Chemical Characteristics Of Selenium In Paddy Soils And Genotypic Differences And Mechanism Of Selenium Absorption And Accumulation Of Rice In Hainan Province

Selenium (Se) is an essential nutrient element for human beings. Many diseases may be caused by Se deficiency or Se excess. But it is widespread throughout the world for Se deficiency in soil. About 72% counties'soil is severely lack of Se or in a low state of Se content in China. Eating Se-enriched agriculture products is the most economic, safe and convenient way to get enough Se for people who are in the Se-deficiency areas. It has been reported that Se is an important ingredient of many enzymes and proteins in the organisms. It has biological functions such as improving immunity of human body, anti-aging and preventing cancer. Therefore, many researches have been focused on the development of Se-enriched agriculture products.A wide distribution of Se-enriched soils was found in the geological survey of Hainan province in 2006, accounting for about 27% of land area of Hainan province. As rice is a staple food of human, the development of Se-enriched rice is the first choice for the utilization of Se-enriched soils in Hainan.Se content in Soil, especially available Se content is the main factor that affects Se content of rice. Significant genotypic difference of Se absorption and accumulation capacity has been demonstrated by a lot of studies. Therefore planting rice cultivars with high capacity of Se absorption and accumulation in high Se content in soils may be a useful way for the utilization of Se-enriched soil resources of Hainan Province.Representative paddy soils were sampled from 18 counties of Hainan province to determine total Se contents, available Se contents, and their spatial distribution characters. Screening experiment of main cultivar of late and early rice was also carried out. The mechanisms of Se enrichment were also analyzed by comparing Se absorption and accumulation characteristics of Se-enriched rice cultivars with common ones. These results could provide scientific basis for regionalization of Se-enriched rice, and promote the development of high efficiency tropical agriculture by transforming the resources superiority into economic one.Following results were obtained and some progresses were made:1. Chemical characteristics of Se of paddy soil in Hainan province280 representative paddy soils were collected from 18 counties of Hainan province. And their total Se contents, available Se contents, heavy metals i.e. Hg, Cd, Cr, Pb and As contents were determined while basic chemical and physical properties were analyzed. The results showed that total Se contents of the top layer paddy soils were range from 0 to 1.532 mg/kg. The mean value was 0.211mg/kg. Se contents in Soils which had a medium or higher level (>0.175mg/kg) account ed for 47.5%. Available Se contents were from 1.453 to 13.619μg/kg and average available Se content was 5.72μg/kg. Soils which had a medium or high level (>6μg/kg) accounted for 43%. The contents of 5 heavy metals were also low, which were lower than the second grades of the standard values (National Environmental Quality Standard of Soils.), except that few of Hg, Cd and Cr content were higher than the second grades of the standard value.The variation of Se contents in paddy soil was great in the counties of Hainan province. High Se-content paddy soils were mainly distributed in the north-east areas. There were generally two areas with high Se contents. One was from Wenchang, Qionghai to Wanning which were located in the east of Hainan. Haikou, Chenmai, Dingan and Dunchang were also included in this area. The other area was partial areas of Baoting county and Sanya city. Se contents of paddy soils in these areas were bigger than 0.2mg/kg. Scattered distribution of paddy soils with high available Se content could be seen in Haikou, Wenchang, Wangning, Lingshui, Changjiang and partial areas of Dongfang. Paddy soils with high contents of heavy metals were mainly distributed in Haikou and its neighboring areas, high contents could also be found in partial areas of Baisha, Changjiang, Dongfang and Sanya. The heavy metal contents of paddy soil in eastern area of Wenchang were low, while low heavy metal contents were also observed in Wuzhishan and Qiongzhong in the middle of the province as well as in Baoting and Lingshui in the southeast of the province. Therefore, the most suitable areas for Se-enriched rice planting were put forward in partial areas of Wenchang, Baoting and Lingshui according to the total Se content, available Se content and 5 kinds of heavy metals content of paddy soils. Secondly Qiongzhong and Wuzhishan were also suitable areas.2. Screening extractant of available Se in paddy soils in Hainan provinceThree representative paddy soils with different available Se content were collected from Dingan County. And usual extractants such as H2O,NaHCO3 and KH2PO4 were used in the cultivation experiment of rice seedling and ryegrass seedling to screen determination method of available Se content of paddy soil. The results showed that the mean coefficient of variation of available Se contents extracted with KH2PO4 was small (6.81%) and the available Se contents was significantly correlated with Se accumulation of rice plants (p<0.01) and the time of extracting was short. Therefore KH2PO4 was suitable extractant of available Se in paddy soils in Hainan. And the extracting concentration ought to be 0.1mol/L at soil and water ratio of 1:5. Firstly, an 2h extracting should be taken, then the solution would be heated in a lightly boiling water bath for 105mins, finally atomic fluorescence spectroscopy (afs) would be used for the determine of available Se content.3. Screening Se-enriched rice cultivars of Hainan provinceFiled experiments were carried out to screen Se-enriched cultivars from late rice cultivars (63 cultivars, include main cultivars and cultivars which were going to be popularized) and early rice cultivars (12 main cultivars). The results showed that Se contents in brown rice and polished one of different cultivars were obviously different. The max values was 4-5 times of the min ones. Therefore, the screening of Se-enriched rice cultivars was necessary.Se-enriched rice cultivars was obtained when a range of Se content from 0.1 mg/kg to 0.3mg/kg was took as Se-enriched foods standard from late rice cultivars, i.e. Gufengyou8312, BoⅡyou510, Nanfengyou61208, Jiefengyou629, Jing5youl82, Rexiangl, Nanfengyou/6256 and BoⅡyou629. In early rice cultivars, Hongtaiyou996,Ⅱyou128, Congyou629,Ⅱyou202 and Teyou57353 were Se-enriched cultivars.4. Response of Rice cultivars to Se sprayingField experiment was conducted to study the effect of Se spraying on main early rice cultivars. The results showed that Se content in brown rice and polished rice is improved by 43%and 34%, respectively, compared with control treatment. And the differences are significant. Therefore, Se spraying might be an effective way to improve Se content of rice in the soil with low Se content.Rice cultivars had different response to Se spraying. As compared with control, Se contents of brown rice of Congyou629 and Teyou5735 are significantly improved, which indicates that these two cultivars were choosed to plant in the soil with low Se content.5. Genotypic differences and the mechanisms of soil Se absorption and accumulation of rice cultivars.Se-enriched rice cultivars such as Gufengyou8312, Jiefengyou629 and common cultivar of BoIIyou510 were selected for pot experiment which was carried out with natural soil with high Se content and low one. The results showed that Se accumulation in plant could be greatly affected by Se content of soil.So it was better to plant Se-enriched rice cultivar in the soil with high Se content.In soils with high Se content, the accumulation of Se in Se-enriched rice cultivars mainly occurred before heading stage. Se accumulation in common cultivars was almost linearly increased in their whole growth period. For these cultivars, a lot Se were absorbed and accumulated after heading stage, still. Se-enriched cultivars had high capacity of Se absorption and they are favorable for the translocation Se into eatable parts.In low Se content soils, the total accumulations of Se in Se-enriched and common cultivars have no obvious difference. However, almost all Se contents in brown rice and polished rice of Se-enriched cultivars are extremely significantly or significantly higher than that of common cultivars, which indicates that Se-enriched rice cultivars have high capacity to translocate Se into eatable parts.The common cultivars were sensitive to high Se content soil. So their growth was inhibited in soil environment with high Se content. In the soil with high Se content, a lot Se might be fixed in cell wall after absorbed from soil, which could keep Se-enriched cultivars from damage of high concentration of Se.