| The interaction between different elements is an effective means to control the accumulation of heavy metals in crops.Phosphorus（P）and zinc（Zn）,as essential nutrient elements of plants,have obvious interaction with cadmium（Cd）.Therefore,it is one of the economic and feasible methods to control the accumulation of Cd in crops by applying an appropriate amount of P fertilizer and Zn fertilizer and determining a reasonable concentration ratio of P\Zn\Cd according to the different Cd pollution degrees of soil.In this paper,the single factor level design of alternately fixing two factors is adopted,and three groups of treatments are set up:the first group:fixing P 150 mg·kg-1,Zn 10 mg·kg-1,and setting Cd of 0～5 mg·kg-1series level;Group 2:fix the addition of P 150 mg·kg-1and Cd 1 mg·kg-1,and set the Zn level of 0～50 mg·kg-1series;Group 3:Fixed the dosage of Zn10 mg·kg-1and Cd 1 mg·kg-1,and set the P level of 0～250 mg·kg-1series.Group 3 consisted of 18 treatments,and another control treatment（CK）was set up for 19 treatments.Taking purple soil widely distributed in southwest China as the object,through pot experiment of Brassica campestris L.,combined with analysis of forms and availability of P,Zn,and Cd in soil,crop yield,activities of antioxidant enzymes,distribution and accumulation of cadmium in plants,the influence of interaction among P,Zn and Cd on Brassica campestris L.growth and Cd accumulation and its related mechanism was discussed,and the target was determined.The main conclusions are as follows:（1）Applying appropriate amount of phosphate fertilizer and zinc fertilizer can ensure the safe production of Cd-contaminated cultivated vegetables.When the addition of exogenous Cd was 0.3～0.6 mg·kg-1,the yield of Brassica campestris L.was higher than CK treatment,indicating that low cadmium promoted the growth of Brassica campestris L..However,when the amount of Cd was 1 mg·kg-1,the yield of Brassica campestris L.decreased significantly,and a large amount of cadmium accumulated in plants,which significantly inhibited the growth of Brassica campestris L..However,when the amount of exogenous Zn was 10～50 mg·kg-1and the amount of P was 50～250 mg·kg-1,the accumulation of cadmium in Brassica campestris L.decreased significantly.The ratio of addition of exogenous P and Zn to cadmium pollution degree shows that whenω（Cd）:ω（Zn）:ω（P）is 1:10:200,the yield of Brassica campestris L.is the highest,55.72 g·pot-1,and the cadmium content in edible part is low,24.88μg·kg-1,which is lower than the national limit standard of Cd in green leafy vegetables of 50μg·kg-1.Although the application of exogenous phosphorus and zinc significantly promoted the growth of Brassica campestris L.,excessive phosphorus and zinc inhibited the growth of Brassica campestris L..Whenω（Cd）:ω（Zn）:ω（P）is 1:100:150 and 1:10:250,the yield of Brassica campestris L.begins to decline,and whenω（Cd）:ω（Zn）:ω（P）is 1:100:150,the accumulation of cadmium in Brassica campestris L.will also be promoted.Therefore,the accumulation of cadmium in Brassica campestris L.can be controlled by controlling the appropriate application amount of phosphorus and zinc according to different levels of cadmium pollution,and the safe production of Brassica campestris L.can be realized.（2）The changes in plant growth and cadmium content are closely related to the forms of phosphorus,zinc,and cadmium in soil,and the interaction of phosphorus,zinc,and cadmium in soil significantly affects the interaction of phosphorus,zinc,and cadmium in plants.When the amount of Cd was 0.3～0.6 mg·kg-1,the contents of available zinc and available phosphorus in soil increased compared with the treatment without Cd.Therefore,under low cadmium treatment,the yield of Brassica campestris L.increased more than CK treatment.However,with the increase of Cd addition,the contents of available phosphorus and zinc in the soil decreased,so the yield of Brassica campestris L.decreased significantly.Low cadmium will promote the increase of available phosphorus and available zinc content in the soil,while the application of the appropriate amount of P and Zn can reduce the available cadmium content in the soil.The available cadmium content in soil decreased when the amount of exogenous Zn was 100 mg·kg-1,but increased when the amount of Zn was less than 100 mg·kg-1.The cadmium content in the underground part of Brassica campestris L.is the lowest when the amount of exogenous Zn is 100 mg·kg-1,which is consistent with the change trend of soil,but the cadmium content in the underground part is the lowest when the amount of exogenous Zn is 20 mg·kg-1,which indicates that the cadmium content in the underground part is more closely related to the available cadmium content in soil.With the addition of exogenous P,the content of available cadmium in soil decreased significantly,with the addition of exogenous P of 250 mg·kg-1,which was completely consistent with the change trend of cadmium content in Brassica campestris L..The cadmium content in Brassica campestris L.was the lowest when the addition of exogenous P was 250 mg·kg-1,but the yield of Brassica campestris L.with the addition of exogenous P of 200 mg·kg-1was higher than that with the addition of P of 250 mg·kg-1,and the cadmium content in edible part was higher than that of P250.The content of available zinc in soil has no obvious regularity with the addition of exogenous Cd and P.When the addition of Cd is 0.3～0.6 mg·kg-1,the addition of P is 150mg·kg-1and the addition of Zn is 20～100 mg·kg-1,the content of available zinc in the soil is the highest.The addition of exogenous Cd significantly increased the cadmium content in Brassica campestris L.,and the cadmium in the underground part of Brassica campestris L.was more sensitive to the available cadmium content in the soil,and the change of cadmium content by exogenous Cd,Zn and P was more obvious.The changing trend of zinc content in the shoot of Brassica campestris L.with the addition of exogenous Cd was obvious,but the zinc content in the shoot decreased significantly.With the addition of exogenous Zn,the zinc content of Brassica campestris L.increased significantly,which was consistent with the change of available zinc content in soil.When P was added in the range of 100～200mg·kg-1,the zinc content in the underground part of Brassica campestris L.was significantly higher than that in the non-P treatment,and the rest of P addition decreased the zinc content in Brassica campestris L..（3）There is a significant correlation between zinc and cadmium forms and soil available forms,and the changing trend of exchangeable forms and carbonate bound forms with cadmium pollution degree and the addition of P and Zn is consistent with that of available forms.Tessier method was used to classify the forms of cadmium and zinc into exchangeable,carbonate-bound,iron-manganese oxide-bound,organic-bound,and residue-bound forms.Among them,the exchangeable state is closely related to the soil available state.With the addition of exogenous Cd,the available cadmium in soil increased significantly,but the proportion of exchangeable cadmium decreased first and then increased.When the amount of Cd is 1 mg·kg-1,the proportion of exchangeable Cd is the highest,and when the amount of Cd is 3 mg·kg-1,the proportion of residue is the highest.With the addition of exogenous Zn,the proportion of exchangeable Cd increased significantly,the residue increased first and then decreased,and the proportion of other forms decreased.When the amount of Zn is 5 mg·kg-1,the proportion of residue state is the highest,and when the amount of Zn is 100 mg·kg-1,the proportion of exchangeable state is the highest,which is contrary to the change of available cadmium content in the soil.With the addition of exogenous P,the proportion of exchangeable cadmium decreased significantly,while the proportion of residual cadmium increased.Different from the distribution of cadmium forms,the order of zinc forms in soil from more to less is iron-manganese oxide bound state,residue state,organic matter bound state,carbonate bound state,and exchangeable state,which indicates that zinc has poor mobility in soil.Exchangeable,carbonate-bound,and Fe-Mn-oxide-bound zinc all increased first and then decreased with the addition of exogenous Cd,and the highest proportion was found when the addition of Cd was 0.6 mg·kg-1,which was consistent with the changing trend of available zinc content in the soil.With the addition of exogenous Zn,the proportion of exchangeable Zn increased significantly,and other forms did not change regularly.When the addition amount of P is 150 mg·kg-1,the proportion of residue and organic bound forms is the lowest.（4）P and Zn alleviate the toxic effects of Cd on Brassica campestris L.through different mechanisms.P mainly reduces the absorption of cadmium by Brassica campestris L.by reducing the availability of cadmium in soil and improving the stress resistance of Brassica campestris L.,while Zn mainly inhibits the accumulation of cadmium in Brassica campestris L.by promoting the dilution of crop growth and physiological antagonism in plants.When the level of exogenous Cd reached 1 mg·kg-1,the anti-oxidative stress ability of Brassica campestris L.was significantly inhibited,the activities of CAT and POD decreased,and MDA accumulated a lot.When the stress of exogenous Cd was 1 mg·kg-1,the activity of CAT increased significantly when Zn was 5～50 mg·kg-1and P was 50～200mg·kg-1.The activity of POD is not affected by the addition of exogenous Zn.When the amount of P is 250 mg·kg-1,the activity of POD increases.However,MDA content decreased significantly with the addition of exogenous P,but when the amount of P was greater than 200 mg·kg-1,MDA content increased significantly.CA activity increased significantly with the addition of exogenous Zn.Under cadmium stress,CA activity increased instead of decreasing.When the amount of P was 100～200 mg·kg-1,the activity of CA was significantly higher than that of the treatment without P.This is consistent with the change of zinc content in the underground part of Brassica campestris L..Under phosphorus treatment,the content of available cadmium and the proportion of exchangeable cadmium in soil decreased significantly,so the cadmium in Brassica campestris L.decreased significantly.And phosphorus significantly promoted the growth of Brassica campestris L.,so the cadmium content in plants was diluted and the cadmium concentration in Brassica campestris L.decreased.At the same time,under phosphorus treatment,CAT activity increased significantly,MDA content decreased,and the resistance of Brassica campestris L.to cadmium toxicity increased,so Brassica campestris L.could be safely produced under cadmium pollution.Low zinc treatment did not affect the cadmium content of Brassica campestris L.,but high zinc(50 mg·kg-1)treatment significantly reduced the cadmium content of Brassica campestris L.,because the yield of Brassica campestris L.increased significantly and cadmium in Brassica campestris L.was diluted.In addition,under the treatment of high zinc,the activity of CAT increased,and the activity of CA increased significantly,while CA and CAT could enhance the stress resistance of plants.Therefore,high zinc treatment can reduce cadmium accumulation in Brassica campestris L.. |
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