Carbon dots can strongly promote photosynthesis in lettuce (Lactuca sativa L.)

Abstract

Carbon dots (CDs) have great potential in biological applications due to their excellent fluorescence properties, low cytotoxicity, and biocompatibility. However, whether they can promote photosynthesis of plants by in vitro amendment, thus having a potential application in agriculture, still remains unknown. In this study, we systematically investigated the possible application of CDs at concentrations of 10, 20, 30 and 40 mg L⁻¹ in promoting photosynthesis of lettuce (Lactuca sativa L.). The results showed that after a 7 day root exposure, CDs entered the plant roots, and they were translocated to the stems and leaves. Amendment with CDs promoted the photosynthesis of lettuce plant by increasing the chlorophyll content, chloroplast activity, photosystem II (PSII) performance and Rubisco enzyme activity, and 30 mg L⁻¹ was the optimal concentration. Specifically, 10–40 mg L⁻¹ CDs increased the chlorophyll content by 9.8, 15.3, 50.5 and 27.6% respectively, relative to the control. CDs promoted light absorption and utilization and energy conversion efficiency in chloroplasts, indicated by the fact that their amendment enhanced the intensity of the absorption spectra and the fluorescence emission spectra of chloroplasts. CDs also enhanced the PSII activity by accelerating the electron transfer rate. The Rubisco enzyme activity in lettuce leaves was also increased by 17.2–51.9% by the amended CDs, which was beneficial for production of carbohydrates. Consequently, 10–40 mg L⁻¹ CDs increased the soluble sugar content in lettuce leaves by 5.8–16.2%, and significantly raised the shoot dry weight by 11.8, 14.8, 36.4 and 19.5%, respectively, relative to the control. However, amendment with CDs significantly decreased the content of nutrient elements (K, Ca, Mg, P, Fe, Mn and Zn) in lettuce plant, which was ascribed to the biomass dilution effect, and verified that the photosynthesis enhancement was not achieved by altering the uptake of nutrient elements. These findings highlight that CDs are a promising nano-fertilizer for plant growth by promoting photosynthesis.