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.