Cadmium Accumulation And Tolerance Characteristics In High Cadmium Accumulating Rice

Heavy metal pollution is one of the most serious environmental problem in the world, the cadmium (Cd) due to the high toxicity and easy to be absorbed by plants were focused on the people. However, these hyperaccumulators had their weaknesses and shortcomings, such as grew slowly and small, which made them difficult to harvest mechanically. Furthermore, the hyperaccumulators found in our existing resources which grew in temperate regions, such as in wet and warm conditions, and do not be able to endure the heat-resisting and may be susceptible to certain diseases. Therefore, many experts turned the direction of phytoextraction technology to field crops. The high Cd accumulating rice possessed a great Cd accumulation and its cultivation system was highly mechanized and well-established, which may suggest that rice as a promising cultivar for phytoextraction of Cd in Cd-contaminated soils. In this research, the two different Cd accumulation rice of the high Cd accumulating rice (Lu527-8 and Lu17-9) and the normal rice (Lu527-4) were grown in soils with different Cd treatments to evaluate Cd accumulation, physiological and tolerance characteristics. The research contents are as follows:(1) The biomass and Cd concentration of the Lu527-8 and Lu17-9 were significant higher than that of the Lu527-4 at each stages when the Cd concentration in soil was 20 mg·kg-1. With growth period prolonged, a significant increase (p< 0.05) was observed in shoot and the whole plant biomass of the Lu527-8 and Lu17-9 under the high Cd treatment. There is a smaller rise at tillering-jointing stage while a greater increase at jointing-maturity stage. The Cd absorption capacity coefficient in both the Lu527-8 and Lu17-9 were higher than that in the Lu527-4 at each growth period (p< 0.05). With growth period prolonged, the Cd accumulation rate of both the Lu527-8 and Lu17-9 reached a maximum in the filling stage. The Cd accumulation in shoot and the whole plant significantly increased (p< 0.05), there is a greater increase at jointing-filling stage. The Cd accumulation of the high Cd accumulating cultivars at jointing-heading stage and heading-filling stage was significantly higher than other growth stages. The whole plant Cd accumulation of the Lu527-8 and Lu 17-9 with high Cd treatment at jointing-heading stage and heading-filling stage accounted for 15.30%,40.45%,39.70% and 20.74% of the whole growth period. With growth period prolonged, the purification rate in shoot and the whole plant significantly increased (p< 0.05), but the difference was not significant after the filling stage (p> 0.05). The purification rate in whole plant of Lu527-8 and Lu17-9 reached 4.88% and 4.39% at filling stage, which was 2.44 and 2.21 times higher than the Lu527-4, respectively. This study establishes that high Cd accumulating cultivars show great abilities of Cd accumulation and purification from soils contaminated by high Cd, and the filling stage is the optimum harvest stage of rice cultivars with high Cd accumulation. Thus, high Cd accumulating cultivars are the worthy candidates for phytoextraction of Cd-polluted farmlands.(2) The Cd content of soluble fraction, cell wall fraction and organelle fraction in two rice cultivars were significantly increased when the Cd treatments increased. Then, the Cd content in subcellular fraction of the Lu527-8 and Lu17-9 were significantly higher than that of the Lu527-4. The subcellular Cd content in roots showed soluble fraction> cell wall fraction> organelle fraction; the subcellular Cd content in stems and leaves were expressed cell wall fraction> soluble fraction> organelle fraction. The percentage of cell wall fraction in Lu527-8 and Lu17-9 reached 68.86% and 72.44% at jointing stage when the Cd concentration in soils reached 20 mg·kg-1. The photosynthetic pigments content of two rice cultivars was significant increased at jointing and filling stage under different Cd treatments. Meanwhile, the chlorophyll content of Lu527-8 and Lu17-9 were significant higher than Lu527-4. When the Cd concentration was 20 mg·kg-1, the chlorophyll a content of Lu527-8 and Lu17-9 reached 1.94 and 1.88 mg·g-1, which was 1.41 and 1.36 times higher than Lu527-4, the chlorophyll b content of Lu527-8 and Lu17-9 reached 1.09 and 1.06 mg·g-1, which was 1.88 and 1.83 times higher than Lu527-4. Meanwhile, the carotenoid content of Lu527-8 and Lu17-9 were significant higher than Lu527-4 at jointing and filling stage. The MDA and soluble sugar content of the Lu527-8 and Lu17-9 were significant increased under Cd stress, but the soluble protein content were significant decreased under different Cd treatments. In addition, the high Cd accumulating rice have a stronger ability of separation, photosynthetic pigment synthesis ability and cell osmotic regulation ability.(3) The fine lateral root morphological, coarse lateral root morphological and adventitious root morphological of two rice cultivars were increased in response to the Cd concentration increased, and the Lu527-8 and Lu17-9 were significant higher than that of the Lu527-4. Meanwhile, there are significant difference between total absorbing surface area and active absorbing surface area in roots. The total absorbing surface area of root in Lu527-8 and Lu527-4 reached 0.40-0.61 and 0.39-0.62 cm2-plant-1 which were significant higher than Lu527-4. Both of the active absorbing surface area of root in the Lu527-8 and Lu17-9 reached 0.20 cm2`plant-1 when the Cd concentration was 10 mg·L-1. The different two rice cultivars of the root oxidation ability and root reducing capacity were significant decreased with the Cd concentration increased. And then, the root oxidation ability and root reducing capacity of Lu527-8 and Lu527-4 were significant higher than the Lu527-4. The root oxidation ability of the Lu527-8 and Lu17-9 were 1.52 and 1.49 times higher than the Lu527-4; The root reducing capacity of the Lu527-8 and Lu17-9 were 1.28 and 1.31 times higher than the Lu527-4. This indicated the root of the high Cd accumulating rice have stronger root extension ability and active absorption area that will help plant growth better under Cd stress.(4) The proline content of two rice cultivars in different organs were significant increased with the Cd concentration increased. The proline content in different organs showed stems> roots>leaves and the Lu527-8 and Lu17-9 were significant higher than that of the Lu527-4. Meanwhile, the proline content of the Lu527-8 and Lu17-9 in root reached maximum value were 1.67 and 1.89 times higher than the Lu527-4. The thiols compounds content of the Lu527-8 and Lu17-9 were significant higher than the Lu527-4 and in different organs showed stems> roots>leaves. Meanwhile, the non-protein thiol, glutathione and phytochelatin content in Lu527-8 and Lu17-9 reached maximum value when the Cd concentration was 10 mg·L-1. The non-protein thiol content of Lu527-8 and Lu17-9 in roots reached 7.19 and 6.85 ?g·kg-1 which were 1.23 and 1.17 times higher than the Lu527-4. The glutathione content of Lu527-8 and Lu17-9 in roots reached 3.19 and 3.03 ?g·kg-1 which were 1.32 and 1.26 times higher compared to the Lu527-4. The phytochelatin content in root of the Lu527-8 and Lu17-9 were significant higher than the Lu527-4 when the Cd concentration were 2 and 5 mg·L-1, respectively. However, there were no significant difference between the high Cd accumulating rice and the normal rice. In addition, the high Cd accumulating rice have a stronger ability of phytochelatin synthesis...