| The facility agriculture in Northeast China have a serious threat of heavy metal pollution due to the huge demand for vegetable production and improper human operation.Cadmium?Cd?is the most harmful to the environment,and some of the facility soil has been exposed to moderate or light Cd pollution.Chinese cabbage,one of the leafy vegetables,has many cultivars,which is easy to cultivate but very easy to accumulate Cd.The difference of cultivars and environment may cause the Cd content in the edible part of some products to exceed the standard without significant toxicity response during the cultivation process.Therefore,investigating the Cd tolerance and accumulation characteristics of different Chinese cabbage cultivars in typical black soil,and screening low-Cd accumulation cultivars with certain Cd tolerance have very important theoretical and practical significance for ensuring the safety of agricultural production.The experiment used 20 kinds of main cultivated Chinese cabbage cultivars in Northeast China as materials,and carried out a potting simulation of black soil exogenous Cd pollution in the facility greenhouse,and set up seven levels of contamination:CK?no Cd added?,0.05,0.10,0.15,0.30,0.60,1.20 mg·kg-1.In order to evaluate the differences in Cd tolerance and accumulation among the tested cultivars under different levels of Cd contamination,we conducted the dose-effect analysis,species sensitivity analysis and determination of Cd accumulation in roots and leaves.On this basis,through the hydroponic test of seedlings,a comprehensive analysis of the Cd tolerance characteristics of the low-accumulation cultivars obtained from the screening was conducted,and 5 treatments of CK?no Cd added?,10,50,100,and 200?M were set.The influencing factors and toxic response of Cd accumulation in seedlings were investigated,and the changes of cell submicrostructure under Cd stress were discussed.The main test results are as follows:?1?The changes in the biomass of Chinese cabbage cultivars under different Cd concentrations in soil showed significant differences of dose-effect among the tested cultivars.These cultivars of65%had“hormesis”when the Cd concentration in soil was 0.05 mg·kg-1.After that,with the increase of Cd concentration,all cultivars began to have growth inhibition of different degrees,and the tolerance thresholds of Cd for these were different.The log-logistic model was used to analyze the Cd tolerance of the cultivars,and the difference between the predicted value and the actual value was the smallest in this model.The JF had the highest Cd tolerance threshold,which was 1.204mg·kg-1,followed by BCCRQJ,which was 1.036 mg·kg-1.But QT was the most sensitive to Cd,and the tolerance threshold was only 0.469 mg·kg-1.?2?The accumulation in the edible part?leaf?of 20 Chinese cabbages under different Cd concentrations in soil was significantly different.And difference was greatest when the Cd concentration in soil was 1.2 mg·kg-1,followed by 0.6 mg·kg-1.Under these two levels of Cd contamination:the accumulation of Cd in the edible part of JF was the lowest,which was lower than the national food safety standard limit of 0.2 mg·kg-1?leafy vegetables?,the translocation and bioconcentration factors were both less than 1,so JF was identified as a Cd-tolerant low-accumulation cultivar.BCCRQJ had the highest extraction rate of Cd,was 18.64%,which was identified as a Cd-tolerant high-accumulation cultivar.It had a high Cd bioconcentration ability and an extremely high edible risk.The Cd accumulation in the leaf and root was positively correlated significantly with Cd concentration in soil,and the extraction rate of Cd was negatively correlated significantly?p<0.05?.?3?The relationship between Cd uptake rate in roots of Chinese cabbage at seedling stage and the level of Cd contamination can be fitted with Michaelis-Menten equation.The Cd uptake during the growth of Cd-tolerant low-accumulation seedlings was restricted by the root,and the affinity of Cd in roots was lower than that of Cd-sensitive seedlings.Both the exposure time and dose of Cd have significant effects on the growth of seedlings?p<0.05?.Under the same concentration of Cd stress,the exposure time was longer with the higher Cd accumulation in the root and leaf of seedlings.While the same exposure time,the Cd concentration was higher with the higher Cd accumulation in the root and leaf of seedlings.The exposure time had little effect on the Cd root-to-leaf translocation of the Cd-tolerant low-accumulation seedling.The effects of exposure time and dose on the Cd accumulation in the root and leaf of JF were limited.In this experiment,when the Cd concentration was higher than 127.4?M,the uptake capacity of the Cd-tolerant low-accumulation seedling root tended to be stabilize and no longer enhanced.?4?Cd was very toxic to Chinese cabbage seedlings and significantly inhibited its growth.When the Cd concentration increased from 0?M to 200?M,the fresh weight of JF leaves decreased by 72.9%,and QT decreased by 90.8%.Cd of exchangeable form was less harmful to Cd-tolerant low-accumulation seedling of Chinese cabbage,and 64.7%of its Cd was the low-toxic,stable chelate and residual forms.Cd also enhanced the activities of CAT,POD,and SOD in roots and leaves of seedlings at a certain concentration.The Cd-sensitive cultivar,QT,had the strongest antioxidant enzyme activity when treated with 100?M Cd.Cd mainly inhibited the SOD activity in the roots of Chinese cabbage seedlings,and the activity of SOD in the roots of both seedlings was the strongest at 10?M Cd.Cd-induced oxidative damage of Chinese cabbage seedlings was very serious.The contents of H2O2,O2-,MDA and FPRO were positively correlated with Cd concentrations in soil?p<0.05?.JF was more able to resist oxidative stress induced by Cd.?5?Submicroscopic observation showed Cd had significant cytotoxic effects on Chinese cabbage seedlings.Chloroplasts,mitochondria,and other organelles swelled,and membrane permeability changed,and plastoglobuli increased at low Cd concentrations;exudation of cell contents,plasmolysis,cell wall and membrane were significantly damaged at high Cd concentrations.Cells of JF were less damaged under the same Cd concentration.Most cells of JF had thicker cell wall and more intact organelles than QT under the same magnification.It demonstrated that JF had higher Cd tolerance at the cellular level. |
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