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

Abstract

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.

Figure 1

Plant phenotype (a), root activity (b), and chlorophyll (c,d) in different sucrose concentrations. Scale bar 10 cm in (a). Data refers to means ± SE (n = 4). Bars with the same letter are no statistically significantly difference at P < 0.05 by Duncan’s new multiple range method.

Figure 2

Leaf MDA content (a), SOD activity (b) and anthocyanin content (c) in different sucrose concentrations. Data refers to means ± SE (n = 4). Bars with the same letter are no statistically significantly difference at P < 0.05 by Duncan’s new multiple range method.

Figure 3

Leaf total nitrogen (a), ammonium (b), soluble protein (c), and free amino acids (d) in different sucrose concentrations. Data refers to means ± SE (n = 4). Bars with the same letter are no statistically significantly difference at P < 0.05 by Duncan’s new multiple range method.

Figure 4

Glutamine synthase (GS, a), glutamate synthase (GOGAT, b), NADH-dependent glutamate dehydrogenase (NADH-GDH, c), glutamic–oxalacetic transaminase (GOT, d), and glutamic–pyruvic transaminase (GPT, e) activities in different sucrose concentrations. Data refers to means ± SE (n = 4). Bars with the same letter are no statistically significantly difference at P < 0.05 by Duncan’s new multiple range method.

Figure 5

Total soluble sugar (a), reducing sugar (b), sucrose (c), and trehalose (d) in different sucrose concentrations. Data refers to means ± SE (n = 4). Bars with the same letter are no statistically significantly difference at P < 0.05 by Duncan’s new multiple range method.

Figure 6

Invertase (IV, a) and isocitrate dehydrogenase (ICDH, b) activities in different sucrose concentrations. Data refers to means ± SE (n = 4). Bars with the same letter are no statistically significantly difference at P < 0.05 by Duncan’s new multiple range method.

Figure 7

Simplified representation of the effect of exogenous Suc concentration on C and N metabolism in A. paniculata. (a) Effects of 1 mmol L⁻¹ Suc on plant metabolites and enzyme activities in comparison to the control. (b) Effects of 10 mmol L⁻¹ Suc on plant metabolites and enzyme activities in comparison to 1 mmol L⁻¹ Suc. Red lines and boxes represent significant increase, green lines and boxes represent significant decrease, and yellow boxes represent unchanged. NH₄⁺ and anthocyanin were highlighted by blue wireframe, because they are key products of N and C metabolism, respectively, whose changes were closely related to the differential responses of plants to exogenous Suc.