The influence of combined carbon and nitrogen application on the growth and metabolism of Ilex rotunda

When cultivating Ilex rotunda Thunb., the blind application of nitrogen fertilizer often resulted in poor growth and negatively impacted yield and quality. To explore the balancing effect of carbon fertilizer on excessive nitrogen fertilizer and improve the yield and quality of I. rotunda, two-year-old cuttings were sprayed with different concentrations of urea and α-ketoglutarate (α-KG) in 12 treatment groups. After 12 weeks of treatment, the growth, photosynthetic indexes, main physiological indexes and metabolome of I. rotunda were determined and analyzed. The results showed that applying appropriate concentrations of α-KG and urea to I. rotunda enhanced its photosynthetic performance, evidenced by increased net photosynthetic rate and stomatal conductance, reduced intercellular carbon dioxide concentration, and improved specific leaf area of I. rotunda. The activity of photosystem Ⅱ (PS Ⅱ) was enhanced. The activities of enzymes related to GS-GOGAT cycle were increased, and the co-accumulation of carbon and nitrogen metabolites was promoted; it effectively mitigated the adverse effects of excessive nitrogen on the plant. The application of 4 g·L−1 urea + 40 mg·L−1 α-KG (N2KG1) and 4 g·L−1 urea + 80 mg·L−1 α-KG (N2KG2) increased the levels of α-KG and citric acid. N2KG1 and N2KG2 had the most positive effects on the photosynthetic performance, physiological and biochemical indexes, and accumulation of carbon and nitrogen metabolites in I. rotunda leaves. Therefore, the exogenous application of appropriate concentrations of α-KG and urea can promote the growth of I. rotunda, enhance its adaptability to environmental conditions, and improve plant quality. This provides a theoretical basis for cultivating high-quality I. rotunda specimens.