Abstract: Rice plants are positively influenced by oxygen and nitrogen forms. However, little is known about the effects of oxygenation and nitrogen forms on root biological characteristics and cadmium (Cd) accumulation in rice plants. In this study, two rice cultivars Shenliangyou 1813 (SLY 1813) and Hanyou 73 (HY 73) were planted under external oxygenation and different nitrogen forms, including 100% of ammonium nitrogen, 100% of nitrate nitrogen, and 50% of ammonium nitrogen plus 50% of nitrate nitrogen. The experiment was conducted in hydroponic conditions for two weeks. A Cd concentration of 0.5 mg·L^-1 was applied to the nutrient solution during the experiment. Root biological characteristics such as physiological, morphological, anatomical parameters and Cd accumulation rate of aboveground part at the end of the experiment were measured. The results showed that plant height, aboveground biomass, tillering number, and root biological characteristic parameters (including root activity, root length, root surface area, root volume, number of aerenchyma, number of root vascular bundles, and distance from the epidermis to the pericycle) were significantly increased in aerobic conditions compared with those without oxygenation under all nitrogen forms (P<0.05). Across the treatments of nitrogen forms, ammonium and nitrate mixed nitrogen forms improved the growth performance of shoot and root when compared with single ammonium and nitrate nitrogen treatments in both cultivars. The accumulated Cd content of shoots was the lowest under the combined mode of oxygenation treatment and mixed nitrogen forms, but was the highest in the treatment of single nitrate nitrogen or ammonium nitrogen without oxygenation. Shoot Cd content was significantly negatively correlated with root length, root surface area, root volume, and number of root vascular bundles (P<0.01). These results suggest that improved root growth capacities by applying oxygenation and mixed nitrogen forms could reduce Cd accumulation under Cd stress in rice seedlings. This study would provide guidance for the safe production of rice in Cd-contaminated paddy fields.

Key words: rice, Cd stress, nitrogen form, oxygenation, root biological characteristics