High nitrogen inhibits biomass and saponins accumulation in a medicinal plant Panax notoginseng

Abstract

Nitrogen (N) is an important macronutrient and is comprehensively involved in the synthesis of secondary metabolites. However, the interaction between N supply and crop yield and the accumulation of effective constituents in an N-sensitive medicinal plant Panax notoginseng (Burkill) F. H. Chen is not completely known. Morphological traits, N use and allocation, photosynthetic capacity and saponins accumulation were evaluated in two- and three-year-old P. notoginseng grown under different N regimes. The number and length of fibrous root, total root length and root volume were reduced with the increase of N supply. The accumulation of leaf and stem biomass (above-ground) were enhanced with increasing N supply, and LN-grown plants had the lowest root biomass. Above-ground biomass was closely correlated with N content, and the relationship between root biomass and N content was negatives in P. notoginseng (r = -0.92). N use efficiency-related parameters, NUE (N use efficiency, etc.), N_C (N content in carboxylation system component) and P _n (the net photosynthetic rate) were reduced in HN-grown P. notoginseng. SLN (specific leaf N), Chl (chlorophyll), N_L (N content in light capture component) increased with an increase in N application. Interestingly, root biomass was positively correlated with NUE, yield and P _n. Above-ground biomass was close negatively correlated with photosynthetic N use efficiency (PNUE). Saponins content was positively correlated with NUE and P _n. Additionally, HN improved the root yield of per plant compared with LN, but reduced the accumulation of saponins, and the lowest yield of saponins per unit area (35.71 kg·hm^-2) was recorded in HN-grown plants. HN-grown medicinal plants could inhibit the accumulation of root biomass by reducing N use and photosynthetic capacity, and HN-induced decrease in the accumulation of saponins (C-containing metabolites) might be closely related to the decline in N efficiency and photosynthetic capacity. Overall, N excess reduces the yield of root and C-containing secondary metabolites (active ingredient) in N-sensitive medicinal species such as P. notoginseng.

Keywords: Biomass; Economic yield; Nitrogen; Panax notoginseng; Saponins.

Figure 1. The nitrogen (N) content of root (A, D), stem (B, E) and leaf (C, F) in Panax notoginseng grown under different nitrogen fertilization.

Data are mean ± standard deviation (SD) of seven independent biological replicates performed in septuplicate (n = 7). Significant differences are indicated by lowercase letters (one-way ANOVA ; P < 0.05).

Figure 2. Biomass allocation in two- (A) and three-year-old (B) Panax notoginseng grown under different nitrogen fertilization.

RSR is the root to shoot ratio; RMF is the root mass fraction; SMF is the stem mass fraction; LMF is the leaf mass fraction. Data are mean ± SD of five independent biological replicates performed in quintuplicate (n = 5). Significant differences are indicated by lowercase letters (one-way ANOVA; P < 0.05).

Figure 3. The yield of per plant (A, C) and economic yield (B, D) in two- (A, B) and three-year-old (C, D) Panax notoginseng grown under different nitrogen fertilization.

Data are mean ± SD of five independent biological replicates performed in quintuplicate (n = 5). Significant differences are indicated by letters (one-way ANOVA; P < 0.05).

Figure 4. Nitrogen use efficiency (NUE) in two- (A) and three-year-old (B) Panax notoginseng grown under different nitrogen fertilization.

Data are mean ± SD of seven independent biological replicates performed in septuplicate (n = 7). Significant differences are indicated by letters (one-way ANOVA; P < 0.05).

Figure 5. The content of leaf nitrogen and chlorophyll in two- (A, B) and three-year-old (C, D) Panax notoginseng grown under different nitrogen fertilization.

Specific leaf nitrogen (SLN), total chlorophyll content (Chl), chlorophyll a content (Chl a), chlorophyll b content (Chl b). Data are mean ± SD of five independent biological replicates performed in quintuplicate (n = 5). Significant differences are indicated by lowercase letters (one-way ANOVA; P < 0.05).

Figure 6. The photosynthetic nitrogen use efficiency (A, C) and photosynthetic nitrogen allocation (B, D) in two- (A, B) and three-old-year (C, D) Panax notoginseng grown under different nitrogen fertilization.

PNUE, photosynthetic nitrogen use efficiency; N_photo, N content in photosynthetic apparatus; N_L, N content in light harvesting component; N_B, N content in bioenergetics component; N_C, N content in carboxylation component. Data are mean ± SD of five independent biological replicates performed in quintuplicate (n = 5). Significant differences are indicated by lowercase letters (one-way ANOVA; P < 0.05).

Figure 7. Saponins content of Panax notoginseng root grown nitrogen regimes (A, D). The saponins yield per plant grown under nitrogen regimes (B, E). Yield of saponins per area (C, F).

Saponin type: notoginsenoside R₁, ginsenoside Rd, ginsenoside Rg₁, ginsenoside Re, and ginsenoside Rb₁. Total saponin is the sum of R₁, Rd, Rg₁, Re, and Rb₁. Data are mean ± SD of seven independent biological replicates performed in septuplicate (n = 7). Different lowercase letters among nitrogen regimes indicate significant difference (one-way ANOVA, P < 0.05).

Figure 8. Pearson correlation coefficients heatmap of all parameters evaluated in Panax notoginseng grown under nitrogen regimes.

Pearson correlation coefficients of 27 parameters of P. notoginseng under different nitrogen levels. Mediumorchid indicates positive correlation; blue indicates negative correlation. The value in each box represents the correlation coefficient. N (plant), plant total nitrogen content; L (root), root length; P_n, net photosynthetic rate under saturated light; SLN, specific leaf nitrogen; Chl, chlorophyll; Chl a, chlorophyll a; Chl b, chlorophyll b; N (root), N content in root; m (root), root biomass; N (stem), N content in stem; m (stem), stem biomass; N (leaf), N content in leaf; m (leaf), leaf biomass; S (leaf), leaf area; PNUE, photosynthetic N use efficiency; NUE, N use efficiency; LCP, light compensation point; LSP, light saturation point; NAE, N agronomic efficiency; NUPE, N uptake efficiency (NUPE); RNF, recovery of N fertilizer; NCR, N contribution rate; NPFP, N partial factor productivity.

Figure 9. Principal component analysis (PCA) using all parameters evaluated in P. notoginseng grown under nitrogen regimes.

Principal component analysis of 20 parameters of P. notoginseng under different nitrogen levels. P_n, net photosynthetic rate under saturated light; SLN, specific leaf nitrogen; Chl, chlorophyll; Chl a, chlorophyll a; Chl b, chlorophyll b; N (root), N content in root; m (root), root biomass; N (stem), N content in stem; m (stem), stem biomass; N (leaf), N content in leaf; m (leaf), leaf biomass; S (leaf), leaf area; PNUE, photosynthetic N use efficiency; NUE, N use efficiency; LCP, light compensation point; LSP, light saturation point.

Figure 10. A model was proposed to explain the interaction between high N and the accumulation of biomass and C-containing secondary metabolites in a N-sensitive medicinal species, such as P. notoginseng.

The root of N-sensitive medicinal plants adopts a “survival strategy” of inhibiting root growth under N excess, and more biomass is allocated into above-ground at the expense of root biomass by inhibiting photosynthetic capacity and N use efficiency. The reduction in C/N ratio caused by the lower N use efficiency and photosynthetic capacity result in a suppressed accumulation of saponins (C-containing metabolites) under N excess. Arrows and blunted lines designate positive and inhibitory interactions, respectively. The blue and red arrows indicate down- and up-regulation.