Microstructural and physiological responses to cadmium stress under different nitrogen forms in two contrasting Populus clones

Although nitrogen (N) influences plant response to cadmium (Cd) stress, little information has been suggested on how N forms affect Cd distribution and tolerance in Populus clones with different sensitivities to N deficiency. In this study, we examined physiological, chemical and microstructural traits involving in Cd accumulation, distribution and tolerance under different N forms (NH4+-N and NO3--N). We found that N limitation-insensitive clone Nanlin 1388 had stronger Cd tolerance than N limitation-sensitive clone Nanlin 895 under both N forms, which is associated with Cd elaborate arrangement in tissues and/or cellular compartments, as well as an enhanced antioxidation capacity. NH4+ supply significantly promoted Cd uptake and accumulation in leaves, especially in Nanlin 895, when compared to NO3- supply. Furthermore, NO3- and NH4+ differently regulated Cd distribution in leaves and roots of Nanlin 1388 and Nanlin 895. NO3- supply promoted Cd distribution in leaf epidermis, especially in the upper epidermis of leaf blades and veins in Nanlin 1388. In contrast, NH4+ induced more Cd distribution in the leaf lower epidermis and vascular tissues for Nanlin 895. These results collectively suggested that Nanlin1388 had stronger Cd tolerance, especially under NO3- supply, which provided new insights to engineer woody plants for phytoremediation.