Does ammonium nitrogen affect accumulation, subcellular distribution and chemical forms of cadmium in Kandelia obovata?

Heavy metals and nutrients are commonly found in mangrove sediments, but the effect of nutrients on heavy metals in mangrove plants is not clear. A study quantifying the effects of ammonium nitrogen (NH₄ ⁺-N) on the accumulation, subcellular distribution and chemical forms of cadmium (Cd) in Kandelia obovata seedlings were conducted. The experiment consisted of four levels of NH₄ ⁺-N (0, 10, 50 and 100 mg L⁻¹) in each of which consisted of four Cd levels (0, 1, 5 and 10 mg L⁻¹). The results showed that NH₄ ⁺-N magnified the Cd toxicity due to reduced plant biomass, especially with 10 mg L⁻¹ Cd and 100 mg L⁻¹ NH₄ ⁺-N supply. NH₄ ⁺-N, especially at 100 mg L⁻¹, enhanced the concentration and accumulation of Cd in root but its role on Cd translocation from root to stem and leaf was limited, probably due to low translocation factor. At subcellular level, Cd mainly accumulated in root cell wall but its fractionation depended on Cd levels. Under the stress of 1 and 5 mg L⁻¹ Cd, 50 mg L⁻¹ NH₄ ⁺-N supply improved transfer of Cd from root cell wall into cell, and increased pectate and protein integrated forms of intracellular Cd to alleviate Cd toxicity. Under the stress of 10 mg L⁻¹ Cd, NH₄ ⁺-N supply promoted the deposition of Cd on root cell wall to restrain its transfer to root cell, which was verified by the reduced levels of pectate and protein integrated forms of Cd in root cell. Thus, NH₄ ⁺-N supply improved immobilization of Cd in roots and alleviated Cd toxicity through integration with pectate and protein as well as cell wall combinations in root of K. obovata.