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        Cadmium (Cd) pollution poses a threat to the cultivation of the medicinal plant Panax notoginseng in Yunnan Province. Under conditions of exogenous Cd stress, a field experiment was conducted to understand the effect of lime application (0.750, 2250 and 3750 kg bm-2) and oxalic acid spray (0, 0.1 and 0.2 mol l-1) on Cd accumulation. and antioxidant action Systemic and medicinal components affecting Panax notoginseng. The results showed that quicklime and foliar spraying with oxalic acid could increase Ca2+ levels in Panax notoginseng under Cd stress and reduce Cd2+ toxicity. The addition of lime and oxalic acid increased the activity of antioxidant enzymes and altered the metabolism of osmoregulators. CAT activity increased most significantly, increasing 2.77 times. The highest activity of SOD increased by 1.78 times when treated with oxalic acid. The content of MDA decreased by 58.38%. There is a very significant correlation with soluble sugar, free amino acid, proline, and soluble protein. Lime and oxalic acid can increase calcium ions (Ca2+), decrease Cd, improve stress tolerance in Panax notoginseng, and increase total saponins and flavonoid production. The content of Cd was the lowest, 68.57% lower than in the control, which corresponded to the standard value (Cd≤0.5 mg/kg, GB/T 19086-2008). The proportion of SPN was 7.73%, which reached the highest level of each treatment, and the content of flavonoids increased significantly by 21.74%, reaching the drug standard value and the best yield.
        Cadmium (Cd), as a common contaminant in cultivated soil, migrates readily and has significant biological toxicity1. El Shafei et al. 2 reported that Cd toxicity affects the quality and productivity of the plants used. In recent years, the phenomenon of excess cadmium in the soil of cultivated land in southwest China has become very serious. Yunnan Province is China’s Biodiversity Kingdom, among which medicinal plant species rank first in the country. However, the rich mineral resources of Yunnan Province inevitably lead to heavy metal contamination of the soil during the mining process, which affects the production of local medicinal plants.
        Panax notoginseng (Burkill) Chen3 is a very valuable perennial herbal medicinal plant belonging to the genus Araliaceae Panax ginseng. Panax notoginseng root promotes blood circulation, eliminates blood stasis and relieves pain. The main production site is Wenshan Prefecture, Yunnan Province 5. Cd contamination was present on more than 75% of the soil area in the planting area of ​​Panax notoginseng and exceeded 81-100% at various locations6. The toxic effect of Cd also greatly reduces the production of medicinal components of Panax notoginseng, especially saponins and flavonoids. Saponins are a class of aglycones, among which aglycones are triterpenoids or spirosteranes, which are the main active ingredients of many Chinese herbal medicines and contain saponins. Some saponins also have valuable biological activities such as antibacterial activity, antipyretic, sedative and anticancer activity7. Flavonoids generally refers to a series of compounds in which two benzene rings with phenolic hydroxyl groups are linked through three central carbon atoms, and the main core is 2-phenylchromanone 8. It is a strong antioxidant, which can effectively remove oxygen free radicals in plants, inhibit exudation of inflammatory biological enzymes, promote wound healing and pain relief, and lower cholesterol levels. It is one of the main active ingredients of Panax Ginseng. Solving the problem of soil contamination with cadmium in the production areas of Panax notoginseng is a necessary condition for ensuring the production of its main medicinal components.
        Lime is one of the common passivators for fixing cadmium soil contamination in situ. It affects the adsorption and deposition of Cd in the soil and reduces the biological activity of Cd in the soil by increasing the pH and changing the soil cation exchange capacity (CEC), soil salt saturation (BS), soil redox potential (Eh)3,11 efficiency. . In addition, lime provides a large amount of Ca2+, which forms ionic antagonism with Cd2+, competes for root adsorption sites, prevents Cd transport to the shoot, and has low biological toxicity. With the addition of 50 mmol l-1 Ca under Cd stress, Cd transport in sesame leaves was inhibited and Cd accumulation was reduced by 80%. Numerous related studies have been reported on rice (Oryza sativa L.) and other crops12,13.
        Spraying the leaves of crops to control the accumulation of heavy metals is a new method of dealing with heavy metals in recent years. The principle is mainly related to the chelation reaction in plant cells, which causes heavy metals to be deposited on the cell wall and inhibits the uptake of heavy metals by plants14,15. As a stable dicarboxylic acid chelating agent, oxalic acid can directly chelate heavy metal ions in plants, thereby reducing toxicity. Studies have shown that oxalic acid in soybeans can chelate Cd2+ and release Cd-containing crystals through trichome apical cells, reducing body Cd2+ levels16. Oxalic acid can regulate soil pH, increase superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities, and regulate infiltration of soluble sugar, soluble protein, free amino acids, and proline. Metabolic modulators 17,18. Acidic substances and excess Ca2+ in oxalate plants form calcium oxalate precipitates under the action of germ proteins. Regulation of Ca2+ concentration in plants can effectively regulate dissolved oxalic acid and Ca2+ in plants and avoid excessive accumulation of oxalic acid and Ca2+19,20.
        The amount of lime applied is one of the key factors affecting the effect of the restoration. It has been established that the consumption of lime ranges from 750 to 6000 kg·h·m−2. For acidic soils with pH 5.0-5.5, the effect of lime application at a dose of 3000-6000 kg·h·m-2 was significantly higher than at a dosage of 750 kg·h·m-221. However, excessive application of lime will cause some negative effects on the soil, such as large changes in soil pH and soil compaction22. Therefore, we set the CaO treatment levels as 0, 750, 2250 and 3750 kg·h·m−2. When oxalic acid was applied to Arabidopsis, Ca2+ was found to be significantly reduced at 10 mM L-1, and the CRT gene family influencing Ca2+ signaling was strongly responsive20. The accumulation of some previous studies allowed us to determine the concentration of this experiment and continue to study the interaction of exogenous additives on Ca2+ and Cd2+23,24,25. Thus, this study aims to investigate the regulatory mechanism of the effects of topical lime application and foliar spraying of oxalic acid on the Cd content and stress tolerance of Panax notoginseng in Cd-contaminated soils, and to further explore the best ways and means of medicinal quality. guarantee. Exit Panax notoginseng. It provides valuable information to guide the expansion of herbaceous cultivation in cadmium-contaminated soils and the provision of high-quality, sustainable production to meet market demand for medicines.
        Using the local variety Wenshan notoginseng as material, a field experiment was conducted in Lannizhai (24°11′N, 104°3′E, altitude 1446m), Qiubei County, Wenshan Prefecture, Yunnan Province. The average annual temperature is 17°C and the average annual rainfall is 1250 mm. Background values ​​of the studied soil: TN 0.57 g kg-1, TP 1.64 g kg-1, TC 16.31 g kg-1, RH 31.86 g kg-1, alkaline hydrolyzed N 88.82 mg kg -1, effective P 18.55. mg kg-1, available K 100.37 mg kg-1, total Cd 0.3 mg kg-1 and pH 5.4.
        On December 10, 6 mg/kg Cd2+ (CdCl2 2.5H2O) and lime (0.750, 2250 and 3750 kg h m-2) were applied and mixed with the topsoil 0–10 cm in each plot, 2017. Each treatment was repeated 3 times. The experimental plots were located randomly, the area of ​​each plot was 3 m2. One year old Panax notoginseng seedlings were transplanted after 15 days of cultivation in soil. When using shading nets, the light intensity of Panax notoginseng in the shading canopy is about 18% of the normal natural light intensity. Grow according to local traditional growing methods. By the maturity stage of Panax notoginseng in 2019, oxalic acid will be sprayed as sodium oxalate. The concentration of oxalic acid was 0, 0.1 and 0.2 mol l-1, respectively, and the pH was adjusted to 5.16 with NaOH to mimic the average pH of the debris filtrate. Spray the upper and lower surfaces of the leaves once a week at 8 am. After spraying 4 times, 3 year old Panax notoginseng plants were harvested at week 5.
        In November 2019, three-year-old Panax notoginseng plants treated with oxalic acid were collected in the field. Some samples of 3-year-old Panax notoginseng plants to be tested for physiological metabolism and enzymatic activity were placed in freezer tubes, quickly frozen in liquid nitrogen, and then transferred to a refrigerator at -80°C. The part of the mature stage must be determined in the root samples for Cd and the content of the active ingredient. After washing with tap water, dry at 105°C for 30 min, hold the mass at 75°C and grind the samples in a mortar. keep.
        Weigh 0.2 g of dried plant samples into an Erlenmeyer flask, add 8 ml HNO3 and 2 ml HClO4 and stopper overnight. The next day, the funnel with a curved neck is placed in a triangular flask for electrothermal decomposition until white smoke appears and the decomposition solution becomes clear. After cooling to room temperature, the mixture was transferred into a 10 ml volumetric flask. The Cd content was determined on an atomic absorption spectrometer (Thermo ICE™ 3300 AAS, USA). (GB/T 23739-2009).
        Weigh 0.2 g of dried plant samples into a 50 ml plastic bottle, add 10 ml of 1 mol l-1 HCL, close and shake for 15 hours and filter. Using a pipette, draw up the required amount of filtrate for the appropriate dilution and add the SrCl2 solution to bring the Sr2+ concentration to 1 g L–1. The Ca content was determined using an atomic absorption spectrometer (Thermo ICE™ 3300 AAS, USA).
       Malondialdehyde (MDA), superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) reference kit method (DNM-9602, Beijing Pulang New Technology Co., Ltd., product registration number), use the corresponding measurement kit No.: Jingyaodianji (quasi) word 2013 No. 2400147).
        Weigh out 0.05 g of the Panax notoginseng sample and add the anthrone-sulfuric acid reagent along the side of the tube. Shake the tube for 2-3 seconds to thoroughly mix the liquid. Place the tube on the test tube rack for 15 min. The content of soluble sugars was determined using UV-visible spectrophotometry (UV-5800, Shanghai Yuanxi Instrument Co., Ltd., China) at a wavelength of 620 nm.
        Weigh 0.5 g of a fresh sample of Panax notoginseng, grind it to a homogenate with 5 ml of distilled water and centrifuge at 10,000 g for 10 minutes. Dilute the supernatant to a fixed volume. The Coomassie Brilliant Blue method was used. The content of soluble protein was determined using spectrophotometry in the ultraviolet and visible regions of the spectrum (UV-5800, Shanghai Yuanxi Instrument Co., Ltd., China) at a wavelength of 595 nm and calculated from the standard curve of bovine serum albumin.
        Weigh 0.5 g fresh sample, add 5 ml 10% acetic acid to grind and homogenize, filter and dilute to constant volume. Chromogenic method using ninhydrin solution. The content of free amino acids was determined by ultraviolet-visible spectrophotometry (UV-5800, Shanghai Yuanxi Instrument Co., Ltd., China) at a wavelength of 570 nm and calculated from the standard leucine curve.
        Weigh 0.5 g of a fresh sample, add 5 ml of a 3% solution of sulfosalicylic acid, heat in a water bath and shake for 10 minutes. After cooling, the solution was filtered and diluted to a constant volume. The acid ninhydrin chromogenic method was used. Proline content was determined by UV-visible spectrophotometry (UV-5800, Shanghai Yuanxi Instrument Co., Ltd., China) at a wavelength of 520 nm and calculated from the proline standard curve.
        The content of saponins was determined by high performance liquid chromatography (HPLC) in accordance with the Pharmacopoeia of the People’s Republic of China (edition 2015). The basic principle of HPLC is to use a high-pressure liquid as the mobile phase and to apply a highly efficient separation technology on a stationary phase column for ultrafine particles. Operating skills are as follows:
        HPLC conditions and system suitability test (Table 1): Gradient elution was carried out according to the following table, using silica gel bound with octadecylsilane as a filler, acetonitrile as mobile phase A, water as mobile phase B, and the detection wavelength was 203 nm. The number of theoretical cups calculated from the R1 peak of Panax notoginseng saponins should be at least 4000.
       Preparation of the reference solution: Accurately weigh ginsenosides Rg1, ginsenosides Rb1 and notoginsenosides R1, add methanol to obtain a mixed solution of 0.4 mg ginsenoside Rg1, 0.4 mg ginsenoside Rb1 and 0.1 mg notoginsenoside R1 per ml.
        Test solution preparation: Weigh out 0.6 g of Sanxin powder and add 50 ml of methanol. The mixture was weighed (W1) and left overnight. The mixed solution was then lightly boiled in a water bath at 80° C. for 2 hours. After cooling, weigh the mixed solution and add the resulting methanol to the first mass of W1. Then shake well and filter. The filtrate was left for determination.
       The content of saponin was accurately absorbed by 10 µl of the standard solution and 10 µl of the filtrate and injected into HPLC (Thermo HPLC-ultimate 3000, Seymour Fisher Technology Co., Ltd.)24.
        Standard curve: determination of Rg1, Rb1, R1 mixed standard solution, chromatography conditions are the same as above. Calculate the standard curve with the measured peak area on the y-axis and the concentration of saponin in the standard solution on the abscissa. Plug the measured peak area of ​​the sample into the standard curve to calculate the saponin concentration.
        Weigh out a 0.1 g sample of P. notogensings and add 50 ml of 70% CH3OH solution. Sonicate for 2 hours, then centrifuge at 4000 rpm for 10 minutes. Take 1 ml of the supernatant and dilute it 12 times. The content of flavonoids was determined by ultraviolet-visible spectrophotometry (UV-5800, Shanghai Yuanxi Instrument Co., Ltd., China) at a wavelength of 249 nm. Quercetin is a standard abundant substance8.
        Data was organized using Excel 2010 software. Analysis of variance of data was evaluated using SPSS Statistics 20 software. Picture drawn by origin Pro 9.1. The calculated statistics include the mean ± standard deviation. Statements of statistical significance are based on P<0.05.
        In the case of foliar spraying with the same concentration of oxalic acid, the Ca content in the roots of Panax notoginseng increased significantly with increasing lime application (Table 2). Compared to no lime application, the Ca content increased by 212% at 3750 kg ppm lime without oxalic acid spray. At the same lime application rate, the calcium content slightly increased with increasing sprayed oxalic acid concentration.
        The content of Cd in the roots varied from 0.22 to 0.70 mg/kg. At the same spray concentration of oxalic acid, the content of 2250 kg hm-2 Cd decreased significantly with increasing lime application rate. Compared to the control, when spraying the roots with 2250 kg gm-2 lime and 0.1 mol l-1 oxalic acid, the Cd content decreased by 68.57%. When applied without lime and 750 kg hm-2 lime, the Cd content in the roots of Panax notoginseng decreased significantly with increasing oxalic acid spray concentration. With the introduction of 2250 kg of lime gm-2 and 3750 kg of lime gm-2, the content of Cd in the root first decreased and then increased with an increase in the concentration of oxalic acid. In addition, 2D analysis showed that Ca content in Panax notoginseng root was significantly affected by lime (F = 82.84**), Cd content in Panax notoginseng root was significantly affected by lime (F = 74.99**) and oxalic acid. (F = 74.99**). F = 7.72*).
        With an increase in the application rate of lime and the concentration of spraying with oxalic acid, the content of MDA significantly decreased. No significant difference was found in the MDA content between Panax notoginseng roots treated with lime and 3750 kg g/m2 lime. At application rates of 750 kg hm-2 and 2250 kg hm-2 lime, the MDA content in 0.2 mol l-1 oxalic acid when sprayed was 58.38% and 40.21% lower than in non-sprayed oxalic acid, respectively. The content of MDA (7.57 nmol g-1) was the lowest when 750 kg of hm-2 lime and 0.2 mol l-1 of oxalic acid were added (Fig. 1).
        Effect of leaf spraying with oxalic acid on malondialdehyde content in Panax notoginseng roots under cadmium stress [J]. P<0.05). Same below.
        With the exception of application of 3750 kg h m-2 of lime, no significant difference was observed in the SOD activity of the Panax notoginseng root system. When using lime 0, 750 and 2250 kg hm-2, the activity of SOD when spraying 0.2 mol l-1 oxalic acid was significantly higher than in the absence of treatment with oxalic acid, which increased by 177.89%, 61.62% and 45 .08% respectively. SOD activity (598.18 units g-1) in the roots was highest when treated without lime and sprayed with 0.2 mol l-1 oxalic acid. At the same concentration without oxalic acid or sprayed with 0.1 mol l-1 oxalic acid, SOD activity increased with increasing amount of lime application. SOD activity significantly decreased after spraying with 0.2 mol L–1 oxalic acid (Fig. 2).
       Effect of leaf spraying with oxalic acid on the activity of superoxide dismutase, peroxidase, and catalase in Panax notoginseng roots under cadmium stress [J].
        Similar to SOD activity in roots, POD activity in roots (63.33 µmol g-1) was highest when sprayed without lime and 0.2 mol L-1 oxalic acid, which was 148.35% higher than control ( 25.50 µmol g-1). . POD activity first increased and then decreased with increasing oxalic acid spray concentration and 3750 kg h m −2 lime treatment. Compared to treatment with 0.1 mol l-1 oxalic acid, POD activity decreased by 36.31% when treated with 0.2 mol l-1 oxalic acid (Fig. 2).
        Except for spraying 0.2 mol l-1 oxalic acid and applying 2250 kg hm-2 or 3750 kg hm-2 lime, CAT activity was significantly higher than the control. CAT activity of treatment with 0.1 mol l-1 oxalic acid and treatment with lime 0.2250 kg h m-2 or 3750 kg h m-2 increased by 276.08%, 276.69% and 33 .05% respectively compared to no oxalic acid treatment. The CAT activity of roots (803.52 µmol g-1) treated with 0.2 mol l-1 oxalic acid was the highest. CAT activity (172.88 µmol g-1) was the lowest in the treatment of 3750 kg hm-2 lime and 0.2 mol l-1 oxalic acid (Fig. 2).
        Bivariate analysis showed that Panax notoginseng CAT activity and MDA significantly correlated with the amount of oxalic acid or lime spraying and both treatments (Table 3). SOD activity in roots was highly correlated with lime and oxalic acid treatment or oxalic acid spray concentration. Root POD activity correlated significantly with the amount of lime applied or with the simultaneous application of lime and oxalic acid.
        The content of soluble sugars in root crops decreased with an increase in the application rate of lime and the concentration of spraying with oxalic acid. There was no significant difference in the content of soluble sugars in the roots of Panax notoginseng without the application of lime and with the application of 750 kg·h·m−2 of lime. When applying 2250 kg hm-2 lime, the content of soluble sugar when treated with 0.2 mol l-1 oxalic acid was significantly higher than when spraying with non-oxalic acid, which increased by 22.81%. When applying lime in the amount of 3750 kg·h·m-2, the content of soluble sugars significantly decreased with an increase in the concentration of spraying with oxalic acid. The soluble sugar content of the 0.2 mol L-1 oxalic acid spray treatment was 38.77% lower than that of the treatment without oxalic acid treatment. In addition, spray treatment with 0.2 mol l-1 oxalic acid had the lowest soluble sugar content of 205.80 mg g-1 (Fig. 3).
       Effect of leaf spraying with oxalic acid on the content of total soluble sugar and soluble protein in the roots of Panax notoginseng under cadmium stress [J].
        The content of soluble protein in the roots decreased with an increase in the application rate of lime and oxalic acid. In the absence of lime, the content of soluble protein in the spray treatment with 0.2 mol l-1 oxalic acid was significantly lower than in the control, by 16.20%. When applying lime 750 kg hm-2, no significant difference in the content of soluble protein in the roots of Panax notoginseng was observed. At a lime application rate of 2250 kg h m-2, the content of soluble protein in the oxalic acid spray treatment of 0.2 mol l-1 was significantly higher than in the non-oxalic acid spray treatment (35.11%). When lime was applied at 3750 kg h m-2, the soluble protein content decreased significantly with increasing oxalic acid spray concentration, and the soluble protein content (269.84 µg g-1) was lowest when treated at 0.2 mol l-1. 1 spraying with oxalic acid (Fig. 3).
        No significant difference in the content of free amino acids in the roots of Panax notoginseng in the absence of lime was found. With an increase in the concentration of spraying with oxalic acid and a lime application rate of 750 kg hm-2, the content of free amino acids first decreased and then increased. Application of treatment with 2250 kg hm-2 lime and 0.2 mol l-1 oxalic acid significantly increased the content of free amino acids by 33.58% compared with no treatment with oxalic acid. With an increase in the concentration of spraying with oxalic acid and the introduction of 3750 kg·hm-2 of lime, the content of free amino acid decreased significantly. The content of free amino acids in the 0.2 mol L-1 oxalic acid spray treatment was 49.76% lower than in the treatment without oxalic acid treatment. The content of free amino acid was maximum when treated without treatment with oxalic acid and amounted to 2.09 mg/g. The content of free amino acids (1.05 mg g-1) was lowest when sprayed with 0.2 mol l-1 oxalic acid (Fig. 4).
       Effect of leaf spraying with oxalic acid on the content of free amino acids and proline in the roots of Panax notoginseng under conditions of cadmium stress [J].
        The content of proline in the roots decreased with an increase in the application rate of lime and oxalic acid. There was no significant difference in the proline content of Panax notoginseng in the absence of lime. With an increase in the concentration of spraying with oxalic acid and lime application rates of 750, 2250 kg hm-2, the content of proline first decreased and then increased. The proline content in the 0.2 mol l-1 oxalic acid spray treatment was significantly higher than the proline content in the 0.1 mol l-1 oxalic acid spray treatment, which increased by 19.52% and 44.33%, respectively. When applying 3750 kg·hm-2 of lime, the content of proline significantly decreased with an increase in the concentration of spraying with oxalic acid. The content of proline after spraying with 0.2 mol l-1 oxalic acid was 54.68% lower than without oxalic acid. The content of proline was the lowest and amounted to 11.37 μg/g upon treatment with 0.2 mol/l oxalic acid (Fig. 4).
        The content of total saponins in Panax notoginseng was Rg1>Rb1>R1. There was no significant difference in the content of the three saponins with increasing concentration of oxalic acid spray and no lime (Table 4).
        The content of R1 when spraying 0.2 mol l-1 oxalic acid was significantly lower than in the absence of spraying oxalic acid and using lime 750 or 3750 kg·h·m-2. With an oxalic acid spray concentration of 0 or 0.1 mol l-1, there was no significant difference in the R1 content with an increase in the lime application rate. At a spray concentration of oxalic acid of 0.2 mol l-1, the R1 content of 3750 kg hm-2 of lime was significantly lower than that of 43.84% without lime (Table 4).
        The content of Rg1 first increased and then decreased with increasing concentration of spraying with oxalic acid and lime application rate of 750 kg·h·m−2. At a lime application rate of 2250 or 3750 kg h m-2, the Rg1 content decreased with increasing oxalic acid spray concentration. At the same spray concentration of oxalic acid, the content of Rg1 first increased and then decreased with an increase in the lime application rate. Compared to the control, except for three spray concentrations of oxalic acid and 750 kg h m-2, the Rg1 content was higher than the control, the Rg1 content in the roots of other treatments was lower than the control. The Rg1 content was highest when sprayed with 750 kg gm-2 lime and 0.1 mol l-1 oxalic acid, which was 11.54% higher than the control (Table 4).
        The content of Rb1 first increased and then decreased with increasing concentration of spraying with oxalic acid and lime application rate of 2250 kg hm-2. After spraying 0.1 mol l–1 oxalic acid, the Rb1 content reached a maximum of 3.46%, which is 74.75% higher than without spraying oxalic acid. With other lime treatments, there was no significant difference between different oxalic acid spray concentrations. When sprayed with 0.1 and 0.2 mol l-1 oxalic acid, the content of Rb1 first decreased, and then decreased with increasing amount of lime added (table 4).
        At the same concentration of sprayed oxalic acid, the content of flavonoids first increased and then decreased with an increase in the application rate of lime. No lime or 3750 kg hm-2 lime sprayed with various concentrations of oxalic acid had a significant difference in flavonoid content. When lime was applied at a rate of 750 and 2250 kg h m-2, the content of flavonoids first increased and then decreased with an increase in the concentration of spraying with oxalic acid. When treated with an application rate of 750 kg hm-2 and sprayed with 0.1 mol l-1 oxalic acid, the content of flavonoids was the highest and amounted to 4.38 mg g-1, which is 18.38% higher than lime at the same the same application rate. without spraying with oxalic acid. The content of flavonoids during spraying with oxalic acid 0.1 mol l-1 increased by 21.74% compared with treatment without spraying with oxalic acid and lime treatment with 2250 kg hm-2 (Fig. 5).
       Effect of oxalate foliar spraying on flavonoid content in Panax notoginseng roots under cadmium stress [J].
        Bivariate analysis showed that the soluble sugar content of Panax notoginseng significantly correlated with the amount of lime applied and the concentration of oxalic acid sprayed. The content of soluble protein in root crops significantly correlated with the application rate of lime, both lime and oxalic acid. The content of free amino acids and proline in the roots significantly correlated with the application rate of lime, the concentration of spraying with oxalic acid, lime and oxalic acid (Table 5).
        The content of R1 in the roots of Panax notoginseng significantly correlated with the concentration of spraying with oxalic acid, the amount of applied lime, lime and oxalic acid. The flavonoid content correlated significantly with the concentration of sprayed oxalic acid and the amount of lime applied.
        Many amendments have been used to reduce plant Cd by immobilizing Cd in soil, such as lime and oxalic acid30. Lime is widely used as a soil additive to reduce cadmium content in crops31. Liang et al. 32 reported that oxalic acid can also be used to restore soils contaminated with heavy metals. After applying various concentrations of oxalic acid to contaminated soil, soil organic matter increased, the cation exchange capacity decreased, and the pH value increased by 33. Oxalic acid can also react with metal ions in the soil. Under Cd stress, the Cd content in Panax notoginseng increased significantly compared to the control. However, when lime was used, it decreased significantly. In this study, when applying 750 kg hm-2 lime, the Cd content in the root reached the national standard (Cd limit: Cd≤0.5 mg/kg, AQSIQ, GB/T 19086-200834), and the effect when applying 2250 kg h m−2 of lime works best with lime. The application of lime created a large number of sites of competition between Ca2+ and Cd2+ in the soil, and the addition of oxalic acid could reduce the Cd content in the roots of Panax notoginseng. However, the Cd content of Panax notoginseng roots was significantly reduced by the combination of lime and oxalic acid, reaching the national standard. Ca2+ in the soil is adsorbed on the root surface during mass flow and can be taken up by root cells through calcium channels (Ca2+-channels), calcium pumps (Ca2+-AT-Pase) and Ca2+/H+ antiporters, and then horizontally transported to root xylem 23. Content Root Ca was significantly negatively correlated with Cd content (P<0.05). The content of Cd decreased with an increase in the content of Ca, which is consistent with the opinion about the antagonism of Ca and Cd. Analysis of variance showed that the amount of lime significantly influenced the Ca content in the roots of Panax notoginseng. Pongrac et al. 35 reported that Cd binds to oxalate in calcium oxalate crystals and competes with Ca. However, regulation of Ca by oxalate was not significant. This showed that the precipitation of calcium oxalate formed by oxalic acid and Ca2+ was not a simple precipitation, and the co-precipitation process can be controlled by various metabolic pathways.