Nitrogen physiology and metabolism of two contrasting Catalpa bungei genotypes in response to limited nitrogen availability

To investigate N physiology and metabolism of different Catalpa bungei (C. bungei) genotypes in response to low N availability, saplings of a genotype (Cbu22-03) with high N uptake efficiency and a genotype (CbuGu) with low N uptake efficiency were exposed to 10, 100, or 1000 μM NH4NO3. The greater root biomass and fine-root surface area in Cbu22-03, combined with its higher net influxes of NO3- and NH4+ and upregulated transcript levels of CbuNRT1.1 and CbuAMT1.2, corresponded to the increased NO3- and NH4+ concentrations and total N amounts in roots compared with CbuGu. Accordingly, δ15N in both roots and leaves of Cbu22-03 was lower than that of CbuGu. Low N supply led to decreased net influxes of NO3- and NH4+, and reduced NO3-/NH4+ concentrations of roots and leaves, NR activities, and total N amounts, while inducing the up-regulation of CbuNRT1.1, CbuAMT1.2, and CbuAHA2 transcript levels, which likely enhanced nitrogen use efficiency (NUE) in both genotypes. These data suggest that Cbu22-03 shows greater capacities of N uptake and assimilation than CbuGu under the three N conditions, and both genotypes of C. bungei decreased the processes of N metabolism in response to low N availability.