Introduction

Salinization is one of the main threats to crop production in arid regions, and it causes severe reduced biomass and productivity (Taïbi et al. 2016). Salinization also alters various biochemical and physiological processes, thereby resulting in a metabolic imbalance in plants (Li et al. 2016). Generally, salinization negatively affected plants in three ways as follows: (1) the soil has a high concentration of salt leads to high osmotic potential in plants; (2) imbalance of ions in cells such as Na+ and Cl− leads to ion toxicity in plants; (3) excessive production of reactive oxygen species (ROS) leads to oxidative stress in plants (Salimi et al. 2016; Ahmadi et al. 2018).

Osmotic regulation is one of the biochemical changes in plants under salt stress. It promotes the protection of plant cell structure, prevents ionic toxicity, and improves water absorption through the synthesis and accumulation of a large number of compatible solutes in plants (Khan et al. 2015; Zhang et al. 2018). Many compatible solutes such as carbohydrates, amino acids and proteins which are products of carbon (C) and nitrogen (N) metabolism (Liu et al. 2014b, a). C and N metabolisms are very sensitive to environmental stress that generally decreases chlorophyll contents, depresses photosynthesis, alters expression of genes and activities of enzyme, which thus inhibit growth (Flores et al. 2004; Debouba et al. 2006; Cuellar-Ortiz et al. 2008). When plants are subjected to salt stress, the cell membrane is firstly affected and the membrane permeability is increased (Liu et al. 2015). Other significant effects of salt stress are changes in soluble sugar content and in invertase (INV) activities, sucrose synthase (SS), sucrose phosphate synthase (SPS) (Fernandes et al. 2004). Moreover, the yield of secondary metabolites is closely related to the growth conditions, and plants usually produce more secondary metabolites under moderate environmental stresses (Kleinwächter and Selmar 2013). Thus, C and N metabolism and secondary metabolites are of vital importance for stress tolerance.

Methyl jasmonate (MeJA), as an inducer or signal transduction agent, is involved in many physiological and biochemical processes of plants (Fan et al. 2016; Zaid and Mohammad 2018). MeJA also elicits beneficial effects on active components in plant. Takahashi and Hara (2014) sprayed MeJA on leaf number of plants and found that MeJA promotes the accumulation of starch by up-regulating the expression genes of starch biosynthetic in Arabidopsis thaliana. Furthermore, MeJA induces the accumulations of monoterpene and sesquiterpene by stimulating oxygenated linalool and (E)-β-farnesene or forming traumatic resin (TDs) ducts in xylem and foliage of Norway spruce (Martin et al. 2002, 2003). Zaid and Mohammad (2018) studied the interactive effect of MeJA and N in reducing the toxicity of Cd in an important medicinal and aromatic plant.

Glycyrrhiza uralensis Fisch. (G. uralensis) is one of the most widely used herbal medicine and food additives (Egamberdieva et al. 2017). Generally, wild G. uralensis resource grows in arid and salinization areas and therefore has the ability to adapt to drought and salt conditions. Nevertheless, studies and practices proved that the salt tolerance ability of cultivated G. uralensis is lower than wild plants (Zhang et al. 2018). Actually, soil salinization and water resource shortage are universal in regions of G. uralensis cultivation. Thus, improving the salt and drought tolerance of cultivated G. uralensis has become an important goal for sustainable development. However, there is limited information on MeJA’s response to C and N metabolism and secondary metabolites to abiotic stresses including salt stress, and thus in light of the important role of MeJA in plant stress response, this research was conducted to study the effects of MeJA on the growth parameters, key metabolites and key enzyme activities related to C and N metabolism, and the contents of bioactive components in G. uralensis under salt stress.

Materials and methods

Plant materials and growth conditions

The seeds of G. uralensis, we utilized were provided by Ningxia Academy of Agriculture and Forestry Sciences, China, Yinchuan, Ningxia Province, China, and it is identified as G. uralensis by Ming Li Professor.

The seeds of G. uralensis were steeped with 85% H2SO4 for 45 min to break the seed coat, then surface sterilized with 0.1% H2O2 for 10 min, cleaned 3 times using distilled water and imbibed in distilled water for 8 h at 25 °C. Eighty seeds of uniform size that were pre-treated were planted in plastic boxes (6 × 12 × 12 cm) filled with 1400 g autoclaved sand medium that were pre-irrigated. The treatments include control group (CK), with 370 mL of distilled water; salt stress group (S), with 370 mL of distilled water containing 75 mM NaCl; salt stress combined with methyl jasmonate group (S + MeJA), with 370 mL of distilled water containing 75 mM NaCl and 30 μM MeJA. Seedling growth conditions were maintained at 12 h/12 h, 28 °C/20 °C (day/night).

A weighing method (every afternoon weighing and watering) was used to control saturation moisture content 65–75% for all treatments. Three replications per treatment were used, and all pots were randomly arranged and periodically rotated to minimize the effects of environmental heterogeneity. Whole G. uralensis seedlings from all treatments were collected in the morning at 45 day after treatment for determination of all parameters.

Determination of carbohydrates

Soluble sugar and sucrose were determined according to the method of Loutfy et al. (2012). Specifically, 0.2 g of fresh seedlings were extracted with 10 mL of distilled water at 100 °C for 30 min, then the homogenate was centrifuged at 3000 g for 10 min, and 0.5 mL of supernatant added 5 mL of ketone concentrated sulfuric acid reagent at 100 °C for 1 min.

Finally, the absorbance was measured at 630 nm. In addition, a standard curve was plotted with 0–100 mg of glucose, and standard curves as Y = 0.00847X−0.0059. For sucrose, 0.2 g fresh seedlings were extracted with 4 mL distilled water, then homogenate was centrifuged at 3000 g for 10 min and supernatant (1 mL) contained 2 M of NaOH (1 mL), then kept in boiling water bath for 10 min, stop the reaction by adding 1 mL of 0.1% (w/v) resorcinol, and 10 M of HCl, stop the reaction by keeping in boiling water bath for 8 min. Finally, the absorbance was measured at 500 nm. In addition, the standard curve was plotted with 0–100 mg of glucose, and standard curves as Y = 0.00847X−0.0059 for soluble sugar and Y = 0.019X + 0.0009 for sucrose.

Determination of N forms

To determine NO3−, NH4+ and NO2−, 0.2 g fresh seedlings were homogenized with deionized water, centrifuged at 4 °C and 8300 g for 10 min. The NO3− and NH4+ in the supernatant were quantified photometrically at 410 and 630 nm, respectively, and KNO3 and (NH4)2SO4 were used as standard, respectively (Zhang et al. 2017). The NO2− in the supernatant was quantified photometrically at 520 nm.

C and N metabolism-related enzyme extraction and assays

Invertase (INV) activity was determined as described by Yang et al. (2004). 0.2 g fresh seedlings were homogenated with chilled distilled water for 3 h at 4 °C and the mixture was centrifuged at 8000 g for 10 min at 4 °C. INV activity was assayed using the DNS method, which measures reducing sugar content at 540 nm.

To measure sucrose synthase (SS) and sucrose phosphate synthase (SPS) activities, 0.2 g fresh seedlings were homogenized with 50 mM HEPES–NaOH (pH 7.5) and the mixture was centrifuged at 12000 g for 10 min at 4 °C. The assay mixture of SS containing the supernatant, 50 mM HEPES–NaOH (pH 7.5), 50 mM MgCl2, 100 mM uridine diphosphoglucose (UDPG) and 100 mM fructose. After incubation at 30 °C for 30 min, 2 M NaOH was added to the assay mixture to terminate the reaction, then, the mixture was boiled for 10 min and cooled. After that, 30% (w/v) HCl and 0.1% (w/v) m-dihydroxybenzene were added, the mixture was shaken thoroughly, kept in a water bath for 10 min at 80 °C, and once the mixture was cooled, its absorbance was measured at 480 nm. SPS activity was determined in the same way as SS activity; the only difference is that fructose was substituted by fructose-6-phosphate (Xie et al. 2021).

To measure nitrate reductase (NR) activity, 0.2 g fresh seedlings were homogenized with 3 mL of 25 mM potassium phosphate buffer (pH 7.5) containing 10 mM L-cysteine and 1 mM EDTA-Na2 and centrifuged at 8300 g for 10 min at 4 °C. The NR activity in the supernatant was quantified photometrically at 540 nm based on the reduction of nitrate to nitrite during 30 min at 25 °C (Du et al. 2008).

To measure the activities of glutamine synthetase (GS) and glutamate synthetase (GOGAT), 0.2 g fresh seedlings were homogenized with 3 mL of 50 mM Tris–HCl buffer (pH 8.0, containing 2 mM Mg2+, 2 mM DTT, and 0.4 M sucrose) and the mixture was centrifuged at 8300 g for 10 min at 4 °C, and the supernatant as enzyme extract. GS activity was assayed by monitoring the formation of glutamyl hydroxamate at 540 nm after reacting with acidified ferric chloride (Liu et al. 2014a, b). GOGAT activity was measured by estimating the oxidation of NADH at 340 nm was assayed according to Wang et al. (2018).

Determination of glycyrrhizic acid, liquiritin and licochalcone A contents

The determination was carried out by HPLC on the C18 column (4.6 mm × 15 cm, 5 μm). The mobile phase consisted of acetonitrile-1% acetic acid with 0.6 mL·min−1 of the flow rate, and the temperature of column was 25 °C. To determine the quantitative assay using the external standard method.

Determination of total flavonoids, total saponins and total polysaccharide contents

For determination of total flavonoids, the powder sample (0.05 g) was extracted with 80% ethanol for 40 min at 40 °C in an ultrasonic bath and the supernatant was used as the extract. 400 μL extract contained 1 mL 70% methanol and 2 mL 10% sodium hydroxide for 10 min, then added 1 mL 70% methanol again, finally the supernatant was measured photometrically at 410 nm.

For determination of total saponins, the powder sample (0.05 g) was extracted with 80% (containing 0.3% ammonia water) ethanol for 80 min at 40 °C in an ultrasonic bath and the supernatant used as the sample extract. 200 μL extract contained 0.25 mL 5% vanillin glacial acetic acid solution and 0.8 mL perchloric acid, react at 55 °C for 20 min in a water bath, then added 2 mL glacial acetic acid under temperature, the absorbance of the supernatant at 590 nm was measured.

For determination of total polysaccharide, the powder sample (0.05 g) was extracted with 80% ethanol for 80 min at 40 °C in an ultrasonic bath and the supernatant was used as the sample extract. 40 μL extract contained 0.5 mL 5% phenol solution, then added 2.5 mL concentrated sulfuric acid, finally the supernatant was measured photometrically at 490 nm.

Statistical analysis

Each treatment of the experiment was designed completely randomly. SPSS 17.0 software (SPSS, Inc., Chicago, IL, USA) was used for principal component analysis (PCA) and Pearson’s correlation analysis. Significant differences were tested using the least significant difference (LSD) test at P < 0.05. Mean values and standard errors (SEs) were presented.

Results

Effects of MeJA on growth in salt-stressed G. uralensis seedlings

Exposure of G. uralensis plants to salt stress leads to a significant decline in growth parameters including main root length, main root diameter and secondary root number (Fig. 1), and reduced by 28.11%, 23.04% and 48.01%, respectively compared with control. Salt stress also decreased plant height, leaf number and biomass yield by 46.67%, 31.75% and 56.19%, respectively, relative to control. Supply of MeJA showed an enhancement by 23.61%, 16.04% and 24.58% at axial root length, secondary root numbers and biomass yield as compared with salt stress, respectively (Table 1).

Fig. 1
figure 1

Effect of MeJA on growth property of G. uralensis grown under salt stress. CK, control group; S (75 mM NaCl), salt stress group, S + MeJA (75 mM NaCl + 30 μM MeJA), salt stress combined with methyl jasmonate group

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Table 1 Effects of MeJA on growth and biomass in salt-stressed G. uralensis seedlings
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A significant negative correlation was observed between soluble sugar and main root length, main shoot diameter, secondary root numbers and leaf number, respectively. A significant negative correlation was observed between sucrose and main root diameter, also a significant negative correlation was observed between NO3− and plant height, leaf number, respectively. In addition, a significant positive correlation was observed between NR and plant height, a significant negative correlation was observed between NR and shoot diameter (Table 2).

Table 2 Linear correlations (Pearson’s coefficient) between growth and physiochemical characteristics in G. uralensis
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A significant positive correlation was observed between leaf number and glycyrrhizic acid and glycyrrhizin, while a significant positive correlation was observed between biomass and glycyrrhizic acid, glycyrrhizin and licochalcone A. A significant positive correlation was observed between total flavonoids and leaf number and biomass yield (Table 3).

Table 3 The correlations analysis between glycyrrhizic acid, liquiritigenin, licochalcone A and growth in G. uralensis
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Effects of MeJA on carbohydrates contents in salt-stressed G. uralensis seedlings

Salt stress enhances soluble sugar and sucrose contents by 69.74% and 36.05% in salt-stressed G. uralensis plants relative to control. MeJA further induces an enhancement of soluble sugar and sucrose contents by 7.80% and 8.69%, respectively, as compared with plants treated with salt only (Fig. 2).

Fig. 2
figure 2

Effect of MeJA on the soluble sugar, sucrose, SS, SPS and INV in G. uralensis grown under salt stress. Values are means ± SE (n = 6). CK, control group; S (75 mM NaCl), salt stress group; S + MeJA (75 mM NaCl + 30 μM MeJA), salt stress combined with methyl jasmonate group. The different letters within the different treatments indicate the significant difference at P ≤ 0.05

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A significant positive correlation was observed between soluble sugar, SS and SPS (Table 2).

Effects of MeJA on the activities of INV, SS and SPS in salt-stressed G. uralensis seedlings

The activities of SS, SPS and INV in G. uralensis plants were not affected by salt stress compared to control. MeJA triggered the enhancement of SS and SPS activities compared to salt stress, and increased by 43.08% and 66.74%, respectively, compared to salt stress (Fig. 2).

Effects of MeJA on NR, GS and GOGAT activities in salt-stressed G. uralensis seedlings

The results regarding the impacts of MeJA on NR, GS and GOGAT activities in salt-stressed G. uralensis plants are depicted in Fig. 4. Reduction in NR and GOGAT activities were observed in G. uralensis plants under salt condition compared to control. MeJA increases NR, GS and GOGAT activities by 31.06%, 8.42% and 47.38% in G. uralensis plants, respectively, as compared with salt alone (Fig. 3).

Fig. 3
figure 3

Effect of MeJA on NO2−, NO3−, GOGAT, NH4+, GS and NR in G. uralensis grown under salt stress. Values are means ± SE (n = 6). CK, control group; S (75 mM NaCl), salt stress group; S + MeJA (75 mM NaCl + 30 μM MeJA), salt stress combined with methyl jasmonate group. The different letters within the different treatments indicate the significant difference at P ≤ 0.05

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Effects of MeJA on NO2 −, NO3 − and NH4 + concentrations in salt-stressed G. uralensis seedlings

Salt stress triggered the increase of NO2− and NH4+ contents by 21.76% and 47.43%, respectively, compared to control. Supplementation of MeJA to G. uralensis plants displayed a remarkable increase in NO3− and NO2− contents, which rose by 21.67% and 28.99%, respectively, as compared to salt alone. In addition, MeJA effectively reduced NH4+ content compared to salt alone (Fig. 3).

Effects of MeJA on the contents of glycyrrhizic acid, liquiritin and licochalcone A in salt-stressed G. uralensis seedlings

Salt stress induced the decreased of glycyrrhizic acid content by 12.72%, respectively, compared to control in G. uralensis plants, but glycyrrhizin and licochalcone A contents unchanged compared with the control. MeJA significantly increased glycyrrhizic acid and licochalcone A contents by 12.04% and 10.01%, respectively, compared to salt stress (Fig. 4).

Fig. 4
figure 4

Effect of MeJA on the contents of glycyrrhizic acid, liquiritin and licochalcone A, and total flavonoids, saponins and polysaccharide in G. uralensis grown under salt stress. Values are means ± SE (n = 6). CK, control group; S (75 mM NaCl), salt stress group; S + MeJA (75 mM NaCl + 30 μM MeJA), salt stress combined with methyl jasmonate group. The different letters within the different treatments indicate the significant difference at P ≤ 0.05

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A significant positive correlation was observed between glycyrrhizic acid and licochalcone A and glycyrrhizin (Table 3).

Effects of MeJA on contents in total flavonoids, total saponins, total polysaccharide in salt-stressed G. uralensis seedlings

Salt stress triggered the decrease of total flavonoids and polysaccharides contents by 27.83% and 37.01%, respectively, compared to control in G. uralensis plants, while salt stress also increased total saponin content. MeJA increased total flavonoids and total polysaccharides contents by 15.75% and 41.82%, respectively, compared to salt stress (Fig. 4).

PCA of growth and physio-biochemical characteristics in G. uralensis

PCA divided the total variance into five PCs with the largest contribution (88.799%) (Table 4). The higher eigenvectors of PC I are soluble sugar, SS, SPS and GS. The higher eigenvectors of PC II are INV and NR. The higher eigenvectors of PC III is NO3−. The higher eigenvectors of PC IV are sucrose and NO2−. The higher eigenvectors of PC V is GOGAT (Fig. 5).

Table 4 Eigenvectors and percentages of accumulated contribution of principal components
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Fig. 5
figure 5

Effect of MeJA in salt-stressed G. uralensis seedlings by carbon and nitrogen metabolism. Sucrose synthase (SS), sucrose phosphate synthase (SPS) and sucrose of carbon metabolism were analyzed, and nitrate (NO3−), ammonium (NH4+), nitrate reductase (NR), glutamate synthase (GOGAT), nitrite (NO2−) and glutamine synthetase (GS) of nitrogen metabolism were also detected. The pointing of red arrows in the figure represent the changes of enzymes and substances in carbon and nitrogen metabolism which were mainly affected by MeJA

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Discussion

Salt stress usually causes a severe reduction in plant biomass. In this study, the length and diameter of main root and the number of secondary root of G. uralensis seedling significantly reduced by salt stress, which is consistent with previous reports on various plants such as wheat (Bot et al. 2013), tomato (Latef et al. 2011), pepper (Latef et al. 2014) and others (Duan et al. 2013; Mostofa et al. 2015; Ahmad et al. 2016). MeJA can alleviate the inhibition effect of salt stress on plant growth (Ahmadi et al. 2018). The present results showed that MeJA significantly increased main root length, secondary root numbers and biomass while did not have a significant effect on shoot diameter of G. uralensis seedling subjected to salt stress, indicating that MeJA application alleviates the inhibited effect of salt to the growth of root and biomass, which is consistent with previous results in cowpea (Sadeghipour 2017) and strawberry (Faghih et al. 2017).

Osmotic stress affects plant growth and biomass under salt stress condition. To respond to the harmful impact of osmotic stress induced by salt stress, plants produce and accumulate higher levels of compatible solutes in the cytosol and other organelles (Latef and Miransari 2014). Compatible solutes such as proline, soluble sugar and soluble protein are mainly accumulated in response to salt stress in G. uralensis (Lu et al. 2013; Zhang et al. 2021). Among them, soluble sugar maintains cell homeostasis and improves plant tolerance to osmotic stress (Ahmad et al. 2016; Bai et al. 2013). In which sucrose plays an important role in plant structure and metabolism and it is also involved in responses to various stresses (Qiu et al. 2014). In this study, a significant accumulation trend in soluble sugar and sucrose was recorded in G. uralensis seedling exposed to salt stress (Fig. 2), which is similar to previous results in Zea mays L. (Feng et al. 2002), Lupinus albus L. (Fernandes et al. 2004) and Nitraria tangutorum (Liu et al. 2016). A previous study reported that MeJA enhanced the protective property of the osmolytes in peach (Yu et al. 2016). Moreover, the present results showed that the increase in contents of soluble sugars accompanied by the decrease in growth parameters under salt treatment, which explain the negative Pearson’s coefficient of soluble sugars for the growth parameters (Table 2). Our results also showed that application of MeJA to salt-stressed G. uralensis seedlings induced an increase in sucrose level, which perhaps provide better protection to G. uralensis seedlings subjected to osmotic stress. Therefore, MeJA may be an effective method for protecting plants against osmotic stress through regulating carbohydrate accumulation.

Accumulation of carbohydrates is closely relevant to the activities of INV, SS and SPS that are involved in sucrose synthesis and metabolism. Salt stress affected the levels of INV, SS and SPS activities in various plants such as Cicer arietinum (Kaur et al. 2003), Lotus japonicus (Miguel et al. 2008), and Triticum aestivum L. (Fresneau et al. 2007). In the present study, INV activity increased in G. uralensis under salt stress with the increased sucrose content, while the activities of SS and SPS had slight change in G. uralensis, indicating that INV plays a vital role in buffer regulation of the carbohydrates contents for salt-induced osmotic stress. This is supported by the previous results in Dwarf bamboo (Liu et al. 2014a, b) and Cicer arietinum (Kaur et al. 2003). Moreover, MeJA markedly enhanced INV, SS and SPS activities, which is in harmony with the results in Hericium erinaceus (Wang et al. 2011). These results suggested that MeJA could further alleviate salt stress by inducing INV, SS and SPS activities, and then these enzymes could affect carbohydrate levels of G. uralensis under salt stress (Table 2).

The regulation of N metabolism is an important property to responding to stress, which affects almost all physiological processes in plants. Salt stress interferes with NO3− uptake and decreases NO3− contents in Triticum aestivum L. (Elbaki et al. 2017), Citrullus lanatus (Yang et al. 2013), Helianthus annuus L. (Silva et al. 2014) and Cucumis sativus L. (Shao et al. 2015). In this study, salt stress has no effect on NO3− content in G. uralensis seedlings, this may be due to the NO3−supply and transfer rate from the vacuole into the cytoplasm, which is supported by the previous found in Fargesia denudata (Liu et al. 2014a, b). Increased NO3− assimilation is beneficial for improving the resistance of plants under stresses via regulating stomatal opening and the synthesis of osmotic substances (Yi et al. 2014; Wilkinson et al. 2007). Interestingly, MeJA application significantly increased NO3− content in G. uralensis under salt stress. It suggests that MeJA enhanced NO3− absorption in G. uralensis roots which might improve osmotic regulation, thus strengthen the adaptation to salt stress. NR convert NO3− to NO2−, then NiR catalyze the conversion of NO2− into NH4+, further supply more substrate for proline synthesis (Khan et al. 2015). Previous studies showed that salt stress decreased the NR activity in Populus simonii (Meng et al. 2016), soybean (Farhangi-Abriz and Torabian 2017) and wheat (Elbaki 2017) under salt stress. Moreover, MeJA can increase N assimilation in Triticum aestivum L. by increasing NR and glutamate synthase (GS) activity (Kaya et al. 2021). Present study results found that salt stress significantly decreased NR activity but increased NO2− content in G. uralensis seedlings. However, MeJA application increased NR activity and NO2− content in salt-stressed G. uralensis, which in harmony with the results in Solanum lycopersicum (Singh et al. 2016). These results suggest that MeJA can promote the conversion of NO3− to NO2− by increasing the activity of NR, which might facilitating the assimilation of NO3− and the synthesis of amino acid subsequently (Kaya et al. 2021).

Excessive NH4+ from both NO3− reduction and photo-respiration is toxic to plant cells (Thomas and Hilker 2000). In the present study, salt stress caused a significant increase in NH4+ content, which is consistent with the results in tomato (Manai 2012) and cucumber (Shao et al. 2015). Interestingly, MeJA application decreased NH4+ content in salt-stressed G. uralensis and this may mitigate the cytotoxicity of excess NH4+ induced by salinity. The conversion of non-toxic organic N is closely associated with GS/GOGAT cycle, which is the major route for NH4+ assimilation (Yang et al. 2010). Our result observed decreased GOGAT activity in G. uralensis under salt stress, the results were consistent with those in cucumber (Shao et al. 2015) and soybean (Zilli et al. 2008). These results suggested that the GS/GOGAT cycle is severely blocked in plants subjected to salt stress condition. Whereas, MeJA remarkably improved GOGAT and GS activities in G. uralensis under salt stress, suggesting that MeJA could reduce NH4+ accumulation by accelerating the interconversion of glutamine (Glu) and glutamic acid (Gln). Taken together, we concluded that MeJA promoted N assimilation and removal of the excess NH4+ by increasing the activities of NR, GS and GOGAT under salt stress, which helps to maintain the balance of N metabolism in G. uralensis exposed to salt stress. In addition, MeJA significantly up-regulated GDH in rice, which plays a role in the re-assimilation of the excess ammonium induced exposed to stress (Wu et al. 2019). The effect of MeJA on GDH activity should be considered subsequently in this study to help clarify in more deeply the mechanisms by which MeJA improves nitrogen metabolism under salt stress.

Secondary metabolism is the result that the interaction between plants and environments in the long-term evolution, and it is closely correlated with the strengthening of plant tolerance to stresses (Xiao-Hong et al. 2005; Wang et al. 2015). The production of anthraquinone, phenolics and flavonoids was increased in adventitious roots of Morinda citrifolia under salt stress (Baque et al. 2010). The contents of secondary metabolites in Swertia chirata Buch.-Ham were remarkably increased under salt stress (Abrol et al. 2012). Salt stress also remarkably enhanced the total phenolics and γ- and δ-tocopherols levels in wild almond species (Sorkheh et al. 2012). In this study, the results found that salt stress significantly decreased glycyrrhizic acid content in G. uralensis plants, but has no effect on glycyrrhizin and licochalcone A contents. Salt stress also decreased total flavonoids and polysaccharides contents in G. uralensis plants, suggesting that salt stress remarkably regulated the synthesis and accumulation of secondary metabolites. However, salt stress significantly increased the total saponin content that perhaps a responsive strategy for G. uralensis seedlings adapt to salt stress by producing more secondary metabolites, further promoting plant growth. Previous studies have found that the application of exogenous substances (plant growth regulators) can effectively promote the synthesis of secondary metabolites in plants (Yu et al. 2018). In Catharanthus roseus L., MeJA promoted the accumulation of secondary metabolites (Ruiz-May et al. 2011). In Polygonum multiflorum, MeJA enriches arachidonic acid and linoleic acid metabolisms, and biosynthesis of stilbenoid by inducing transcriptome changes (Liu et al. 2015). Our results showed that MeJA remarkably enhanced glycyrrhizic acid, licochalcone A contents, total flavonoids and polysaccharides contents compared in salt-stressed G. uralensis seedlings. These results indicated that MeJA could induce the accumulation of secondary metabolites in G. uralensis seedlings, the possible reason is that MeJA regulates the key enzymes related to the secondary metabolites and its gene expression in G. uralensis which need further research.

Conclusions

MeJA could relieve the harmful effects caused by salt stress on G. uralensis seedlings through affecting C and N metabolisms that further trigger the accumulation of secondary metabolites. Specifically, MeJA significantly increased carbohydrate contents by enhancing the activities of related enzymes in C metabolism in G. uralensis. Moreover, the increase in soluble sugar and sucrose induced by MeJA plays a protective role in osmotic regulation of salt-stressed G. uralensis. Meanwhile, MeJA enhanced the N metabolism that could induce G. uralensis seedlings produce more amino acids and total N to overcome osmotic damage. Furthermore, MeJA significantly increased glycyrrhizic acid, licochalcone A, total flavonoids and total polysaccharides contents in salt-stressed G. uralensis. Thus, MeJA can effectively regulate different metabolites in G. uralensis to relieve osmotic pressure under salt stress to improve growth. Further research should investigate mechanisms of molecular by which MeJA affect secondary metabolism, including signaling pathway to regulate biosynthesis gene expression. The positive MeJA effects on G. uralensis for growth and the accumulation of secondary metabolites also need to be tested under field conditions.

Author contribution statement

XH Z conceived and designed the experiments; X M, XX Y, GC C and DY L performed the experiments; X M wrote the manuscript; XX Y made the figure; ZG G provided guidance on the whole manuscript, and all authors read and approved the final manuscript and post no conflicting interest.