| Rice is one of the most important crops in the world,and it is primary staple food for more than 50%people all over the world.In China,there is over 60%people making rice as staple food.The charicteristic of the dietary structure among the people in China makes rice as the main resource of cadmium absorbed by humans.Along with fast development of economy and society,cadmium pollution of the paddy field become more and more serious,which poses a severe threat to health of residents.Therefore,studying on the inhibition of cadmium uptake and transportation to the grains is of great significance.In this study,rice cultivars with different grain cadmium accumulation ability were used to investigate the effects of selenium and silicon on mineral nutrition,subcellular and chemical forms cadmium content,cadmium content in iron plaque,cadmium absorption in rice and the differences between the two rice cultivars.Moreover,the mechanisms of selenium and silicon on inhibiting cadmium transportion to the grains were studied.The main results are as follows:1.Screening of indica hybrid rice cultivars with different grain cadmium accumulation ability and the relationship of selenium and silicon content with grain cadmium contentIn order to study the relationship of cadmium content in rice stem,leaf and panicle and translocation coefficient of cadmium from stem and leaf to panicle at heading stage with cadmium content in brown rice.Also the relationship between selenium or silicon content at heading stage and cadmium content in brown rice was also studied.The results showed that,the cadmium content in brown rice among 55 indica hybrid rice cultivars had significant differences,ranges from 0.89 to 2.88 mg/kg,Luxiangyou104 had the greatest cadmium content and Qyou9 had the lowest cadmium content.The cadmium content of stem,leaf and panicle and translocation coefficient of cadmium from stem or leaf to panicle at heading stage also had significant differences among the 55 rice cultivars.So did the selenium and silicon content.The result of the correlation analysis showed that there was a positive correlation between cadmium content in brown rice and cadmium content in stem(r=0.416**),leaf(r=0.565**),panicle(r=0.495**)at heading stage.The positive correlation of cadmium content in brown rice and translocation coefficient of cadmium from stem(r=0.285*)and leaf(r=0.323**)to panicle at heading stage was also confirmed.The silicon content in stem(r=-0.750**),leaf(r=-0.834**),panicle(r=-0.628**)and brown rice had a negative correlation with the cadmium content in brown rice,so did the selenium content except that in the stem at heading stage(r=-0.323**in leaf and r=-0.403**in stem).2.Effects of nitrogen and silicon on dry matter accumulation and cadmium uptake by indica hybrid riceA field experiment was conducted in cadmium contaminated paddy rice field to investigate the effects of combined application of nitrogen and silicon on growth and cadmium accumulation in hybrid rice cultivar Gangyou725.In the present study,three nitrogen(0,90,180 kg/hm2)and/or silicon(0,150,300 kg/hm2)levels were applied,dry matter weight,nitrogen accumulation in rice shoot,rice yield and its component factors,cadmium content and accumulation amount in rice root and shoot and cadmium content in polished rice were investigated.The results showed that under 3 nitrogen application levels,silicon could improve the root dry matter weight significantly.But it only improved the shoot dry matter weight at 0 and 90 kg/hm2 nitrogen treatments.Under the three nitrogen treatments,the nitrogen accumulation in shoot of the rice was increasing with the silicon levels applying in the field.The yield component factors were significantly affected by combined application of nitrogen and silicon.Treatornents with 150 kg/hm2 silicon could increase the effective panicle numbers under the same nitrogen levels,but decrease the 1000-grain weight compared to the treatments with 0 kg/hm2 silicon treatmet.Under 0 kg/hm2 nitrogen treatment,applying 150 and 300 kg/hm2 silicon improved the spikelets per panicle significantly compared to the 0 kg/hm2 silicon treatment.Under each nitrogen level,rice yield was higher in 150 and 300 kg/hm2 silicon treatments compared with 0 kg/hm2 silicon treatment,but the effect was not significant under 180 kg/hm2 nitrogen treatment.The cadmium contents in root,stem,panicle and the cadmium accumulation amount were lower in treatment with 150 and 300 kg/hm2 silicon treatments than those in 0 kg/hm2 silicon treatment under each nitrogen level,but the cadmium content in 150 and 300 kg/hm2 silicon treatments was higher than in 0 kg/hm2 silicon treatment.The cadmium content in polished rice was decreased significantly by the silicon under the three nitrogen levels,and the differences were significant between the 150 kg/hm2 and 300 kg/hm2 silicon treatments under 0 and 90 kg/hm2 nitrogen treatments,however,there was no differences between the two silicon treatments under 180 kg/hm2 nitrogen treatment.Based on the rice yield and cadmium content in panicle,combined application of 180 kg/hm2 with 150 kg/hm2 silicon could both improve rice yield and decrease cadmium uptake in edible part by rice.3.Alleviation role and effects of selenium on mineral nutrients in indica hybrid rice seedlings under cadmium stressA hydroponic experiment was conducted to study the alleviation role of selenium in two indica hybrid rice cultivars with different grain cadmium accumulation ability under 0,0.1 and 1 mmol/L cadmium concentrations.The results showed that,selenium significantly increased the biomass and chlorophyll a content of the two rice seedlings under cadmium stress,but decreased the chlorophyll b content in Chuanguyou2348.The contents of ascorbic acid and glutathione in the two rice seedlings also increased in the selenium treatments,but those were higher in Chuanguyou2348 than in Yixiangyou2115,showed the differences between the two cultivars.Under 1 mmol/L cadmium stress,selenium promoted the synthesis of ascorbic acid and glutathione to alleviate cadmium stress in Chuanguyou2348,but accelerated the synthesis of phytochelatins in Yixiangyou2115 to mitigate cadmium stress.Selenium also mitigated the injuries of the cadmium stress to the root,which kept root activity in higher level than those without selenium treatments.Selenium treatment inhibited the absorption of cadmium in the two cultivars directly and decreased the contents of calcium and zinc in shoot of the two cultivars under cadmium stress,but induced different reactions on copper and magnesium contents in the two rice shoots.To the roots,selenium increased the calcium and copper contents,but decreased the zinc and magnesium contents in the two cultivars under cadmium stress conditions.4.Effects of Na2SeO3 and Na2SiO3 on cadmium uptake and content in two different indica hybrid rice cultivars at jointing stageA pot experiment was conducted to explore the effects of Na2SeO3 and Na2SiO3 on cadmium uptake by two rice cultivars with different grain cadmium accumulation ability at jointing stage.The results showed that,Na2SiO3 significantly increased the pH of the rhizosphere and depressed the growth of the root in the two rice cultivars.Na2SeO3 improved Chuanguyou2348 root biomass weight,but had no effect on Yixiangyou2115.The cadmium content in the iron plaque of Chuanguyou2348 was significantly higher than that in Yixiangyou2115.Na2SeO3 and Na2SiO3 decreased the cadmium content in the iron plaque by 8.5%and 14.1%in Yixiangyou 2115 compared to the control treatment,and decreased by 12.9%and 39.4%in Chuanguyou 2348 compared to the control.Exogenous Na2SeO3 or Na2SiO3 decreased the cadmium content in the root in the two rice cultivars,the cadmium translocation coefficient from root to shoot also decreased in the treatments with Na2SeO3 or Na2SiO3,then decreased the cadmium content in the shoot.Exogenous Na2SeO3 and Na2SiO3 also decreased the total cadmium accumulation in the two rice cultivars,but the effects of the two treatments on cadmium accumulation in different parts were cultivar depended.The percentages of shoot cadmium accumulation in both cultivars were decreased under the two treatments significantly compared to the each control.In Yixiangyou2115,Na2SeO3 and Na2SiO3 mainly increased the percentage of cadmium in the iron plaque,but increased the cadmium accumulation percentage in iron plaque and root of Chuanguyou2348.Correlation analysis showed that the accumulation of cadmium in rice shoot and root increased with the cadmium content in the iron plaque.Exogenous Na2SeO3 or Na2SiO3 decreased the cadmium content and accumulation in rice root and shoot,then decreased the cadmium content of the grain at mature stage in both cultivar.Chuanguyou2348 had a bigger decline than Yixiangyou2115 under the two treatments between the two cultivars and Na2SiO3 treatment was more effective than Na2SeO3 in reducing cadmium content in the grain between the two cultivars.5.Effects of selenium and silicon on the contents of cadmium and mineral elements in indica hybrid rice growing under cadmium stressIn this experiment,two indica hybrid rice cultivars,Yixiangyou2115 and Chuanguyou2348 were grown in cadmium contaminated paddy field to investigate the effects of selenium and silicon on cadmium and some mineral element contents in the two rice cultivars.The results showed that,selenium and silicon treatments had significant effects on cadmium,calcium,copper,magnesium,zinc,selenium and silicon contents in the two cultivars.The concentrations of mineral elements were higher in Chuanguyou2348 than those in Yixiangyou2115 except selenium and silicon,selenium and silicon contents were higher in Yixiangyou2115 than those in Chuanguyou2348.Selenium and silicon treatments decreased the accumulation of cadmium,copper,magnesium in the stem,copper in panicle of Yixiangyou2115 and cadmium in leaf of Chuanguyou2348,elevated the accumulation of calcium in leaf of two cultivars and in stem of Chuanguyou2348.Silicon with+6 ?mol/L selenium treatment decreased zinc content in the stem,cadmium content in the leaf of Yixiangyou2115 and magnesium content in the stem,calcium content in the panicle of Chuanguyou2348,improved the accumulation of copper and zinc in leaf of Yixiangyou2115.Silicon with 12 ?mol/L selenium treatment suppressed the accumulation of cadmium,zinc in the stem of Chuanguyou2348 and zinc in the panicle of Yixiangyou2115,increased the accumulation of copper in the stem and magnesium,zinc in the panicle of Chuanguyou2348.All selenium and silicon treatments decreased Cd content in Yixiangyou2115 panicle,but 12?mol/L selenium treatment had little effect on reducing cadmium content in panicle of Chuanguyou2348 compared to its control.Under 6 ?mol/L selenium treatment,the cadmium content in the brown rice of Yixiangyou2115 was 2.0 mg/kg.However,Chuanguyou2348 got the minimum value under silicon with 6 ?mol/L selenium and 12?mol/L selenium treatments.6.Effects of selenium and silicon on cadmium subcellular distribution and its chemical forms in indica hybrid riceThe cadmium subcellular distribution and chemical forms in Yixiangyou2115 and Chuanguyou2348 under selenium or silicon treatments were studied by the method of sucrose differential centrifugation and chemical reagent stepped extraction.The results showed that,spraying selenium mainly affected the HAc and HCi extractable fraction in rice root and silicon reduced all the fraction cadmium contents,the HC1 extractable cadmium content in Yixiangyou2115 was 10.63 mg/kg under selenium spraying treatment and showed significant differences with the control.The HAc and HCI extractable cadmium contents in Chuanguyou2348 were 27.83 mg/kg and 16.42 mg/kg under selenium spraying tratment and significantly higher than the control.The mixed using of selenium and silicon affected the chemical forms in the roots depend on the cultivars.The ethanol,deionized water,HAc and HCI extractable fraction were higber under mixed using of selenium and silicon than silicon treatment alone.Selenium and silicon could decrease the contents of high activity cadmium and the total accmulation amount.The percentages of ethanol and deionized water extractable cadmium decreased treatment and the percentage of HC1 extractable cadmium increased under the mixed usng of selenium and silicon.The cadmium in the cell mainly distributed in the cell wall and soluble fraction.Selenium mainly increased cadmium content in the soluble fraction and silicon mainly increased cadmium content in the cell wall compared to their control.The effects of mixed using of silicon and selenium on cadmium subcellular distribution depend on cultivars and different organs.7.Effects of selenium and silicon on the physiological and biochemical characteristics of indica hybrid rice under cadmium stressThe rice cultivar with high grain cadmium accumulation ability Chuanguyou2348 and low grain cadmium accumulation ability Yixiangyou2115 were planted in cadmium contaminated paddy field to study the effects of selenium and silicon on the physiological and biochemical characteristics in rice leaf and upmost internode.The results showed that,selenium increased the non-protein thiols,glutathione and phytochelatins content in the leaf and selenium was more effective than selenium with silicon treatment.The effects of silicon on the contents of glutathione and non-protein thiols were cultivar depended,they were decreased by 6.4%and 12.3%compared to the control in Yixiangyou2115,but increased by 8.5%and 5.3%compared to the control in Chuanguyou2348.Selenium and silicon increased the cellulose content in the upmost internode of the two cultivars,but selenium treatment was not statistically significant.The silicon treatments(alone or with selenium)increased the cellulose content significantly,it increased by 11.4%and 17.4%compared to the control in Yixiangyou2115 and increased by 11.5%and 20.0%compared to the control in Chuanguyou2348.Selenium and silicon could increased the AsA and soluble protein content in rice,but there were significant differences between the two cultivars.The activity of the GSH-Px was activated by selenium and silicon in leaf,and selenium was more effective than silicon did,mixed using of selenium and silicon treatment had the highest activity of GSH-Px.Selenium and silicon decreased the cadmium content in leaf and panicle in both cultivars,but there was no effect of selenium on cadmium content in the upmost internode.Selenium treatment alone had no effect on the translocation of cadmium from leaf to panicle in Yixiangyou2115,but could decreased the translocation of cadmium from internode to panicle.All the three treatments could decrease the both translocation coefficients in Chuanguyou2348. |
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