ObjectivesThe research will elucidate the regulation mechanisms of NO₃^– re-utilization by 1-am-inocyclopropane-1-carboxylic acid (ACC) and reveal the physiological mechanisms of nitrogen (N) redistribution at later growth stage of Brassica napus.

MethodsHigh N efficient variety Xiangyou15 (No.27) and low N efficient variety 814 (No.6) were cultured in sand under 15 mmol/L nitrogen condition. ACC (50 mL 100 μmol/L) was irrigated every 7 days and the corresponding Arabidopsis thaliana wild type (col.0) and mutant plants (nrt1.5) under greenhouse and environmental chamber respectively, to investigate the regulation mechanisms of NO₃^– re-utilization and its relationship with nitrogen use efficiency (NUE) during the later growth stages.

ResultsThe relative expression of BnNRT1.5 was significantly inhibited by ACC and senescence. Compared with the control treatment, NO₃^– distribution ability in phloem sap was decreased significantly, resulted in NO₃^– content of the lower leaves was increased significantly, NO₃^– content of middle leaves was decreased significantly, but there was no significant changes of NO₃^– content in upper leaves under ACC treatment, thus lead to the N content of plant tissues and seeds were increased significantly, and NUE based on biomass and seeds yield were decreased significantly. It was suggested that the N re-utilization capability is significant regulated by NRT1.5 gene. Using Arabidopsis thaliana wild type (col.0) and mutant (nrt1.5) plants to further elucidate the above results, compared with the wild type (col.0), higher proportion of NO₃^– accumulated in the older leaves, lower proportion of NO₃^– redistributed from older leaves to new leaves through phloem sap in mutant nrt1.5 plants during the later growth stage, redistribution ability of NO₃^– from older leaves to new leaves during senescence decreased significantly.

ConclusionsN re-utilization efficiency was regulated by ACC significantly at the later growth stages of Brassica napus, redistribution contents of NO₃^– in phloem sap was decreased in Arabidopsis mutant (nrt1.5) or Brassica napus under ACC treatment, higher proportion of NO₃^– accumulated in older leaves and cannot be re-utilized efficiently at the later growth stages. Therefore, NRT1.5 is an effective means to improve the crop NUE by increasing N redistribution and re-utilization during the later growth stage.