Engineered ZnO nanoparticles (NPs) exist in particle and ion states in aqueous solutions, but information on the effect of different states of Zn on plant development is limited. In this study, we exposed lettuce seedlings to different Zn states. The 10 mg L−1 ZnO NP dispersion containing 0.74 mg L−1 Zn2+ ions had an inhibitory effect on root elongation. However, root elongation was not inhibited by 0.74 mg L−1 Zn2+ ions dissociated from ZnCl2. Additionally, the ZnO NP-removed Zn ion solution containing 0.74 mg L−1 Zn2+ had little effect on root elongation. Water-insoluble SiO2 and TiO2 NP dispersions had no effect on root elongation, suggesting that the phytotoxicity of ionizable metal oxide NP dispersions, such as ZnO, negatively affected root elongation. The Zn content in lettuce roots incubated in ZnO NP dispersions was 7.8-fold higher than that in ZnCl2 solution-incubated roots. Moreover, 20 mg L−1 ZnO NP dispersion reduced chlorophyll contents in the lettuce seedlings, and all plants died after transplanting into a medium lacking ZnO NPs. Inhibition of root elongation by ZnO NP dispersions and ZnCl2 solutions was accompanied by changes in the polysaccharide content of roots, and especially water-soluble components of the cell walls were accumulated. Excess intracellular zinc increased 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 NPs, a specific mechanism is suggested whereby accumulation of zinc in roots results in inhibition of chlorophyll synthesis.