Alleviation Of Exogenous Silicon On Cadmium Toxicity In Wheat Seedlings

Due to heavy metals contamination in farmlands,cadmium?Cd?in agricultural products exceeded the Chinese Food Hygiene Standard,which could pose a potential threat to human helth.Silicon?Si?has been proved to improve the stress resistance of crops,especially play important roles in alleviation of toxicity of heavy metals.Wheat is an important grain crop in the north of China.However,substantial proportion of wheat grains produced in Cd-contaminated soil were polluted,and have attracted much attention.Hence,the hydroponic experiment was conducted to study the role and mechanism of alleviation of Si on wheat seedlings under Cd stress.Growth status,photosynthesis parameters,element uptake,Cd²⁺ flux,subcellular distribution and chemical forms of Cd and antioxidant system were determined,aiming to provide a means to develop effective strategies that contribute to increase the safety of crop plants,which are grown in Cd-contaminated areas.1.Cd stress significantly inhibited wheat growth,resulting in dwarf plants,yellowing leaves,short root system and other symptoms.Si increased shoot biomass of wheat under low Cd level?5 ?M?,but had no effect on biomass of wheat under high Cd level?20 ?M?.The role of Si on root morphology parameters did not reach the significant level.Cd toxicity significantly decreased the photosynthesis parameters of wheat leaves.However,Si supply increased the photosynthetic rate,while did not affect other parameters.These results suggested Si application could improve the growth of wheat by increasing photosynthesis.2.In situ measurements of Cd²⁺ flux showed that Si pretreatment and transient treatment both significantly decreased Cd²⁺ uptake.Si supply also decreased the concentration and accmulation of Cd in shoot and root of wheat under high Cd level,while had no effect on root to shoot translocation of Cd.Moreover,Si supply enhanced the Fe transport,but reduced the translocation of Mn.The uptake and translocation of Zn and Cu were affected little by Si addition.3.Cd mainly existed in cell wall fractions and soluble fractions,and Cd content in organelle fractions was little.Si supply affected little on subcellular fraction of Cd in wheat under low Cd level.However,under high Cd level,Cd contents in subcellular fractions were decreased by addition of Si,and the proportion of Cd in cell wall fraction was enhanced in leaves,but reduced in roots.Application of Si increased the content and proportion of the ethanol-extractable Cd both in leaves and roots under Cd stress,however,other chemical forms of Cd contents and proportions were decreased,especially in the Na Cl-extractable Cd contents.These results suggested that Si supply could improve the tolerance of wheat by compartmentation of Cd,but it also depends on plant organs and Cd treatment level.4.Cd stress significantly increased the contents of malondialdehyde?MDA?and hydrogen peroxide?H₂O₂?in wheat leaves,and decreased the activities of SOD,POD and CAT.Si treatment significantly decreased the contents of MDA and H₂O₂,and increased the activities of POD and SOD?low Cd level?,but did not affect CAT activity.These results indicated that the Si addition could significantly enhance the antioxidant defense ability of wheat leaves and improve their tolerance to Cd.