Objective Ammonium nitrogen reduces crop absorption of cadmium (Cd), while nitrate nitrogen increases Cd absorption. We studied the effects and mechanisms of supplying these two forms of nitrogen on cadmium uptake in rice.

Methods A hydroponic experiment was conducted using the japonica rice variety "Zhonghua 11" (Oryza sativa L., ZH11). Based on Hoagland nutrient solution, we set up nutrient solutions with nitrate nitrogen and ammonium nitrogen at a concentration of 1.25 mol/L, including treatments with and without 0.5 μM Cd, resulting in four treatments overall. After 21 days of growth in the treatment solution, we sampled to investigate rice growth indicators, the content of iron (Fe), zinc (Zn), and cadmium (Cd) ions in the above-ground and root parts, subcellular Fe and Cd content in roots, and the expression of genes related to metal ion transport. We analyzed the interactions among various physiological indicators using structural equation modeling.

Results Under ammonium nitrogen conditions, low cadmium stress had no significant effect on rice growth. However, under nitrate nitrogen conditions, low cadmium stress significantly reduced leaf SPAD values, plant height, and dry biomass of both the above-ground and root parts by 46%, 27%, 36%, and 25%, respectively, and the leaves showed symptoms of chlorosis. The SPAD value of the leaves was positively correlated with the Fe content in the above-ground part (R²=0.79), negatively correlated with Cd content, and had no relation to Zn, indicating that the decrease in SPAD values under nitrate nitrogen is due to reduced iron content and increased cadmium content. Under low cadmium stress, the iron content in the surface and internal roots of rice treated with nitrate nitrogen was higher and lower than that treated with ammonium nitrogen, respectively, resulting in a lower transfer coefficient of iron from the root surface to the inner root compared to ammonium nitrogen. Subcellular iron content analysis showed that the iron content in the root cell walls of nitrate nitrogen treatment was higher than that of ammonium nitrogen treatment, while the iron content in organelles and soluble parts was lower than that of ammonium nitrogen, leading to poorer mobility of iron and a lower transfer coefficient of iron from roots to above-ground parts. Simultaneously, nitrate nitrogen treatment enhanced the expression of root iron uptake transport genes OsIRT1 and OsIRT2.

Conclusion Under low cadmium stress, supplying nitrate nitrogen significantly decreased rice leaf SPAD values, inhibited growth of the roots and above-ground parts, and caused leaf chlorosis due to iron deficiency. In contrast, supplying ammonium nitrogen did not result in iron deficiency and therefore did not affect rice growth. Nitrate nitrogen promoted the formation of iron membranes at the root surface and the accumulation of iron in the cell walls, reducing the transfer coefficient of iron and resulting in insufficient iron supply to the above-ground parts. The feedback of iron deficiency signals within the plant upregulated the expression of genes regulating iron absorption and transport, and the nonspecific absorption of related transport proteins led to a significant increase in cadmium accumulation, inhibiting rice growth and development.