## 4 Materials and Methods
### Transcriptome and Proteome Association Analysis  
The transcriptome and proteome data of Chinese cabbage were jointly analyzed with the assistance of PTM Biotechnology Co., Ltd. (Hangzhou, China). The mRNA information derived from the transcriptome was integrated with the protein informationidentified by the proteome to elucidate the corresponding relationships. The _p_-value and difference multiple of the genes and proteins identified through this joint analysis were compared and analyzed using theorem, theorem repeatability, and Pearson correlation analysis. Furthermore, KEGG pathway-based GSEA analysis, expression cluster analysis, and GO and KEGG enrichment analysis were performed to gain insights into the functional implications of the identified genes and proteins.  
Determination of Ammonium Nitrogen Content, Glutamine Synthetase and Glutamate Dehydrogenase Activities in Plants  
The content of ammonium nitrogen (NH4+) in plants was determined using a Plant Ammonium Nitrogen Content Assay Kit (Boxbio, Beijing, China) according to the manufacturer's instructions. Weigh 0.1 g of Chinese cabbage petiole and combine it with 1 mL of the extraction solution. Homogenize the mixture at room temperature, then centrifuge at 10,000\(\times\)\(g\) at 4 \({}^{\circ}\)C for 10 min, using the supernatant as the test sample. Preheat the spectrophotometer for at least 30 min, set the wavelength to 630 nm, and zero with distilled water. In a 1.5 mL centrifuge tube, add 20 \(\upmu\)L of the sample or extract (blank control), along with 490 \(\upmu\)L of Reagent I and 490 \(\upmu\)L of Reagent II. Thoroughly mix the components and allow the color reaction to occur at 25 \({}^{\circ}\)C for 20 min. Transfer 1 mL of the reaction solution to a 1 mL glass cuvette. Measure the absorbance at 630 nm, record the value as A, and calculate \(\Delta\)A = A (measurement) \(-\) A(blank).  
Glutamine synthetase (GS) activities were measured according to the instructions from Solarbio. Weigh 0.1 g of Chinese cabbage petiole and add 1 mL of the extraction solution. Homogenize the mixture in an ice bath and centrifuge at 8000\(\times\)\(g\) for 10 min at 4 \({}^{\circ}\)C. Collect the supernatant and keep it on ice for subsequent measurements. Preheat the zymograph for 30 min, adjusting the wavelength to 540 nm and zeroing it with distilled water. In the measurement tube, combine 160 \(\upmu\)L of Reagent I, 70 \(\upmu\)L of Reagent III, and 70 \(\upmu\)L of the sample. For the control tube, mix 160 \(\upmu\)L of Reagent II, 70 \(\upmu\)L of Reagent III, and 70 \(\upmu\)L of the sample. After thorough mixing and incubating in a 25 \({}^{\circ}\)C water bath, add 100 \(\upmu\)L of Reagent 4 to both the measurement and control tubes. Allow them to stand for 10 min, then centrifuge the samples at 5000\(\times\)\(g\) for 10 min at room temperature. Transfer 200 \(\upmu\)L of supernatant from the sample into a 96-well UV plate and determine the absorbance at 540 nm, recording it as B. Utilize the formula \(\Delta\)B = B (measurement) \(-\) B (control) to calculate the difference in absorbance between the assay and control samples.  
Glutamate dehydrogenase (GDH) activities were also measured according to the instructions from Solarbio. Weigh approximately 0.1 g of tissue and combine it with 1 mL of extraction solution for ice bath homogenization. Centrifuge at 8000\(\times\)\(g\) and 4 \({}^{\circ}\)C for 10 min, then carefully collect the supernatant, keeping it on ice for subsequent measurements. Preheat the zymograph for at least 30 min, adjusting the wavelength to 340 nm and zeroing it with distilled water. In a 96-well UV plate, add 10 \(\upmu\)L of the sample and 190 \(\upmu\)L of the working solution. Ensure thorough mixing, and immediately record the absorbance value C1 at 340 nm for 20 s, followed by the absorbance value C2 after 5 min and 20 s. Calculate \(\Delta\)C = C1 \(-\) C2.