Studies On Characteristics Of Lead Uptake And The Control Of Agronomy Practice In Rice

Lead (Pb) is one of the hazardous heavy metal pollutants of the environment that originates from various sources like mining and smelting of lead-ores, effluents from storage battery industries, automobile exhausts, fertilizers, pesticides and from additives in pigments and gasoline. Consequently, the levels of Pb increased in soil and environment, meanwhile, Pb in soil and environment could be absorbed by plant, after that entered food chain circle. However the information about the Pb uptake, accumulation and tolerance in rice was very limited. This investigation was carried out to study the Pb uptake and the control of agronomy practice in rice, based on the four levels of lead hydroponic experiments and seven levels of lead pot experiments. The major results were summarized as follows:1. When rice was raised in nutrient solution under different Pb treatments in the medium, the growth of rice seedlings was greatly influenced, especially the root. The index of root tolerance was decreased with the increasing of Pb concentrations, and there were obvious differences in root tolerance index among different rice varieties under the l0mg/kg Pb treatment, so this concentration could be used to screen Pb-tolerant and Pb-sensitive rice. In addition, both the organic acid secretion from root and the root activity increased with the increase of the Pb treatments, which indicated there was a self-protected mechanism in rice root.2. Obvious increase of POD and SOD activities were identified in the rice roots subjected to high Pb concentration treatment. But the chlorophyll content in rice seedlings was decreased, and the content of the MDA which was produced in peroxide reaction was also decreased with high Pb concentration (20mg/kg) in medium. This suggested that the decrease of the chlorophyll content mainly due to the active oxygen damage.3. Our studies on Pb uptake indicated the absorbed Pb was distributed in a specific manner with its greater localization in roots than in shoots. It has been shown that only about 1% Pb was transported from root to up-ground part. In addition, the rice growth and biomass accumulation were greatly effected by the high Pb concentration (>1000mg/kg) in the field, there was little toxic symptoms with Pb concentration under 1000mg/kg. However, the results showed that the Pb content in grain was 3.03-40.56 times higher than the control, which indicated that Pb was a potential toxicelement and a "potential killer" for rice safe quality.4. The Pb contents in grain were not affected by the planting season if there was not heavy Pb stress, but the Pb contents in grain planted in early season were higher than that of the late season when the rice were cultivated in highly Pb-polluted areas, and the difference increased with the increasing of Pb concentrations in soil.5. Under the condition of dryland, the rice grain Pb contents of grain were higher than that of the rice under normal water management, and the rice dryland cultivation might resulted in highly the increase of Pb content in the grain when there was high Pb concentration in environment.6. Positive relationships between Pb and Mn, Pb and Cu were observed in the study , this results might indicated that the effects on rice planted in Pb stressed soil were not resulted from Pb element only, and it might be caused by other elements like Mn and Cu and their interaction.