## 4 Discussion
### N Metabolism of Oilseed Rape Was Inhibited by AT treatment  
Compared with CK, most genes involved in N metabolism were markedly elevated by the NT treatment, though these genes were not induced by the AT treatment. The latest transcriptomics resultsfor N limitation (0.3 mmol/L NO3\({}^{-}\)) in oilseed rape show that genes related to N uptake and allocation, such as _BnaNRT1.1 s_ (_B. napus_ nitrate transporter _1.1 s_), _BnaNRT1.4 s_, _BnaNRT1.5 s_, _BnaNRT1.9 s_, _BnaNRT2.1 s_, _BnaNRT2.4 s_, and _BnaNRT3.1 s_, are induced in roots by N limitation, and N metabolism genes such as _BnaGSIs_ and _BnaGS2s_ are also elevated in roots under N limitation; in contrast, genes such as _BnaNRT1.8 s_, _BnaNIA1s_, and _BnaNIA2s_ are downregulated in roots [33]. These results are not consistent with our findings. The majority of genes related to N metabolism were induced in the roots of oilseed rape exposed to N limitation, whereas these genes were mostly strongly repressed in roots by the N-free (CK) treatment. This might be caused by the different adaptive mechanisms of oilseed rape in response to N limitation or N-free conditions. Overall, increased N uptake and transport and enhanced assimilation of inorganic N into amino acids might be helpful for the adaptability of oilseed rape to N limitation [33]. NT treatment enhanced expression of _BnNRTs_ but AT treatment decreased that of _BnAMTs_ in oilseed rape. Four types of transporters, namely, low- and high-affinity NO3\({}^{-}\) transporters (_NRTs_)/NH4\({}^{+}\) transporters (_AMTs_), take up and reallocate NO3\({}^{-}\) and NH4\({}^{+}\) in angiosperms, respectively [34]. NO3\({}^{-}\) induction of _GmNRT2_ mRNA expression was accompanied by a fourfold increase in net NO3\({}^{-}\) uptake by soybean roots under 100 \(\upmu\)M external NO3\({}^{-}\)[35]. In addition, correlation coefficients between \({}^{13}\)NO3\({}^{-}\) influx from 100 \(\upmu\)M and 5 mM [NO3\({}^{-}\)] suggest that in _A. thaliana_, high- and low-affinity transport systems are mediated primarily by _AtNRT2.1_ and _AtNRT1.1_, respectively [36]. In our physiology experiment, N accumulation in the NT treatment group was significantly higher than that in the AT treatment group. Therefore, upregulated expression of most _BnNRTs_ under NT treatment greatly promoted NO3\({}^{-}\) uptake by oilseed rape. Recent research has demonstrated that downregulation of _AtAMT1_ genes, which mediate the majority of NH4\({}^{+}\) uptake in _Arabidopsis_ roots, is associated with decreased NH4\({}^{+}\) accumulation in the _AtNRT1.1_ mutants _chl1-1_ and _chl1-5_ under conditions of high NH4\({}^{+}\)[29]. The adjustment of oilseed rape to excessive NH4\({}^{+}\) conditions via downregulation of the transcript levels of _BnAMTs_ might be an adaptive mechanism (Figure 8) for reducing NH4\({}^{+}\) uptake. Other research also supports this mechanism. For example, \({}^{13}\)NH4\({}^{+}\) influx and transcript levels of _OsAMT1.3_ in roots decreased within 48 h when plants acclimated to 10 \(\upmu\)m of external NH4\({}^{+}\) for 3 weeks were transferred to 10 mm NH4\({}^{+}\)[37]. NT treatment increased expression of _BnNRT1.5_ while decreasing that of _BnCLC-B_. Long-distance transport and distribution of NO3\({}^{-}\) between roots and shoots are regulated by two genes encoding components involved in long-transport mechanisms. _NRT1.5_ is responsible for NO3\({}^{-}\) xylem loading, and _NRT1.8_ is responsible for NO3\({}^{-}\) xylem unloading [38; 39]. _BnNRT1.5_ expression in the roots of oilseed rape increased significantly with NO3\({}^{-}\) treatment, driving greater NO3\({}^{-}\) long-distance transport from roots to shoots [16], which was beneficial for oilseed rape growth because NO3\({}^{-}\) assimilation efficiency is known to be higher in shoots than in roots due to solar energy utilization by shoots [40].