Evaluation of the Role of Mixed Amino Acids in Nitrate Uptake and Assimilation in Leafy Radish by Using 15N-Labeled Nitrate

In this paper, the role of mixed amino acids in nitrate uptake and assimilation was evaluated in leafy radish by using 15N labeled nitrate. The mixtures of alanine, β-alanine, aspartic acid, asparagines, glutamic acid, glutamine, and glycine were sprayed to plant leaf two or four times. The activity of the enzymes related to the process of NO3− reduction (nitrate reductase, nitrite reductase and glutamine synthetase) was affected differently depending on the application rate of mixed amino acids. Applying mixed amino acids increased the fresh weight, dry weight, and N yield. The NO3− content was reduced to 24–38%, but no significant differences were observed in amino acids and proteins. In addition, the nitrogen derived from fertilizer and the 15N-NO3− recovery rate increased to 2–8% and 15–47%, respectively. These results strongly suggest that the positive effect of mixed amino acids on nitrate uptake and assimilation might be attributed to the regulation on NO3− uptake and assimilation, but not to the preference for amino acids as sources of reduced nitrogen.