| Three hydroponic experiments were carried out under greenhouse condition. Two of the three is to investigate Hg toxicity and accumulation in rice plants grown in solution culture, and to study the interactions between Hg and As in the uptake process. The third experiment was carried out to investigate whether iron plaque could affect mercury uptake by rice plants.In the first experiment all seedlings of rice were exposed to 0, 0.1, 0.5, 2. 5,5.0 mg Hg L-1 with and without As at the concentration of 0.5mg L-1 as Na3AsO4. The results showed that increasing Hg2+ concentrations in the nutrient solution decreased both root and shoot biomass. Hg2+ at concentrations of 1.0 and 2.5 mgL-1 caused 50% reduction in root biomass. A 50% reduction in shoot biomass occurred at Hg2+ concentrations of around 0.5 mg L-1. Nevertheless, 0.5 mg As L-1 has no significant effect on plant yield. Hg accumulated in rice roots, and the Hg concentration factor in roots reached nearly 1900 at 2.5 mg Hg L-1. The addition of As slightly increased the Hg concentration in the roots. However, As concentrations in the roots decreased significantly with increasing Hg concentration in the growth solution to 1.0mg Hg L-1 or 2.5mg Hg L-1. Shoot As concentrations decreased with increasing Hg concentrations in the growth solution, but increased again with further increase in Hg concentration to 2.5 mg L-1. Possible mechanisms of Hg uptake and interactions between Hg and As in the uptake process are also discussed.In the second experiment all seedlings of rice were exposed to 0.5,1.0,1.5mgHg L1 with As at the concentration of 0.5, 1.0, 1.5mg L-1 as Na3AsO4. The photosynthetic rate, transpiration rate and stomatal conductance were measured using a portable photosynthetic system.The data indicated that mercury has significant effect on rice biomass (P<0.001), interaction between Hg and As was observed in terms of root biomass ( P = 0.021) . A 50% reduction in root biomass occurred at concentrations of 1.5 mg Hg2+ L-1 with 1.0 mg As L-1 (0.190 vs. 0.403g ) . Both Hg and As had significant effect on As concentrations in roots and shoots ( P<0.001 ) . As concentrations in roots and shoots significantly increased with increasing As concentrations in nutrient solution. Moreover, increasing Hg concentration ingrowth media significantly reduced As concentration in root but not in shoot. Photosynthesistic rate, stamatal conductance of CO2, transpiration rate were significantly affected by Hg but not As, 50% reduction occurred at 1.0 mg Hg2+L-1 for photosynthesistic rate and stamatal conductance of CO2, the concentrations of 1.5 mg Hg2+ L-1 for transpiration rate.In the third experiment, we investigated the effects of iron plaque on the root surface on mercury uptake by and translocation within rice seedlings grown in nutrient solution in the greenhouse. After iron plaque on rice roots was induced in solution with 20 mg Fe + L-1, the formation of iron plaque was clearly visible as reddish coatings on the root surface after 12h induction. Seedlings with and without iron plaque theatment were transplanted into nutrient solution with 0.5 mg Hg L-1, 1.0 mg Hg L-1 . The result showed that iron plaques had little significant effect on biomass. With iron plaque, Hg concentration of 39.1 mg kg-1, 45.1 mg kg-1 on root surface were observed at concentrations of 0.5 mg Hg2+L-1 and 1.0 mg Hg2+ L-1 respectively, which were 2 to 3 times higher compared with those without iron plaque. At concentrations of 1.0 mg Hg2+L-1, Hg concentrations in shoots was significantly lower with iron plaque compared with those without iron plaque (3.3mg kg-1 vs. 5.4 kg L-1). Iron plaque also showed high affinity to phosphorus, but did not inceased phosphorus concentration in shoot compared with those without iron plaque, possible mechnism was also disculssed in the article.
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