## 3 Discussion
It serves as a vital component of proteins and nucleic acids and plays a crucial role in carbon fixation during photosynthesis as a part of chlorophyll [24]. Plants possess the ability to obtain nitrogen either from the atmosphere as nitrogen (N\({}_{2}\)) or by absorbing soil nitrogen through their roots. The primary uptake of nitrogen by the vegetable root system occurs from inorganic nitrogen sources, including ammonium and nitrate, as well as small organic nitrogen-containing compounds present in the soil. Research has demonstrated the pivotal role of nitrogen in promoting rapid plant growth and enhancing yield. Consequently, traditional agricultural practices have often relied extensively on the applicationof excessive nitrogen fertilizers, aiming to maximize vegetable yields [25; 26]. However, a high nitrogen fertilization rate increased the severity of disease caused by botrytis cinerea in lettuce [27]. In Chinese cabbage, excessive nitrogen can induce petiole spot disease [9]. This dual nature of nutrient effects underscores the delicate balance needed for optimal plant health and productivity. In this study, the analysis of transcriptome and proteome data from resistant and susceptible Chinese cabbage varieties subjected to different nitrogen treatments unveiled the significant involvement of nitrogen metabolic pathways in the process (Figures 3 and 4). The plant nitrogen metabolic pathway facilitates the transport of inorganic nitrogen compounds into leaves through ammonium transporters (AMTs), nitrate transporters (NRT), and amino acid transporters (AATs). Within the plant, nitrate reductase (NR) orchestrates the cytoplasmic reduction of NO3\({}^{-}\) to nitrite (NO2\({}^{-}\)), and nitrite reductase (NiR) converts nitrite (NO2\({}^{-}\)) to NH4\({}^{+}\) in the plastids. The potential toxic accumulation of ammonium nitrogen poses a threat to plant health, underscoring the crucial role of ammonia assimilation in plants. The process of plant ammonia assimilation typically transforms inorganic nitrogen into organic nitrogen through the GS-GOGAT cycle, initiated by glutamine synthetase (GS) and glutamine-2-oxo-glutamate transaminase/glutamate synthetase (GOGAT) [24; 28]. Ammonia assimilation may also occur via glutamate dehydrogenase (GDH) in the presence of high ammonia concentrations. Significant differences in gene expression and enzyme activity of GS and GDH were observed between susceptible and resistant varieties in this study. Additionally, the accumulation of ammonium nitrogen (NH4\({}^{+}\) -N) in sensitive varieties treated with high nitrogen was significantly up-regulated (Figure 5). The emphasis on the importance of ammonia assimilation in the nitrogen metabolism process underscores its role in the onset of petiole spot. It is noteworthy that elevated nitrogen levels exacerbate petiole spot, leading to an increased density of spots, with ammonium nitrogen having a stronger impact than nitrate and amide nitrogen [12]. These findings align with previous research, reinforcing the influence of the nitrogen metabolism pathway on the development of leaf stalk disease in Chinese cabbage. A differential gene analysis of cabbage resistant and susceptible to petiole spot under normal and high nitrogen treatments revealed several genes related to plant ROS production that were enriched (Figure 6). Cells with petiole spots on Chinese cabbage exhibit a higher abundance of ROS compared to cells without petiole spots (Figure 7A). ROS are generated in response to a variety of environmental stresses, causing oxidative damage that may lead to cell death [29]. Moreover, an increase in ROS levels was observed in susceptible cabbage upon staining for ROS, and high nitrogen treatments exacerbated this discrepancy (Figure 7B). Notably, NH4\({}^{+}\) has been recognized as a catalyst for elevated levels of H2O2 and sequent oxidative stress responses in various plant species, including Arabidopsis [30], the aquatic plant _Vallisneria natans_[31], and numerous other plant species [32].