Some Aspects of Nitrogen Assimilation in Wheat Plant (Triticum aestivum L.) as Affected by Differents Nitrate Levels and Bacteria Inoculation

The N metabolism of bread (yocora rojo) wheat (Triticum aestivum L.) plants, grown with different levels of nitrate (0.5, 1, 5 and 10 mM) in the media, was studied in a pot experiment with soil under optimum conditions in a growth cabinet. At all nitrate levels the activity of nitrate reductase (NAR) was higher in wheat shoots rather than the roots, and the proportion of the enzyme in the shoot to root was calculated to be 94 : 6 % respectively, The wheat shoot : root fresh weight ratio was increased (1.3 to 2.6) with elevated nitrate supply. Plants inoculated with Azotobacter chroococcum showed not much change in the distribution of the enzyme with alterations in nitrate supply. Most of the TCA-soluble nitrogen (N) was present as amino acids, nitrate and ammonium-N; and the levels found in both shoots and roots confirmed that internal reserves of N depends on approximate space value (ASV) and type of the N pools within the plant. The abundance of free amino acids in shoot pools and root pools fed with nitrate indicate that under experimental conditions the proportion of N being taken up as dissolved inorganic nitrogen (DIN) was released as dissolved organic nitrogen (DON). The free amino acids detected, including Asp, Glu, Asn, Ser, Gln, Thr and Ala, were remarkably high in the shoot and root. In contrast, others (including Arg, Tyr, Trp, Met, Val, Phe, Iso, Leu, Orn and Lys) showed similar variation pattern between shoot and root. The higher ratio of Gln : Glu in root suggesting most probably due to either a reaction involved in the reduction of the nitrate or to the activation of glutamine synthesis (GS). A conclusion was drawn that the higher level of glutamate than that of glutamine, may have been due to the presence of GDH-dependent glutamate synthesis in the shoot.