Mechanisms Of Cadmium Uptake And Accumulation In Rice Due To Exogenous Phosphorus

Cadmium(Cd)pollution in farmland might cause excessive Cd accumulation in brown rice and harm human health.Containing phosphorus(P)fertilizers and soil passivation remediation materials have been widely used in the remediation of heavy metal-polluted soil.However,the interaction mechanism of P and Cd in crops is rarely studied.In this study,the effects of P on the absorption and transport capacity of Cd under different growth stages(tillering stage,booting stage,milky stage,dough stage,and maturing stage)were studied by hydroponic experiments.And the effect of P on iron plaque and Cd absorption in rice root surface was studied by inducing the formation of iron membrane on root surface of rice by external iron.The main results of this study were as follows:(1)P can alleviate the adverse effect of Cd stress on rice growth and photosynthetic pigment synthesis,and enhance the absorption and transport capacity of Cd in rice.When the concentration of Cd in the solution was 0.2 mg·L-1,the total biomass of rice increased with the increase of P concentration,especially for the biomass of rice under 45.0 mg·L-1 P treatment was significantly(P<0.05)increased by 33.1%as compared with the 10.0 mg·L-1 P treatment.Meanwhile,chlorophyll a and carotenoid contents in rice leaves under the 22.5 and 45.0 mg·L-1 P treatments were significantly(P<0.05)increased by 47.5%and 54.1%,21.9%and 53.1%,respectively as compared with 10.0 mg·L-1 P treatment.When the concentration of Cd was 0.1～0.2 mg·L-1,the concentration of Cd in rice plants increased with the increase of P concentration.In particular,when P concentration was 45.0 mg·L-1,Cd concentration in the stem,leaf,husk,and brown rice were significantly(P<0.05)increased by 101%,149%,96.6%,and 61.8%,respectively,as compared with 10.0 mg·L-1 P treatment.(2)Under the interaction of P and Cd,the key growth stages of the promotion of Cd absorption in rice by P were the booting stage,milky stage,and dough stage.At the booting stage,the daily net accumulation rate of Cd in stems and leaves was significantly(P<0.05)increased with the increase of P concentration.Compared with the 10.0 mg·L-1 P treatment,the daily net accumulation rate of Cd under P concentration of 45.0 mg·L-1 treatment was significantly(P<0.05)increased by 241%and 164%,respectively.When P concentration was 45.0 mg·L-1,the daily net accumulation rates of Cd in roots,stems,leaves,and brown rice were significantly(P<0.05)increased by 127%,169%,353%,and 123%,respectively,as compared with 10.0 mg·L-1 P treatment at milky stage.The daily net accumulation rates of Cd in rice stems and brown rice under P concentration was 45.0 mg·L-1 treatment were significantly(P<0.05)increased by 55.7%and 55.3%as compared with 10.0 mg·L-1 at dough stage.Therefore,the application of chemical fertilizer containing P at booting stage,milky stage and dough stage can promote the absorption and transport of Cd in rice.(3)P can inhibit the absorption of Cd by the iron plaque on the root surface of the rice,that can promote the absorption of Cd by rice.In the process of iron plaque induction,the increase of P concentration in the solution would reduce the proportion of red-brown iron plaque on the root surface of rice and promote the absorption and transport of Cd in rice;After the induction of iron plaque,the increase of P concentration in the solution would inhibit the formation of iron plaque on the root surface of rice and promote the absorption and transport of Cd in rice.When P concentration was 48.0 mg·L-1 Cd content in root iron plaque was significantly(P<0.05)decreased by 61.9%compared with 1.5 mg·L-1 P treatment,while Cd content in stem and leaf of rice was significantly increased by 106%and 71.5%,respectively.The results of scanning electron microscopy(SEM)showed that after the induction of iron plaque,the iron plaque deposits in the root surface of rice decreased significantly with the increase of P concentration.Especially,when the P concentration was 48.0 mg·L-1,the Fe content in the root surface iron plaque decreased significantly by 54.4%compared with 1.5 mg·L-1 P treatment,while the Cd content in the root increased significantly by 96.9%.