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) 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 significantly promoted plant growth. In contrast, 10 mmol L Suc retarded plant growth and increased contents of anthocyanin and MDA and activity of SOD in comparison to 0.5-5 mmol L 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 Suc, which was consistent with root activity, suggesting that exogenous Suc enhanced root N uptake. At 10 mmol L 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 NH 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) increased supply of C-skeleton and promoted N uptake and assimilation in A. paniculata plant, whereas high Suc concentration (e.g. 10 mmol L) uncoupled C and N metabolisms, reduced N metabolism and induced plant senescence.