Effects Of Sulfur Supply On Cadmium Uptake And Accumulation In Rice Seedlings

Sulfur (S) is one of the essential elements for plant growth. Human activities, such as fossil fuel consumption, fertilization, irrigation of wastewater containing S, have greatly perturbed the S cycle in terrestrial ecosystem and resulted in spatial variability of S in soil-plant system. The excessive S was observed in paddy soil at acid rain area, and/or irrigation fields of wastewater containing S, while S deficiency also occurred in some area of China. It was reported that paddy soils have been contaminated by cadmium (Cd) in many mining and developed areas. It becomes more and more urgent to understand the mechanism of Cd uptake and accumulation in rice to control its harm to human health through food chain. In the present study, an effective method for the extraction and element analysis of the iron plaque on the root surface of rice was developed, and pot experiments were conducted to study the effect of S supply on the formation of iron plaque on the rice root surface, microstructure, and Cd uptake by rice. The major results were reported as follows.An effective extraction solution (ACA: ascorbic-citrate-acetic) for the extraction and elemental analysis of iron plaque from the root surface of rice was developed. It has a higher extracting efficiency of Fe, Mn, P than DCB. Moreover, S concentration in the solution could be simultaneously determined together with Fe, Mn, P and other heavy elements. It is the basis of investigating the effect of S on iron plaque formation and their ecological effects.Differences of sulfur supply on micro structure of root surface, iron plaque from the root surface of rice were determined in dependence on sulfur form, sulfur levels. Supply of sulfate-S tended to decrease concentration of Fe in iron plaque from the root surface of rice, otherwise it's opposite for elemental S. There were significant interaction between S stress and Cd pollution to Cd uptake by rice. Significantly differences of sulfur supply on Cd uptake by rice seedlings were demonstrated in dependence on sulfur form, sulfur level, Cd levels. Supply of sulfate and/or elemental sulfur resulted in a decrease of Cd in shoot of rice cultivated in Cd stressed soil; supply of elemental sulfur decreased Cd in roots of rice seedlings cultivated in Cd stressed soils, but supply of sulfate increased Cd in roots of rice seedlings cultivated in Cd stressed soils. Supply of sulfate and elemental sulfur < 30 mg S·kg-1 increased concentrations of Cd in shoots of rice seedlings cultivated in low Cd soils, but excessive sulfur supply resulted in decrease of Cd in shoots of rice seedlings cultivated in low Cd soils.The effect mechanisms of Cd uptake by rice seedling are different under different sulfur forms. The iron plaque is an important barrier to prevent the uptake and transportation of Cd from rice because of a large proportion of plaque-Cd in total Cd uptake by rice whatever to supply elemental sulfur or sulfate. Element S doesn't change the distribution of Cd in rice seedlings, otherwise sulfate changes the distribution significantly. The root Cd of rice seedling increased with the increasing of sulfate supply, showing that under the condition of more sulfate, the effect of the root tissue to prevent the Cd uptake into rice and Cd transportation to shoots increased gradually, and it is another important mechanism to prevent Cd to enter into edible parts.The supply of sulfur and Cd had significant effects on the formation, structure and thickness of iron plaque in different parts of root surface of rice seedling, but there were no significant differences to the concentration of Fe and Mn of plaque for total root. The formation of iron plaque was to accumulate stratified out of the cytoderms on the surface of root, but not to penetrate into the cells on the surface of root.Effect of S on Cd uptake into rice could be related to S induced activity of Cd in soil, S induced transformation of iron plaque formation in the root surface. Therefore, S supply could effectively reduce Cd accumulation in rice exposed to Cd contaminated soils.The mechanism of sulfur to affect Cd uptake was related to the effects of sulfur on iron plaque and Cd activities of soil. The present research demonstrated that sulfur could decrease the Cd uptake by rice in soils contaminated by Cd, which had important theoretical and practical meaning to decrease the Cd transportation from paddy soil with high level of sulfur and Cd in mining area to food chains.