Cadmium And Arsenic Accumulation And Partitioning In Rice (Oryza Sativa L.) And Their Regulation

Recently,soil contamination with heavy metals has become a global problem.With the complexity of the sources of contamination,heavy metal contamination has aroused the widespread concern of scholars at home and abroad.Rice is the main food crop in China and even in Asia and is easy to accumulate heavy metals such as cadmium and arsenic in its grain.However,information is still sacnt and even controversial on the absorption and transportation of heavy metals in plant and the interactions between the metals.A series of experiments with rice were conducted to study the interaction between cadmium and arsenic,the effect of cadmium and arsenic stress on the plant growth,cadmium and arsenic absorption and partitioning,effect of calcium carbonate and sodium silicate on mitigating cadmium and arsenic stresses.In addition,transmission electron microscopy(TEM) was used to analyze root ultrastructure and distribution pattern of cadmium and arsenic in organs and organelles.The results obtained were presented as follows.1.Single and combined cadmium and arsenic treatment decreased the rice biomass.The root and shoots dry mass of rice seedlings were 42.6%and 51.7%of the control, respectively.Under 4 mg·L^-1 As stress the rice height,spike length,1000-grain weight, and grains per panicle were significantly decreased.Significant interaction was observed between Cd and As.As concentrations in roots,shoots,chaff and brown rice decreased significantly with increasing of Cd concentrations in nutrient solution. Moreover,increasing As concentration in the solution significantly reduced Cd concentration.2.There are some interactions among Cd,As and iron.Appropriate iron nutrient supply had a beneficial influence on Cd and/or As stressed rice plant.The data indicated that the combined treatment with Cd and As significantly increased the iron concentration on the root surface.The Cd and As concentrations in roots and shoots were decreased in the treatment with 30 mg Fe²⁺·L^-1 compared with the control,while the Cd and As concentrations in roots and shoots increased slowly in the treatment with 50 mg Fe²⁺·L^-1.Addition of 50 mg Fe²⁺·L^-1 increased the shoot dry mass by 19.4%in comparison with non-Fe²⁺treatment.Cd concentration of 50 mg Fe²⁺·L^-1 treatment in rice shoots was 16.7%ofnon-Fe²⁺ treatment.Moreover,As concentration office shoots at 30 and 50 mg Fe²⁺·L^-1 level were 59.2%and 52.2%of the control,respectively.3.A sand culture experiment was carried out to investigate the effect of water regimes on Cd and As uptake and rice growth.The results indicated that the flooding treatment decreased the Cd and As concentrations in roots and shoots compared with the treatment with moisture content of 75%sand water capacity.4.Two contrasting rice(Oryza sativa L.) cultivars,i.e.Wuyujing3 and Shanyou63,were grown in red soil to study the toxicity of As and Cd(individually or in combination) on rice seedlings.Results showed that both As and Cd treatments decreased root biomass and shoot biomass compared with the corresponding controls(neither Cd nor As treatment).However,the treatment with As significantly mitigated the Cd-induced inhibition of shoot growth for SY63,root and growth for WYJ3.Both As and Cd exposures significantly decreased the SOD and POD activities,GSH and AsA levels, while the As plus Cd treatment mitigated the decreasing effect compared with Cd treatment alone(p＜0.05).The As plus Cd treatment significantly decreased As or Cd levels both in roots and in shoots compared with the As treatment alone,or Cd treatment alone,respectively.The very similar trend was also found in Cd and As contents of different organs office at the maturity stage(p＜0.05).5.A pot experiment was conducted to study the effect of sodium silicate and calcium carbonate on Cd and As uptake by rice grown in a red soil experimentally contaminated with Cd and As.The results showed that addition of silicate or carbonate signifcantly increased the plant height,panicle length,1000-grain weight,and grains per panicle compared with the treatment with Cd and As alone.However,application of sodium silicate and calcium carbonate at higher rates significantly decreased the number of effective tillers and Cd and As contents of different organs office at the maturity stage.6.Silicon treatment increased the SOD and POD activities,biomass production,and the net photosynthetic rate in rice grown hydroponically with Cd or As stress.Silicon increased SOD,POD and PPO activities in rice roots exposure at 50μmol·L^-1 As by 60.7%,49.8%and 19.2%,respectively compared with non silicon treatments. Transmission Electron Microscope(TEM) analysis showed:that serious untrastructural damage was observed in rice roots treated with 50μmol·L^-1 Cd or 100μmol·L^-1 As individually.Plasmolysis,concentrated cytoplasm,ambiguity of organelle were all symptoms of root cells under heavy metal stress.Some vacuole and irregular thickening cell wall were found on the stressed cells.And there existed some different granules in these cell walls.