Site of free amino acid accumulation in tulip (Tulipa gesneriana) roots during autumn and winter

Tulip (Tulipa gesneriana, cultivar Oxford) bulbs were planted in October and grown in culture solution in a glasshouse under natural conditions. One group was cultivated in a solution containing nitrogen (N) (+N treatment: nitrate-N (NO3--N) 12.0, ammonium-N (NH4+-N) 3.0 mg L-1). Another group was cultivated in a nitrogen-free solution (-N treatment). In the first experiment, the transverse accumulation of free amino acids in the cortex and stele of the tulip roots was measured. In the cortex, the total concentration of free amino acids amounted to 39.4 mg Ng(-1) dry weight (DW) in the +N treatment, but only 5.3 mg Ng(-1) DW in the -N treatment. In the stele, the concentration was 14.4 and 5.8 mg Ng(-1) DW, respectively. Glutamine was the predominant amino acid in the cortex in the +N treatment, reaching 18.1 mg Ng(-1) DW. The concentrations of asparagine and arginine were also higher after the +N than -N treatment. On the other hand, the concentration of 4-methyleneglutamine was not so different between the treatments in either the cortex or stele. In the second experiment, the accumulation of free amino acids at five vertical positions from the base to root tip was analyzed. The total concentration of free amino acids was not so different among the positions. According to statistics, the concentrations of glutamine and asparagine were relatively constant between the +/-N treatments. In the -N treatment, the concentration of 4-methyleneglutamine gradually decreased from the base toward the root tip, and it tended to be higher in the -N treatment than that in the +N treatment.