Foliar Application Of Nanometer Silicon Increases Heavy Metal Resistance In Rice (Oryza Sativa L.) And Its Mechanisms

The contamination of soils with metals is a major environmental problem throughout the world.Soils polluted with metals may threaten ecosystems and human health.Silicon is the second most abundant element on the surface of the earth,and there is increasing evidence that Si has a number of beneficial effects on plant growth under biotic and abiotic stresses.The crossing of inorganic chemistry,environmental chemistry,material chemistry and life science has formed a new study field to utilize environment matter and bio-nanometer matter in agriculture.In the present study,we try to use different nanometer silicon preparation sprayed onto the leaves of rice(Oryza sativa L.) under heavy metal stress,to find a efficient method in the utilize of silicon to reduce the uptake of heavy metal, and discover the mechanism.In pot experiment 1,six rice cultivars were planted in the soil artificially amended with multi-metals(Cd,5 mg/kg;Pb,200 mg/kg;Cu,250 mg/kg,Zn,300 mg/kg),and two kinds of nanometer Si preparation(from TEOS and sodium silicate were foliarly applied to investigate their effects on the growth of rice and heavy metals uptake in rice grains.Foliar application of nanometer silicon preparation can improve the growth of rice,significantly increase the biomass,weight of per 100 grains and spike number of per plant.The distribution of heavy metal in rice was root＞shoot＞grain.The uptake coefficient of heavy metal is Cd＞Zn＞Cu＞Pb whether with or without silicon.Hybridize cultivars accumulated more Cd in grains than that of general cultivars but there is little difference between hybridize cultivars and general cultivars on the accumulation of Zn and Cu.Foliar application of nanometer silicon reduced the accumulation of all heavy metals and effect was more pronounced on hybride than on general cultivars.Pot experiment 2 was conducted to investigate the effects of foliar application of Si on heavy metals uptake in rice(cv.youyou 128) grain under Cd,Pb,Cu and Zn stress.Foliar. application of Si alleviated the toxicity of heavy metals on rice.Compared with application of inorganic Si,the application of organic Si was more positive in alleviating heavy metal toxicity.With the application of Si,the weight of per 100 grains,and weight of per spike were increased significantly(P＜0.05),the accumulation of Cd,Pb,Cu,Zn in the grain were decreased significantly(P＜0.05).The uptake coefficient and the accumulation of heavy metals in grains were also decreased.With the increasing of Cd concentration in soil,the alleviating of heavy metal toxicity by Si application increased.These results indicated that foliar application of Si was a propriety method of alleviating heavy metal toxicity and decreasing metals accumulation in rice.Foliar application of nanometer silicon at 5 mM and 10 mM significantly increased the weight of per 100 grains and biomass were significantly increased.The uptake of heavy metal decreased with increasing application rate of nanometer silicon preparation.The content of Zn and Pb was lower than the threshold concentrations for safely edible food. The application of Si reduced Cu concentration,which was lower than the safely edible threshold.But the Cd concentrion was slightly higher than the safely edible threshold.At the same application rate,organic nanometer silicon was more effective than inorganic nanometer silicon for improving rice growth and decreasing heavy metal accumulation.In the present study,solution culture experiments were also conducted to investigate the influence of 2.5 mM nanometer silicon preparation on growth of rice seedlings under 20μM Cd stress.The results showed that.Si application significantly alleviated the toxicity of Cd in rice.Si reduced the distribution of Ca in the shoots,increased the Mg,Zn,Fe and Cu concentration(P＜0.05),increased chlorophyll content,promoted the growth of rice and increased the biomass of the plants.Application of Si increased Pn and WUE,but decreased Tr,Gs and Ci in leaves of rice seedlings under Cd stress.Si decreased Cd content in the shoots and roots of rice.The concentration of Cd in xylem and the translocation of Cd from roots to shoots were decreased by Si application. The results of applastic fluorescence tracer PTS(trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid) showed the significant reduction of apoplastic transport in nanometer silicon preparation applied plants,and the reduction of fluorescence was more than that of Tr reduced by the application of nanometer silicon,this suggested that Si restrained the apoplastic transport of Cd and alleviated the toxicity of Cd.Si decreased the content of MDA content and affected the activity of activities of SOD, POD,CAT,APX and GR.Activity of SOD in shoots was increased under short-time Cd stress,but decreased later.Activity of SOD in roots was decreased under short-time Cd stress,but increased later.The application of silicon decreased the activity of SOD.Cd stress and silicon increased activity of POD.The activity of POD in roots was higher than that in shoots.Activity of CAT in shoots was increased under short-time Cd stress,but decreased later.Application of silicon increased the activity of CAT in both of shoots and roots.Activity of GR was decreased by Cd,but was increased by silicon.Activity of APX was little affected by Cd or silicon.Total ascorbate,total GSH,the ration of GSH/GSSG was increased by the application of silicon.NPT content was increased in the nanometer silicon preparation applied rice seedlings. So transcription amount for the key enzyme regulating GSH production(γ-ECS) was detected and the result showed that transcription amount forγ-ECS was increased both under Cd stress and silicon application.GSH played a role in Cd tolerance and silicon increased GSH production.