Since soil salinization imposes various adverse effects on plants, research on how to relieve salt stress from plants is extremely urgent. We synthesized a new type of cerium-doped carbon quantum dots by a hydrothermal synthesis method. Characterization shows that the carbon quantum dots have a small and uniform particle size, high stability, high water solubility and biocompatibility. Mung bean seeds were soaked in CDs:Ce solutions under a concentration gradient (0.25, 0.5, 1, 2, 3 mg/ mL) and germinated under salt stress (150 mM NaCl). Compared with salt stress, the addition of CDs solutions effectively enhanced the ability of plants to relieve salt stress. The relieving effect on mung bean plants was the most significant after treatment with 2 mg/mL CDs:Ce, and the main root length, plant height and leaf length in comparison with the case of salt stress increased by 83%, 80%, and 60%, respectively. Chlorophyll content, peroxidase activity, superoxide dismutase activity and catalase activity, total protein content increased by 90%, 77%, 76%, 77% and 76%, respectively, malondialdehyde and proline The content decreased by 83% and 77%. Inductively coupled plasma mass spectroscopy proved mung bean plants absorbed CDs:Ce, but the absorption of NaCl decreased by 21.8%. Fluorescence imaging showed CDs:Ce was absorbed by roots, and transferred from the vascular system and apoplastic pathways to stems and leaf veins, and mainly aggregated in intercellular gaps, the vascular system, leaf veins, cilia and stomata. Stereomicroscopy showed that CDs:Ce induction increased the stomatal opening by 15.7%, and improved metabolic efficiency and NaCl excretion from the plants. Hence, CDs:Ce shows great potential in protecting crops from abiotic stress.

    Abstract
    1 Introduction
    2 Materials and methods
        2.1 Materials
        2.2 Synthesis of CDs:Ce
        2.3 Plant materials, stress treatment, and growth conditions
        2.4 Morphological indices
        2.5 Physiological indices
        2.6 Absorption of CDs:Ce and NaCl by plants
        2.7 Localization of CDs:Ce in mung bean seedlings
        2.8 Statistic analysis
    3 Results and discussion
        3.1 Characterization of CDs:Ce
        3.2 Plant growth and development
        3.3 Chlorophyll contents
        3.4 MDA content
        3.5 Antioxidant enzymes activity
        3.6 Total protein content
        3.7 Proline content
        3.8 Absorption of CDs:Ce and NaCl by plants
        3.9 Localization of CDs:Ce in mung bean seedlings
        3.10 Mechanism of CDs:Ce in relieving salt stress
    4 Conclusion
    Acknowledgements 
    References

• Zihao Dong(School of Life Sciences, Shanxi Normal University, Modern College of Arts and Sciences, Shanxi Normal University)
• Yan Gong(School of Life Sciences, Shanxi Normal University)
• Jie Zhao(School of Life Sciences, Shanxi Normal University)