The water-soluble fluorescent carbon quantum dots (CQDs) are synthesized by utilizing lemon juice as carbonresource via a simple hydrothermal reaction. The obtained CQDs are with an average size of 3.1 nm. They revealuniform morphology and well-crystalline and can generate bright blue-green light emission under UV or blue lightirradiation. We find that the fluorescence from these CQDs is mainly induced by the presence of oxygen-containinggroups on the surface and edge of the CQDs. Moreover, we demonstrate that the as-prepared CQDs can beapplied to imaging plant cells. This study is related to the fabrication, investigation, and application of newlydeveloped carbon nanostructures.