Effects of lime and oxalic acid on antioxidant enzymes and active components of Panax notoginseng under cadmium stress
Contents of Cd and Ca in Panax notogensing rootsThe Ca content of P. notoginseng roots increased significantly with the increase of lime application rates under the same concentration of oxalic acid sprayed on leaves (Table 2). Compared with no lime application, the Ca content was the highest increased by 212% under 3750 kg hm−2 lime without spraying oxalic acid. The content of Ca slightly increased with the increase of oxalic acid spraying concentrations under the same rate of lime application.Table 2 Effects of foliar spraying of oxalic acid on contents of Cd and Ca in roots of Panax notoginseng under Cd stress.Full size tableThe contents of Cd in roots ranged from 0.22 to 0.70 mg kg−1. The content of 2250 kg hm−2 Cd decreased greatly with the increase of lime application rates under the same spraying concentration of oxalic acid. Compared with the control, the root Cd contents decreased by 68.57% under the application of 2250 kg hm−2 lime and 0.1 mol L−1 oxalic acid spraying. The Cd contents of P. notoginseng roots decreased significantly with the increase of oxalic acid spraying concentrations under application of non-lime and 750 kg hm−2 lime. The root Cd contents decreased at first and then increased with the increase of oxalic acid concentrations under the application of 2250 kg hm−2 lime and 3750 kg hm−2 lime. In addition, the Bivariate analysis showed that the Ca content of P. notoginseng roots was significantly affected by lime (F = 82.84**), and the Cd content of P. notoginseng roots was significantly affected by lime (F = 74.99**) and oxalic acid (F = 7.72*).MDA contents and relative antioxidase activitiesThe content of MDA decreased greatly with the increase of the rates of lime application and oxalic acid spraying concentrations. There was no significant difference in the content of MDA in the roots of P. notoginseng with non-lime and 3750 kg hm−2 lime application. Under 750 kg hm−2, 2250 kg hm−2 lime application, the MDA content with 0.2 mol L−1 oxalic acid spraying concentration treatment decreased by 58.38% and 40.21% comparing with non-oxalic acid spraying application, respectively. The content of MDA (7.57 nmol g−1) was the lowest under 750 kg hm−2 lime application and 0.2 mol L−1 oxalic acid spraying treatment (Fig. 1).Figure 1Effects of foliar spraying of oxalic acid on contents of malondialdehyde in roots of Panax notoginseng under Cd stress. Notes The figure legend showed the spray concentration of oxalic acid (mol L−1), different lowercase letters indicate significant differences between treatments at the same lime application rate (P Rb1 > R1. The contents of the three saponins had no significant difference with increase of the concentrations of oxalic acid spraying and no application of lime (Table 4).Table 4 Effects of foliar oxalate application on the percentages of three saponins in roots of Panax notoginseng under Cd stress.Full size tableThe contents of R1 with 0.2 mol L−1 oxalic acid spraying was significantly lower than that without oxalic acid spraying and rates of 750 or 3750 kg hm−2 lime application. Under the concentration of 0 or 0.1 mol L−1 oxalic acid spraying, there was no significant difference in contents of R1 with increase of rates of lime application. Under the concentration of 0.2 mol L−1 oxalic acid spraying, the contents of R1 with 3750 kg hm−2 lime was significantly lower 43.84% than that without lime application (Table 4).The contents of Rg1 increased at first and then decreased with the increase of oxalic acid spraying concentrations and 750 kg hm−2 lime application. Under the application rates of 2250 or 3750 kg hm−2 lime, the contents of Rg1 decreased with the increase of oxalic acid spraying concentration. With the same concentration of oxalic acid spraying, the Rg1 content increased at first and then decreased with the increase of lime application rates. Compared with the control, except that the Rg1 content with three concentrations of oxalic acid spraying and 750 kg hm−2 lime was higher than that of the control, the contents of Rg1 in the roots of P. notoginseng under other treatments was lower than that of the control. The Rg1 content was the highest with 750 kg hm−2 lime and 0.1 mol L−1 oxalic acid spraying treatment, which was higher 11.54% than that of the control (Table 4).The contents of Rb1 increased first and then decreased with the increase of oxalic acid spraying concentration and 2250 kg hm−2 lime application. The content of Rb1 with 0.1 mol L−1 oxalic acid spraying reached the maximum value of 3.46%, which was higher 74.75% than that without oxalic acid spraying treatment. Under other lime application treatments, there was no significant difference among different oxalic acid spraying concentrations. With 0.1 and 0.2 mol L−1 oxalic acid spraying treatments, the contents of Rb1 decreased at first and then decreased with the increase of lime application rates (Table 4).Contents of flavonoidsWith the same concentration of oxalic acid spraying, the content of flavonoids increased at first and then decreased with the increase of the amounts of lime application. There was no significant difference in the content of flavonoids under different concentrations of oxalic acid spraying without the application of lime or 3750 kg hm−2 lime. Under 750 and 2250 kg hm−2 lime application, the content of flavonoids increased at first and then decreased with the increase of the concentration of oxalic acid spraying. Under the treatment of 750 kg hm−2 application and 0.1 mol L−1 oxalic acid spraying, the content of flavonoids was the highest, which was 4.38 mg g−1, which was higher 18.38% than that of the same rate of lime application and without spraying oxalic acid. The content of flavonoids with 0.1 mol L−1 oxalic acid spraying treatment increased by 21.74% compared with that without oxalic acid spraying treatment and 2250 kg hm−2 lime application (Fig. 5).Figure 5Effects of foliar spraying of oxalate on the contents of flavonoids in roots of Panax notoginseng under Cd stress.Full size imageBivariate analysis showed that the content of soluble sugar in P. notoginseng root was significantly relationship with the amount of lime application and the concentration of oxalic acid spraying. The content of soluble protein in root was significantly relationship with lime application rates, both of lime and oxalic acid. The contents of free amino acid and proline in roots were significantly relationship with lime application rates, oxalic acid spraying concentrations, both of lime and oxalic acid (Table 5).Table 5 Variance analysis of the effects of oxalic acid, calcium and cadmium on the contents of multiple medicinal ingredients in the roots of Panax notoginseng (F value).Full size tableThe content of R1 in the root of P. notoginseng was significantly relationship with oxalic acid spraying concentrations, lime application rates, both of lime and oxalic acid. The content of flavonoids was significantly relationship with oxalic acid spraying concentrations, lime application rates. More