**1.6 Objectives**

Regulation of induction of the NO3− uptake and reduction systems by nitrogen metabolites has been attributed to feed−back inhibition (Pal'ove−Balang, 2002). It was found that nitrate uptake rate follows a biphasic relationship with external nitrate concentration, suggesting the existence of at least two different uptake systems (Cerezo et al., 2000). At high external nitrate concentration (> 0.5 mM), a low affinity transport system (LATS), which shows linear kinetics, contributes significantly to the uptake rate and appears to be constitutively expressed and essentially unregulated. At low external concentrations (< 0.5 mM), two high affinity transport systems (HATS) operate, one of these being constitutive whereas the other is induced by nitrate. The HATS for nitrate uptake is sensitive to metabolic inhibitors and appears to be an active transport system (Daniel−Vedele et al., 1998).

Although the regulatory effect of amino acids on nitrate uptake and NR has been examined extensively, its effect on GS has not been examined in detail. Otherwise, a lot of amino acids were investigated about their regulation on nitrate uptake and assimilation, but very little information has been reported about effect of mixed amino acids (MAA).

In fact, there are two possible reasons for the increase of total N content in the plants: preference for amino acids as sources of reduced nitrogen and regulation of amino acids on inorganic nitrogen uptake and assimilation.

The solution experiments were carried out to investigate the regulation of the induction of NO3– uptake, NRA, NiRA and GSA in radish and red pepper by applying mixed amino

(mM)

disappeared from the initially treated solution.

 **uptake** 

300 A0

was found in treatment A2 that showed 305% higher than A0.

A1 A2

**–**

**2.2 Results and discussion** 

**2.2.1 Effect on NO3**

NO

3

0

NO3–. Values are means ± SD (n=5).

100

200


mol g- FW)

Effect of Mixed Amino Acids on Crop Growth 125

**Treatments K+ NO3– Ala β–Ala Asp Asn Glu Gln Gly**  A0 5.25 5.0 ─ ─ ─ ─ ─ ─ ─ A1 6.78 5.0 0.3 0.3 0.3 0.3 0.3 0.3 0.3 A2 13.10 5.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 Table 3. The compositions of the treatment solutions for radish in hydroponic experiment

Plants were harvested 24 h after treatment and separated into roots and shoots for enzymes assay and N content analysis. Net NO3– uptake rates were determined by amount of NO3–

The MAA treatments showed different effect on nitrate uptake depending on the concentrations (Fig. 2). The NO3– uptake in treatment A1 was similar to that of A0 after 8 h exposure to NO3–. However, exposure for longer hours (16 or 24 h) to 0.3 mM MAA

> Time (h) 0 5 10 15 20 25

Fig. 2. Effect of mixed amino acids on the nitrate uptake in radish supplied with 5.0 mM

Several authors reported that free amino acids could down regulate NO3– uptake. It was found that exogenously supplied amino acids and amides could decrease the uptake of NO3– by soybean (Muller and Touraine, 1992); wheat (Rodgers and Barneix, 1993); maize (Ivashikian and Sokolov, 1997; Padgett and Leonard, 1996; Sivasankar et al., 1997); barley (Aslam et al., 2001). In this experiment, the effectiveness of the MAA treatments on NO3– uptake was similar to above references at low MAA treatment rate (0.3 mM MAA, Fig. 2). However, contrary result was found at high MAA treatment rate (3.0 mM MAA, Fig. 2), in

– uptake

inhibited the NO3– uptake by 38% compared with A0. In contrast, the highest NO3

acids (MAA) under the conventional fertilization. These two plants were selected because radish is NO3– preferred crop and red pepper is NH4 + preferred crop. The amino acids used in this experiment were alanine (Ala), β–alanine (β–Ala), aspartic acid (Asp), asparagine (Asn), glutamic acid (Glu), glutamine (Gln) and glycine (Gly). These amino acids were selected for the reasons include: (1) their structural role in proteins, (2) significant effect on NO3– uptake which was found in many works, and (3) considerable amounts in plant phloem and xylem (Caputo and Barneix, 1997; Lohaus et al., 1997; Peeters and Van Laere, 1994; Winter et al., 1992).

In the frame of the studies on the effect of the mixed amino acids (MAA) on nitrate uptake and assimilation, the pot experiments were focused on the role of MAA in process of NO3– uptake and assimilation. In order to distinguish the origin of N in radish, 15N labeled nitrate was used.

In order to develop an approach for more efficient N fertilizer use and to prevent environmental pollution due to nitrate leaching, the aim of the study presented here, is to investigate the effect of amino acid fertilizer (AAF) on nitrate removal in high nitrate soils.
