Adaptation of morphological traits and anatomy structure related to Cd spatial distribution in wild rice Oryza officinalis and Oryza rufipogon roots

Excessive cadmium (Cd) accumulation in rice poses significant risks to public health. Wild rice as the ancestor of cultivated rice is regarded as a treasure trove of rice germplasm resources. Two wild rice Oryza officinalis Wall. and Oryza rufipogon Griff. were used as test materials to understand the response of morphological traits and anatomy structure of wild rice roots to Cd treatments (0, 1, 5, 15, and 30 mg·kg− 1). The results showed that the RRS (ratio of root biomass to shoot biomass) of O. officinalis and O. rufipogon under 30 mg·kg− 1 Cd treatments both reached the highest, while the root surface area and root volume decreased. The SRL (ratio of total root length to total biomass) and SRV (ratio of total root volume to total biomass) under 30 mg·kg− 1 Cd treatment was the lowest. The exodermis cells of wild rice under 30 mg·kg− 1 Cd treatment were large and loosely arranged with some cortical thin-walled cells to be collapsed. The casparian strips and suberin layer were more deepening staining and thicker with root porosity decreased and electron-dense, particle-like structures observed under Cd treatment in root cells. The thickness of the exodermis cells of O. officinalis under 30 mg·kg− 1 Cd treatment increased by 45.6% compare to 0 mg·kg− 1 Cd treatment, the thickness of endodermis cell decreased by 30.4%. Cd contents in roots of O. officinalis and O. rufipogon increased with increase in Cd treatment concentrations with Cd content in order root > shoot. The proportion of soluble component increased with Cd treatment concentrations with in order cell wall (F1) > soluble component (F3) > organelles (F2). Cd content in the root-surface iron plaque of O. rufipogon displayed an initial increase followed by a subsequent decrease. In general, the results suggest that the casparian strips and suberin tissue of wild rice should play important roles to deposit Cd, especially thickness of exodermis of O. officinalis root cell and root-surface iron plaque of O. rufipogon.