Protective roles of carbon quantum dots in improving root physiology of dragonhead under cadmium toxicity

Abstract

Cadmium (Cd) contamination severely restricts plant growth by impairing root development and physiological function. This study was laid out to investigate the potential of carbon quantum dots (CQDs) in alleviating cadmium-induced stress in dragonhead (Dracocephalum moldavica) roots. The effects of different cadmium levels (control, 25 and 50 mg CdSO₄ per kg of soil) and foliar treatments of water and CQDs (2, 4, 6, and 8 mg L⁻¹) on plants were investigated in a factorially arranged experiment. The CQDs markedly improved root performance under cadmium toxicity. Application of CQDs increased plant root length and dry mass by about 16% and 19% at 25 mg kg⁻¹, and 19% and 27% at 50 mg kg⁻¹ of cadmium toxicity, respectively, showing the protective roles of these materials. The CQDs also reduced Cd content in roots by approximately 12%, contributing to improved membrane integrity and reduced stress damage. These protective outcomes were associated with several physiological adjustments. Treatment with CQDs, particularly at 4 and 6 mg L⁻¹, noticeably increased activities of plasma and vacuolar H⁺-ATPases, root metabolic activity, and nutrient content (calcium, magnesium, potassium) in root tissues under cadmium toxicity. Furthermore, CQDs decreased the accumulation of stress-related molecules (abscisic acid, salicylic acid, phytochelatins, lignin, reactive oxygen species, lipid peroxidation, antioxidant enzyme activities, proline, and soluble carbohydrates), indicating lower oxidative and osmotic stress. These findings demonstrate the functional role of CQDs in alleviating cadmium toxicity by supporting root growth and modulating key physiological processes involved in ion transport, hormonal balance, and oxidative defense.