Nitrogen Form Modulates Plant Responses to Heterogeneous Iron Availability in Arabidopsis and Wheat

Plants commonly encounter spatially heterogeneous distributions of nitrogen (N) forms and iron (Fe) in soils, but how N form modifies plant responses to heterogeneous Fe availability remains poorly understood. Using a split-root system, we examined the phenotypic responses of Arabidopsis thaliana and wheat to contrasting Fe availability combined with ammonium (NH4+) or nitrate (NO3−) supply. Plants were exposed to high (100 μM) or low (5 μM) Fe in combination with NH4+ or NO3−, either under homogeneous or spatially heterogeneous conditions. Within each species, NH4+ and NO3− treatments were supplied at equivalent total N concentrations. In both species, high Fe combined with sole NH4+ supply resulted in pronounced reductions in shoot and root growth, whereas spatial presence of NH4+ and NO3− between root compartments was associated with improved growth. Under high-Fe conditions, sole NH4+ supply to both root compartments (HAHA) resulted in pronounced reductions in shoot and root growth, whereas supplying NH4+ and NO3− to separate root compartments (HAHN) was associated with improved growth. Under these conditions, shoot fresh weight in HAHN was 178% higher than that in HAHA in Arabidopsis and 231% higher in wheat. In wheat, root fresh weight under HAHN was 471% higher than under HAHA. Wheat exhibited stronger spatial differentiation of root growth between contrasting nutrient compartments, whereas Arabidopsis showed greater variation in whole-plant growth in response to N form. Chlorophyll accumulation also showed species- and Fe-dependent responses to N form. Overall, these results indicate that N form modifies plant phenotypic responses to heterogeneous Fe–N supply and that wheat exhibits greater root plasticity under spatially heterogeneous Fe–N conditions. The roles of plant Fe and N status, rhizosphere pH, and long-distance signaling in these responses remain to be determined.