Variations in iron plaque, root morphology and metal bioavailability response to seedling establishment methods and their impacts on Cd and Pb accumulation and translocation in rice (Oryza sativa L.)

Global rice production is undergoing a shift from traditional seedling transplanting to direct seeding. There are also serious global challenges caused by heavy metal pollution in grain. To evaluate heavy metal accumulation in rice under different seedling establishment methods, we conducted field experiments with manual transplanting (MT), seedling throwing (ST) and direct seeding (DS). Data revealed that DS method enhances cadmium (Cd) and lead (Pb) accumulation in roots, straw, and brown rice. In particular, the Cd concentration in DS brown rice was 35.4% and 22.5% higher than in MT and ST methods, respectively; the concentration of Pb in DS brown rice was 1.97 times higher than in MT. The reasons for these observations are that DS rice root systems have finer diameters, larger specific surface areas, larger specific root lengths, more root tips, and reducing Fe plaque formation on root surface while DS rice roots formed in the upper soil layer and exhibited higher Cd and Pb bioactivity within the rhizosphere soil. Collectively, here we show for the first time that the shift in seedling establishment methods from transplanting to direct seeding also have played a partial contribution in the observed increased severity of heavy metal pollution in rice levels.