The Dose-Dependent Effect of Carbon Quantum Dots as a Photosynthesis Enhancer on Soybean Plant Growth

When carbon quantum dots (CDs) are used to enhance photosynthesis, they inevitably enter the plant. However, the dose-dependent effects of CDs on plant growth are poorly understood. In this study, we investigated the dose-dependent effects of CDs on soybean growth. CDs were synthesized from citric acid and urea via a hydrothermal procedure. The analysis of the structure, chemical composition, and optical properties revealed that synthetic CDs have a sphere-like shape with rich hydrophilic groups on their surface. CDs exhibited superior upconverted photoluminescence properties and emitted strong fluorescence (exciting wavelength of 220 nm; emitting wavelength of 438 nm). Various concentrations of synthetic CDs (0-1000 mg L^-1), as a photosynthesis enhancer, were applied to soybean plants under hydroculture for 1-10 days. CDs positively affected soybean growth at concentrations less than 200 mg L^-1. However, at higher concentrations (500 or 1000 mg L^-1), they exhibited significant toxicity to plant growth, which was evidenced by the mass accumulation of CDs in damaged leaves. Metabolomic and transcriptomic analyses indicated that CDs at a low concentration (100 mg L^-1) could increase antioxidant and biomass accumulation in soybeans to promote plant growth. This study provided valuable insights into the impacts of CDs on plants in sustainable agricultural practices involving the use of nanomaterials.