Abstract: 【Objective】Understanding of the accumulation characteristics of Cadmium by different rice cultivars in soils and its species sensitivity distribution is necessary for determination of the hazardous concentration of Cd to rice cultivars of 5% of the tested species (HC5), the threshold values (HC5) of Cd to rice species will provide vital information for protection of Cd pollution to different rice cultivars and thereafter the food chains in China.【Method】Two typical paddy soils were collected from southern China, and the soils were polluted by Cd with different Cd addition levels of 0, 1.2, 4.8, 10, 40, and 120 mg•kg-1 in soils. Eighteen different rice cultivars from the main rice producing areas around China were selected for test plant species in this study. The biomass of the rice cultivars, the Cd concentrations in plants and thereafter the bioconcentration factor (BCFs) were investigated using pot experiments in green houses. The log-logistic distribution model was used to determinate the dose-response relationship of Cd toxicity to rice cultivars based on different endpoints. The response relationships of Cd toxicity to18 different rice species were fitted by cumulative probability distribution functions Log-logistic distribution model. The species sensitivity distributions (SSD) of Cd to rice was performed with the Burr-III type distribution model, and the different species sensitivity distribution of Cd toxicity frequency and 95% of rice varieties based on the protection of Cd toxicity thresholds HC5 were obtained and meanwhile, the quantitative relationship of BCFs with the bioavailability of Cd in soils was also determined in this study.【Result】 The biomass of different rice species decreased with the increasing concentrations of Cd in acidic soil from Qiyang, biomass of rice X-42 which was the most sensitive species to Cd stress decreased by 86%, but the biomass of rice Z-120 showed stronger tolerance to Cd stress decreased by 51%. When added at a concentration of 4.8 mg•kg-1, the biomasses of JY-253, J-463, Z-611, J-899, T-15, X-6, T-167 reached the maximum, and the others were at the concentration of 1.2 mg•kg-1. Compared with Cd0, the biomass of different varieties of rice increased by 4%-56%. This showed the stimulating effect of relatively lower level of Cd on rice growth. In alkaline soils from Guangzhou, there was no significant difference in the biomass of different rice species with increasing concentrations of Cd. The biologic concentration factor (BCFs) of Cd to rice species decreased with the increased spiked levels of Cd in both soils. In the low-Cd concentration (1.2 mg•kg-1), BCF in the red soil ranged between 0.0056 (S-974)-0.0133 (T-15), 2.38-fold difference; but in the paddy soil, the range of variation between 0.0018 (L-42, LY-28) -0.0034 (J-899), 1.89-fold difference. However, the BCFs of rice in Qiyang soil was higher than in Guangzhou soil under the same Cd level 2-14 times difference. This may be related to the nature of the soil. Qiyang red soil is an acidic soil, organic matter scarce, highly effective state of the measured Cd contention, and strong poisoned rice, reduced the multi-biomass. On the contrary, Guangzhou paddy soil which is high in pH and more organic matter, increased Cd absorption capacity and holding power, weakening poison rice biomass did not change significantly. Rice X-45 had the highest Cd toxicity threshold concentration (EC50, mg•kg-1), while rice X-42 had the lowest EC50. The EC50 of 18 rice species ranged from 4.30-61.611 mg•kg-1, with the variations differed from 1.0 to 14.32 folds.【Conclusion】Significant differences of response of rice cultivars to Cd stress were observed among the tested rice plant species, and hormesis effect of Cd stress to rice cultivars was also observed in this study although Cd was not included in the nutrients for rice plants. The results indicated that distinct difference existed among the tested rice species according to the accumulation of Burr-III type distribution model, e.g. the result showed that rice X-42 was the most sensitive species to Cd stress with the minimum toxicity threshold among the rice cultivars, on the contrary, rice X-45 showed a strong tolerance to Cd stress in soils. The maximum concentration of half (50%) inhibiting concentration of Cd to rice cultivars with protection of the 95% rice species (HC550%) was 4.93mg•kg-1 as determined by Burr-III model.

Key words: Burr-III model , cadmium , rice cultivars , species sensitivity distributions (SSD) , ecological risk