Effects Of Root Anatomy On Arsenic Uptake By Paddy Rice (Oryza Sativa L.)

Arsenic has attracted more and more attention by environmental scientists since the serious arsenic contamination of groundwater in Bangladesh. Arsenic poisoning in human bodies may be caused by food chain pathways through irrigating paddy rice with arsenic-contaminated groundwater. The impact of arsenic accumulation in rice is especially important as rice is the staple food for people in China. It is of great significance to understand the mechanisms of As uptake by rice, as measures are therefore possibly taken to reduce As level in edible parts. Root system is the major absorption organs for contaminants as well as nutrients, its anatomy has great effect on arsenic accumulation and translocation. Root anatomy has been found to be varied with species, genotypes and rhizosphere conditions. The present study was conducted in hydroponics to investigate root anatomy and its effect on arsenic uptake in different genotypes and induced rhizosphere conditions. The results indicated that arsenic uptake and translocation varied among genotypes and was highly related to root radial oxygen loss (ROL) and iron plaque formation. In addition, root anatomy played an important role on arsenic tolerance and the kinetics of arsenite and arsenate uptake. Finally, alternative rice genotypes and irrigation techniques suitable for arsenic contaminated sites were also suggested.The main findings are as follows:1. Four genotypes (Yanjing 1439, Zhonglou 1, Xiangwangxian 3 and Guichao 2) with great difference in root anatomy were investigated. Aerenchyma, root surface area, ROL and iron plaque formation were found higher in japonica rice than in indica rice. Arsenic (both arsenite and arsenate) tolerance, adsorption, absorption, translocation were higher in japonica rice. Iron plaque formation on different rice genotypes roots was less than 16 g kg^-1when exposed to 30mg/l Fe²⁺ for 24h, which increased arsenic adsorption and absorption, decreased arsenic translocation and increased arsenic tolerance of the genotype of Guichao 2.2. Root anatomy, ROL, iron plaque formation and arsenic uptake and tolerance of Guichao 2 was significantly affected by different rhizosphere conditions. In condition of excessive phosphorus, rice seedlings developed lower root porosity, but more root surface area. ROL and iron plaque formation did not show significant difference. Therefore, arsenic uptake and translocation and tolerance were not affected by P sufficiency. Phosphorus deficiency and sulfide excess induced lower root porosity and root surface area, which decreased arsenic adsorption. Seedlings grown in aerated solutions reduced root surface area. Less developed aerenchyma was found in aerated roots, while cell wall suberization and thickening of the superficial layers were not found as in stagnant roots. ROL of aerated seedlings were increased but iron plaque formation declined due to the significant decrease of root surface area. Arsenic adsorption, absorption and translocation decreased when rice seedlings were pretreated with aeration. Fe-plaque with concentration of 70 g kg^-1 approximately on roots of seedlings precultured in different conditions (P, S, O) enhanced arsenic adsorption but reduced root uptake and translocation to shoot.3. Root anatomy and iron plaque played different roles on kinetics of arsenic uptake. Roots precultured in aerated conditions increased arsenite affinity but decreased arsenite influx comparing with those in stagnant treatments. However, arsenate influx was higher in aerated treatments. Most arsenic was sequestrated on root surface by iron plaque and affected arsenic uptake kinetics. Iron plaque inhibited arsenic absorption by decreasing arsenite influx and decreased arsenite affinity of aerated roots, but did not affect kinetics of arsenate uptake.4. Arsenic accumulation in rice was also affected by its species. More arsenate was adsorbed on root surface than arsenite. On the contrary, more arsenite translocated to shoots than arsenate for different rice genotypes and rhizosphere conditions. Rice roots of different genotypes absorbed more arsenite than arsenate when roots introduced with iron plaque, but on the contrary without iron plaque. In addition, arsenite was more absorbed into roots when roots with or without iron plaque than arsenate.5. Indica rice genotype Guichao 2 can be suggested to grow in paddy soil with arsenic contamination. Field management such as reducing phosphorus while enhancing sulfur fertilizer application, water drainage could be easy way to reduce As uptake in paddy rice grown in contaminated sites.