitrate, they may suffer from methemoglobinemia or carcinoma by converting nitrate to nitrite of nitrosamines. Some countries limit the nitrate

Several authors reported that free amino acids could down regulate nitrate uptake and nitrate content in plant. It was found that exogenously supplied amino acids and amides could decrease the uptake of nitrate by soybean (Muller and Touraine, 1992); wheat (Rodgers and Barneix, 1993); maize (Ivashikina and Sokolov, 1997; Padgett and Leonard, 1993, 1996; Sivasankar et al., 1997); barley (Aslam et al., 2001)(Table 1). Plants appear to have multiple mechanisms for regulating nitrate uptake in addition to amino acids or N-

Work by Breteler and Arnozis (1985) determined that pretreatment of dwarf bean roots with many different individual amino acids inhibited nitrate uptake to varying degrees dependent upon prior exposure of the plants to nitrogen and the specific amino acid treatment. No significant effect of amino acids on nitrate transport was detected when both NO3− and amino acids were present in the bathing solution, and no correlation emerged between inhibition of nitrate uptake and inhibition of nitrate reductase relative to specific amino acids. A more detailed study, presented by Muller and Touraine (1992), demonstrated inhibition of uptake by 50% or greater by alanine, glutamine, asparagines, arginine, β–alanine and serine when soybean seedlings were pretreated for 18 h prior to exposure to NO3−. The mechanisms of inhibition by arginine and alanine appeared to differ, however. Arginine stopped NO3− uptake immediately upon introduction to the uptake solution, kinetically similar to NH4+ inhibition. The authors suggested that this may be the result of a non-metabolic response such as alteration of membrane potentials. Inhibition by alanine was slower to develop, suggesting a metabolic component to the regulation rather

**treatment NO3− supplied NO3<sup>−</sup> uptake Remarks** 

100 0.5 40−120 (Muller, 1992)

1.0 0.3 89−106 Non−starvation

starvation

(Rodger, 1993)

1.0 0.3 50−105 3 days N

**(mM) (%)**  Soybean 10 0.5 5−85 14 amino acids

Maize root 15 5.0 84 (Padgett, 1993) Barley root 1.0 0.1 40−50 (Aslam, 2001) 1.0 10 70

The N status of the plants could also affect the inhibitory effect of amino acids on nitrate uptake. Rodger and Barneix (1993) had supplied amino acids exogenously to N starved or non−starved wheat seedlings. Exogenously supplied amino acids and amides had no effect on the wheat seedlings under well nourishment. However, some of the amino acids and amides supplied seedlings starved of N for 3 days inhibited up to 50% of the nitrate uptake rate.

content in plant material sold for human consumption.

status (Padgett and Leonard, 1993).

than a physical or chemical interference.

**Amino acid** 

Table 1. Effect of amino acid on NO3<sup>−</sup> uptake in several plants

**Plant materials** 

Wheat

in the chloroplast, which is then incorporated into amino acids by the glutamine synthetase−glutamine− 2−oxoglutarate amidotransferase (GS/GOGAT) enzyme system, giving rise to glutamine (Gln) and ultimately other amino acids and their metabolites (Fig. 1; Taiz and Zeiger, 2002). Therefore, NR, NiR and GS constitute the first three enzymes of the nitrate assimilatory pathway. The NR activity is the limiting step of NO3 <sup>−</sup>–N conversion to amino acid synthesis (Campbell, 1999). In most plant species only a proportion of the absorbed nitrate is assimilated in the root, the remainder being transported upwards through the xylem for assimilation in the shoot where it is reduced and incorporated into amino acids (Forde, 2000).

Fig. 1. The main process of nitrate assimilation.
