Nitrogen, despite being essential for the growth of plants, can pose serious threats to the ecological environment when applied excessively as fertilizers. The application of nanomaterials has a catalytic effect on crop growth and a restorative effect on the environment. However, their effect on mitigating ammonium stress in crops is poorly understood. In the present study, the roles of nanoparticles of magnesium oxide (nMgO) and hydroxyapatite (nHA) with different application rates (0, 10, 100, 500, and 1000 mg L−1) on seed germination and seedling growth in water spinach (Ipomoea aquatica Forssk.) and Hami melon (Cucumis melo L.) were investigated, and the ammonium stress mitigating capacity of nanoparticles with the optimal application rate on the two crops was analyzed. The results showed that the application of nMgO and nHA at an optimal rate of 100 mg L−1 significantly promoted seed germination of water spinach, followed by the increase of germination potential, seed germination rate, and germination index, while alleviating the inhibitory effect of NH4+ stress in water spinach. As for the Hami melon, nHA reduced the ammonium stress on seedlings by promoting antioxidant enzyme activity, while nMgO was found to be involved in reducing the root growth of Hami melon seedlings. This study provided a reference on how to select the appropriate type and optimize the application method of nanomaterials that will be used in agriculture in the future.

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