## 4 Discussion
### Oilseed Rape Seedling Growth Was Inhibited by AT treatment  
In the physiological experiment, compared with a high concentration of NO3- as the sole N source, a high concentration of NH4+ significantly reduced oilseed rape biomass, root growth, and N accumulation. These results indicate that a high concentration of NH4+ as the sole N source is a source of stress for oilseed rape but that NO3- or NO3- plus a small amount of NH4+ is a suitable N nutrition environment for growth. Some studies have found that NO3- plus a small amount of NH4+ is beneficial for the growth of crops that prefer NO3-[6] and that NH4+ plus a small amount of NO3- is beneficial for the growth of crops that prefer NH4+[26]. Additionally, some studies have confirmed that a single application of NH4+ or overapplication of NH4+ is conducive to the growth of most dryland crops [5; 6]. As NO3- taken up by plant cells can be stored in large quantities in vacuoles, this form of N is non-toxic  
Figure 7: Quantitative real-time PCR (qRT-PCR) validation of 15 differentially expressed genes (DEGs). (**a**) Transcript levels of 15 DEGs. Vertical bars indicate SDs (n = 3). The columns represent relative expression obtained by qRT-PCR. (**b**) Comparison of relative expression obtained from RNA-Seq data and qRT-PCR. RNA-Seq relative expression (\(x\)-axis) is plotted against qRT-PCR relative expression (\(y\)-axis). The correlation coefficient (R2) is indicated in the figure. FPKM: fragments per kilobase of exon per million fragments mapped. for plants. NO3\({}^{-}\) concentrations in plant cell vacuoles and the cytoplasm range 30-50 mol/m3 and 3-5 mol/m3[27; 28], respectively, whereas NH4\({}^{+}\) is rarely found in vacuoles. Indeed, NH4\({}^{+}\) taken up by plant cells must be combined with organic acids as soon as possible, leading to the synthesis of amino acids or amides and the acidification of cells. Recent studies have suggested that NH4\({}^{+}\) can restrain uptake of K\({}^{+}\), Ca2\({}^{+}\), Mg2\({}^{+}\), and other cations by crops [6]. A high concentration of NH4\({}^{+}\) can also suppress the activities of key enzymes in crop N metabolism, such as _NR_, _GS_, and _GOGAT_ (Glutamate synthase) [29]. Therefore, a high concentration of NH4\({}^{+}\) is likely to result in NH4\({}^{+}\) toxicity in dryland crops. In summary, oilseed rape appears to be a nitrate-preferring crop. The N efficiency of a crop has two meanings. On the one hand, it refers to the amount of N absorbed by crops at the same N supply level; on the other hand, it refers to high use efficiency of the N absorbed and the amount of dry matter produced per unit of N absorbed [30]. In this experiment, compared with the N-inefficient cultivar 814, the N-efficient cultivar 15 displayed better root architecture and higher N use efficiency under the NT treatment. The root system is the main organ for absorbing water and nutrients and is important for organic matter assimilation, transformation, and synthesis [31]. The 15 cultivar showed a better root length, root surface, and root volume than did 814, which led to more N uptake and accumulation in 15 than in 814 under the NT treatment. Our previous work [16] showed that compared with the N-efficient cultivar 15, the N-inefficient cultivar 814 stores more NO3\({}^{-}\) absorbed by the root system in root vacuoles, resulting in a decrease in NO3\({}^{-}\) transported from the root system to the aboveground part. However, to meet the growth and developmental needs of the aboveground part, the root system of the 814 cultivar needs to absorb more NO3\({}^{-}\). In other words, the 814 cultivar must absorb more NO3\({}^{-}\) to produce the same biomass as 15, which leads to lower N use efficiency in the former. GO (Figure S6) and KEGG (Figure S7) analyses of DEGs between 15 and 814 at 48 h after NT treatment (15 vs814(NT)) revealed some differences in gene expression between the two cultivars, especially DEGs related to the cell wall (Table S8) and N metabolism (Table S9).