水稻品種及根部鐵膜對關渡平原土壤中植體砷累積及物種之影響

Abstract

水稻砷污染的問題影響著食品安全及人體健康，因此近年來在全世界受到相當大的關注。位於台灣北部的關渡平原由於地質特性的關係，水稻田土壤受到嚴重的砷污染。儘管土壤中有高濃度的砷，過去的調查卻發現稻米中砷的累積量普遍低於 0.5 mg kg-1，本研究將探討其原因。本研究的目的為，首先探討關渡平原土壤中根部生成之鐵膜對於水稻幼苗砷吸收及累積的影響。接著比較台灣常見的28個水稻品種根部鐵膜生成量及對砷吸收能力之差異對於水稻幼苗砷累積的影響。最後，探討水稻品種間對砷的耐受性、吸收及傳輸之差異對於榖粒砷累積及物種分佈的影響。試驗結果顯示，水稻種植在關渡平原土壤中，根部生成的鐵膜可以阻擋大量砷，降低植體中砷的累積。本試驗證實，在高濃度砷污染的關渡平原土壤中，根部鐵膜為阻擋砷進入水稻植體主要的控制因子。選用的 28 個品種間根部鐵膜生成量及鐵膜累積砷的含量具有顯著差異，並且顯示大部份從土壤吸收的砷 (75.7-92.8 %) 可被累積在鐵膜。然而，由於大部分水稻品種種植於關渡平原土壤根部皆可生成足夠量的鐵膜來阻擋砷，導致品種間鐵膜生成量與植體中砷的累積沒有顯著的負相關。此外，由轉移因子的結果顯示，稉稻中砷由根部轉移至地上部的能力及地上部砷累積大於秈稻，代表砷在植體中的轉移能力對於植體中砷的累積可能也扮演重要的角色。因此，本試驗建議篩選砷吸收能力及轉移能力低的水稻品種種植於砷污染的關渡平原土壤。試驗結果也發現水稻受砷的毒害、根部表面高含量的鐵膜以及選用穀粒砷累積量較低的稉稻品種為關渡平原土壤穀粒砷濃度低的可能原因。然而，本結果同時也發現，水稻雖然種植於低砷濃度的關渡平原土壤，水稻穀粒卻有高濃度砷的累積，此因在水稻正常生長的狀態下，對於砷的吸收及在植體內的傳輸效率較高所導致。此外，水稻穀粒中主要的砷物種為雙甲基砷酸和三價砷， 榖粒中雙甲基砷酸所佔的比例會隨總砷濃度增加而增加，相反的，三價砷則是呈現下降的趨勢。本研究的結果除了有助於釐清影響關渡平原稻米砷累積的因子外，也可瞭解不同品種間砷吸收、傳輸及穀粒砷累積及物種分佈的差異。

The problem of arsenic (As)-contaminated rice affects the food safety and human health, therefore, it received more concerns in recent years around the world. In the Guandu Plain located in northern Taiwan, the paddy soils suffered from serious As contamination due to the geological factors. Despite the high As concentration in the soils, the concentrations of As in rice grains were found to be below 0.5 mg kg-1 based on the past survey, we will investigate the reasons in this study. The objectives of this study were to investigate the effects of iron plaque formation on rice roots on the uptake and accumulation of As in rice seedlings grown in Guandu Plain soils, and to compare the differences in the amounts of iron plaque and capability of As uptake of 28 commonly rice genotypes planted in Taiwan and to investigate the effect on the As accumulation in rice seedlings. Finally, to investigate the influence of the As tolerance, As uptake and translocation capability on the As accumulation and speciation in rice grains among different rice genotypes. The results show that the iron plaque formation on rice roots can sequester most of As uptake from soils, reducing the accumulation of As in rice plants. This study provides evidence that iron plaque is the main controlling factor in limiting the uptake of As into the rice plants grown in Guandu Plain soils. There were significantly differences in the amounts of Fe and As in iron plaque of rice roots among 28 tested rice genotypes, and 75.7-92.8 % of As uptake from soils could be sequestered in iron plaque. However, due to the enough amounts of iron plaque formation on roots of all tested rice genotypes grown in Guandu Plain soils, leading to there were no significant negative correlations between the amounts of Fe in iron plaque and As in rice plants. In addition, the results of translocation factor indicates that the translocation capability of As from roots to shoots and the accumulation of As in shoots of japonica genotypes were higher than indica genotypes, it reveals that the As translocation capability in rice plants may also play a important role in the As accumulation in rice plants. Therefore, low As uptake and translocation capability genotypes of rice selected from this study can be recommended to be grown in As-contaminated Guandu Plain soils. It also found that the As phytotoxicity, high amounts of iron plaque on roots and select the low As accumulation japonica genotypes were the possible causes of the low As concentrations in rice grains grown in Guandu Plain soils. However, it discovered that the high concentrations of As accumulated in rice grains grown in low As concentrations soils, it may result from the high As uptake and translocation efficiency under normal growth conditions. In addition, Arsenic species in rice grains was dimethylarsinic acid (DMA) and arsenite (As(III)), and the percentage of DMA increased with total As concentrations, and conversely, the percentage of As(III) decreases. The results of this study not only help to clarify the reasons of As accumulation in rice grains grown in Guandu Plain soils, but also understand the differences in As uptake, translocation in rice plants, and As accumulation and speciation in rice grains among different rice genotypes.