Differences in Root Surface Iron Plaque Components between Main and Ratoon Crops of Different Rice Varieties and Their Effects on Cadmium Accumulation in Brown Rice
To elucidate the seasonal variation in cadmium (Cd) accumulation in ratoon rice and its relationship with root surface iron plaque, this study compared Cd concentrations in brown rice and the characteristics of iron plaque components (amorphous Fe, Am-Fe; crystalline Fe, Cry-Fe) between the main and ratoon crops of six rice varieties under different stubble heights. A field experiment was conducted in a Cd-contaminated paddy in Liuyang, Hunan (soil total Cd: 0.56 ± 0.06 mg·kg⁻¹). Ratoon crop treatments included low stubble (20 cm) and high stubble (60 cm). Brown rice Cd concentrations varied by variety, season, and stubble height. Low stubble generally increased brown rice Cd in the ratoon crop compared to high stubble; high stubble reduced Cd in most varieties relative to the main crop. Health risk assessment indicated that low stubble in the ratoon crop posed higher non-carcinogenic risk than the main crop and high stubble, while carcinogenic risks exceeded acceptable levels across all treatments. Iron plaque Am-Fe and Cry-Fe concentrations in the ratoon crop were generally lower than in the main crop, with Am-Fe consistently exceeding Cry-Fe. In the main crop, total Fe, Am-Fe, and Cry-Fe on root surfaces were significantly negatively correlated with brown rice Cd (P < 0.05), but correlations were not significant in the ratoon crop. High stubble reduced Cd accumulation and non-carcinogenic risk in most varieties, yet carcinogenic risk remained. Iron plaque significantly impeded Cd uptake in the main crop but its effect weakened in the ratoon crop. Selecting low-Cd-accumulating varieties and optimizing stubble height are key strategies for safe ratoon rice production in Cd-contaminated areas.