Concentration-dependent dual effects of exogenous sucrose on nitrogen metabolism in Andrographis paniculata

The effects of exogenous sucrose (Suc) concentrations (0, 0.5, 1, 5, 10 mmol L^-1) on carbon (C) and nitrogen (N) metabolisms were investigated in a medicinal plant Andrographis paniculata (Chuanxinlian). Suc application with the concentration of 0.5-5 mmol L^-1 significantly promoted plant growth. In contrast, 10 mmol L^-1 Suc retarded plant growth and increased contents of anthocyanin and MDA and activity of SOD in comparison to 0.5-5 mmol L^-1 Suc. Suc application increased contents of leaf soluble sugar, reducing sugar and trerhalose, as well as isocitrate dehydrogenase (ICDH) activity, increasing supply of C-skeleton for N assimilation. However, total leaf N was peaked at 1 mmol L^-1 Suc, which was consistent with root activity, suggesting that exogenous Suc enhanced root N uptake. At 10 mmol L^-1 Suc, total leaf N and activities of glutamine synthase (GS), glutamate synthase (GOGAT), NADH-dependent glutamate dehydrogenase (NADH-GDH) and glutamic-pyruvic transaminase (GPT) were strongly reduced but NH₄⁺ concentration was significantly increased. The results revealed that exogenous Suc is an effective stimulant for A. paniculata plant growth. Low Suc concentration (e.g. 1 mmol L^-1) increased supply of C-skeleton and promoted N uptake and assimilation in A. paniculata plant, whereas high Suc concentration (e.g. 10 mmol L^-1) uncoupled C and N metabolisms, reduced N metabolism and induced plant senescence.