The mechanism by which nitrate nitrogen inhibits the upward transport of iron in rice and exacerbates low cadmium stress

Objectives Ammonium nitrogen (NH₄⁺-N) reduces the uptake of cadmium (Cd) by crops, whereas nitrate nitrogen (NO₃⁻-N) increases Cd uptake. Research on the effects of supplying these two nitrogen forms on cadmium absorption in rice and the underlying mechanisms provides a theoretical basis for developing cadmium-reducing strategies in nitrogen fertilizer management for rice cultivation.

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 μmol/L Cd, resulting in four treatments overall. After 21 days of growth in the treatment solution, we sampled to investigate rice growth indicators, the contents of iron (Fe), zinc (Zn), and Cd in the above-ground and root parts, subcellular Fe and Cd contents 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 was due to reduced Fe content and increased Cd content. Under low Cd stress, the Fe content in root 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 Fe from the root surface to the inner root compared to ammonium nitrogen. Subcellular Fe content analysis showed that the Fe content in the root cell walls and organelles of nitrate nitrogen treatment was higher than that of ammonium nitrogen treatment, while the Fe content in soluble parts was lower than that of ammonium nitrogen, leading to poorer mobility of Fe and a lower transfer coefficient of Fe from roots to above-ground parts. Simultaneously, nitrate nitrogen treatment enhanced the expression of root Fe uptake transport genes OsIRT1 and OsIRT2.

Conclusions Under low Cd 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 Fe deficiency. In contrast, supplying ammonium nitrogen did not result in Fe deficiency. Nitrate nitrogen promoted the formation of Fe membranes at the root surface and the accumulation of Fe in the cell walls, reducing the transfer coefficient of Fe and resulting in insufficient Fe supply in the above-ground parts. The feedback of Fe deficiency signals within the plant upregulated the expression of genes regulating Fe absorption and transport, and the nonspecific absorption of related transport proteins led to a significant increase in Cd accumulation, inhibiting rice growth and development.