Effect of different nitrogen forms on the toxicity of Zn in wheat seedling root: a modeling analysis

Heavy metal stress in culture media is always rhizotoxic. Our study aims to investigate the role of negative potential (ψ ₀) at root cell membrane surface (CMs) on modeling Zn²⁺ toxicity to wheat seedling roots and to examine the effects of different nitrogen forms (NH₄⁺ and NO₃^-) on ψ ₀ and Zn rhizotoxicity. Solution culture experiments were conducted to measure the root elongation and Zn accumulation under Zn²⁺ exposure. The role of two nitrogen forms in affecting Zn²⁺ toxicity was compared, giving particular consideration to ψ ₀ and Zn²⁺ activities at CMs ({Zn²⁺}₀). Results showed that NH₄⁺ alleviates Zn²⁺ rhizotoxicity and NO₃^- increases Zn²⁺ rhizotoxicity. In modeling the rhizotoxicity, root length correlated better with {Zn²⁺}₀ than {Zn²⁺}_b, and the predictive accuracy (r ²) of NH₄⁺ treatment increased from 0.748 to 0.917 when incorporation of {Zn²⁺}₀ and {Ca²⁺}₀ into analysis. Oppositely, ψ ₀ played a limited role in modeling Zn²⁺ rhizotoxicity and bioavailability in NO₃^- treated medium (r ² = 0.609). Moreover, higher concentration of Zn in roots was found in NO₃^- treatment, compared with the NH₄⁺ treatment. ψ ₀ rather than the rhizotoxicity data correlated better with Zn accumulation especially in the NO₃^- treatment (r ² > 0.7), which meant the electrical driving force at CMs playing a dominant role in modeling the metal accumulation. In conclusion, the alleviatory role of NH₄⁺ on Zn toxicity and uptake was well explained and modeled by electrostatic effects at CMs. Though our data do not explore mechanisms for the NO₃^--Zn²⁺ interactions, we propose that ψ ₀ worked better in affecting the driving force for root Zn uptake, than influencing metal bioavailability at CMs.