Foliar spray of carbon quantum dots alleviated cadmium stress in Dracocephalum Moldavica by altering physiological and biochemical responses

Carbon quantum dots (CQDs), characterized by their unique structure and remarkable fluorescence properties, could affect physiological efficiency under heavy metal stress by contributing to metal detoxification and ion homeostasis at the cellular level. Thus, a pot experiment with a factorial arrangement (in three replicates) was laid out to investigate the effects of foliar application of CQDs (0, 2, 4, 6, and 8 mg L−1) under various cadmium levels (0, 25 and 50 mg kg−1) in Dracocephalum moldavica (dragonhead) plants. Foliar application of CQDs with 4 mg L⁻¹ concentration (optimal level) mitigated cadmium stress via an enhancement in vacuolar H+-ATPase activity and nutrient uptake. The CQDs also decreased salicylic acid content, reactive oxygen species generation, lipid peroxidation, and antioxidant enzymes activities, while improved photosynthetic performance, including relative electron transport rate, maximum efficiency of photosystem II (Fv/Fm), Rubisco activity, chlorophyll and flavonoid contents, glucose content, and leaf area. These integrated physiological improvements led to an increase of shoot mass and essential oil yield by approximately 15% and 23% under 25 mg kg⁻¹, and 19% and 31% under 50 mg kg⁻¹ Cd toxicities, respectively. However, foliar treatment of CQDs at concentrations exceeding 4 mg L−1 did not lead to statistically significant improvements in plant performance under toxic conditions. These findings suggest that foliar application in appropriate concentrations can help CQDs' purposeful and economical application in line with boosting the physiological efficiency and productivity of cadmium-stressed dragonhead plants.