Study on the characteristics and differences of content and distribution of 14 elements in different parts of rice grains

Rice is an important food crop in China. The planting area of a double-harvesting model of mid-season rice and ratoon rice is continuously expanding. While striving to create high-yield fields, the nutrition and safety of rice are also receiving attention. This experiment comprehensively considered various influencing factors such as the environment and varieties. A total of 18 varieties of main-season rice and ratoon rice, which were the main varieties at three ecological sites, were used as the test materials. By measuring the content, distribution, accumulation, and translocation coefficient of 14 elements in different parts of the rice grains in the main and ratoon seasons, the common and differential patterns of element accumulation and distribution in the two seasons were summarized and clarified. The results showed that the 14 elements could be divided into two categories. The first category included six elements: Mg, Zn, As, Cu, Se, and Cd. Their concentrations were the highest in the brand, with the distribution rates in the brand ranging from 10.1% to 45.7% for Mg, Cu, and As, and from 50.3% to 78.9% in the endosperm for Zn and Cd. The second category included eight elements: Si, Fe, Mn, Ca, Cr, Ni, Pb, and Co. Their concentrations were the highest in the husk and decreased gradually towards the inner parts of the grain. The accumulation rates in the husk were as high as 59.1% to 99.95%. Comparing different rice seasons, the accumulation of Si, Mg, and Ca in all parts was higher in the ratoon season, while Mn, Cr, Cu, Ni, Co, and Zn were higher in the main season. The total accumulation of Mg was less affected by the rice season. The concentration of elements in the brand varied the most among different ecological sites and rice seasons, while the variation in the endosperm was the smallest. The variability of Mg, Si, Mn, and Zn in the husk was relatively large. The concentration differences of elements in different parts were closely related to their translocation ability. The translocation coefficient indicated that the translocation ability of Ni among different parts was stronger in the ratoon season, while Mg, Cu, Zn, As, and Cd had a stronger ability to translocate from the husk to the brand. Moreover, the main-season rice had a stronger ability to translocate and retain Cd in the endosperm. The principal component analysis results also showed that Mg, Zn, and Cu in the brand, and Ca, Si, Mn, Fe, Cr, and Co in the husk, as well as the harmful elements Pb and Ni, were positively correlated with each other. In conclusion, most elements have similar distribution patterns in different rice seasons, ecological sites, and parts of the rice grain. However, the main season has a greater impact on the concentration and accumulation of elements in different parts compared to the ratoon season. The brand is rich in toxic (As, Cd) and beneficial (Mg, Zn, Cu) elements, and the translocation from the husk to the brand may be the key to regulating the concentration of elements in the edible parts of rice. In the actual production of rice, the selection of appropriate varieties and the degree of rice milling should be based on the local soil heavy metal pollution situation and the characteristics of element accumulation in different varieties to ensure human health and safety.