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 Zn 2+ toxicity to wheat seedling roots and to examine the effects of different nitrogen forms (NH 4 + and NO 3 − ) on ψ 0 and Zn rhizotoxicity. Solution culture experiments were conducted to measure the root elongation and Zn accumulation under Zn 2+ exposure. The role of two nitrogen forms in affecting Zn 2+ toxicity was compared, giving particular consideration to ψ 0 and Zn 2+ activities at CMs ({Zn 2+ } 0 ). Results showed that NH 4 + alleviates Zn 2+ rhizotoxicity and NO 3 − increases Zn 2+ rhizotoxicity. In modeling the rhizotoxicity, root length correlated better with {Zn 2+ } 0 than {Zn 2+ } b , and the predictive accuracy ( r 2 ) of NH 4 + treatment increased from 0.748 to 0.917 when incorporation of {Zn 2+ } 0 and {Ca 2+ } 0 into analysis. Oppositely, ψ 0 played a limited role in modeling Zn 2+ rhizotoxicity and bioavailability in NO 3 − treated medium ( r 2 = 0.609). Moreover, higher concentration of Zn in roots was found in NO 3 − treatment, compared with the NH 4 + treatment. ψ 0 rather than the rhizotoxicity data correlated better with Zn accumulation especially in the NO 3 − 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 NH 4 + on Zn toxicity and uptake was well explained and modeled by electrostatic effects at CMs. Though our data do not explore mechanisms for the NO 3 − -Zn 2+ interactions, we propose that ψ 0 worked better in affecting the driving force for root Zn uptake, than influencing metal bioavailability at CMs.