Chromium (Cr) contamination poses a significant threat to rice safety, yet the mechanisms governing Cr(III) bioavailability under flooded paddy conditions remain poorly understood, particularly the role of chemical speciation. This study investigated the contrasting uptake, translocation, and subcellular distribution of organic-complexed Cr (EDTA-Cr) and inorganic Cr (CrCl 3 ) in hydroponically grown rice. Inorganic CrCl 3 rapidly hydrolyzed into colloidal particles (350.2 nm diameter, 61.1% of total Cr), enhancing root surface adsorption, while EDTA-Cr remained stable in solution due to its high chelation strength. Rice accumulated 7.2-fold more CrCl 3 in roots and 4.0-fold more in shoots compared to EDTA-Cr, with translocation factors (shoot/root ratio) of 0.61 for CrCl 3 versus 0.15 for EDTA-Cr. Subcellular analysis showed 88.0% of CrCl 3 sequestered in root cell walls, whereas 70.0% of EDTA-Cr penetrated organelles and cytosol, triggering early toxicity and limiting translocation. The study elucidates a speciation-dependent bioavailability mechanism: CrCl 3 colloids act as reservoirs, gradually releasing bioavailable Cr 3+ via root exudate-mediated dissolution, while EDTA-Cr bypasses adsorption and directly enters cells, accelerating toxicity thresholds.
Li et al. (Mon,) studied this question.