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Engineered ZnO nanoparticles exist in particle and ion states in aqueous solutions. This study was compared the exposure of different state Zn for lettuce seedlings as in vitro organ culture of root using lettuce seedlings for short time in petri dish. 0.74 mg L− 1 of Zn2+ ions dissociated from ZnCl2 promoted root elongation in lettuce seedlings. However, at the same Zn2+ ion concentration, ZnO nanoparticle dispersions had an inhibitory effect on root elongation. Additionally, 2.40 mg L− 1 of Zn2+ ions in a ZnCl2 solution inhibited lettuce root elongation. Water-insoluble SiO2 and TiO2 nanoparticles dispersions had no effect on root elongation, suggesting that the negative impact of metal oxide nanoparticles on root elongation may be specific to ionizable nanoparticles like ZnO. The Zn content in lettuce roots incubated with ZnO nanoparticle dispersions was 8-fold higher than that in ZnCl2 solution-incubated roots. 20 mg L− 1 ZnO nanoparticle dispersion reduced chlorophyll contents in the lettuce seedlings, and all plants died after transplanting into a medium without ZnO nanoparticles. Inhibition of root elongation by ZnO nanoparticle dispersions and ZnCl2 solutions was accompanied by changes in the polysaccharide content of roots, especially the water-soluble components of cell walls increased. Excess intracellular zinc increases the water-soluble fraction of the cell wall, leading to accumulation of zinc ions and stagnation of root elongation due to abnormal cell wall metabolism. In plants exposed to ZnO nanoparticles, a specific mechanism was clearly identified in which accumulation of zinc to roots results in inhibition of chlorophyll synthesis.