Integrated physiological, transcriptomic, and metabolomic analyses provide insights into zinc-doped carbon quantum dot-mediated alleviation of cadmium toxicity in lettuce.

Cadmium (Cd) accumulation in crops seriously threatens agricultural productivity and food safety, highlighting the need for effective strategies to mitigate Cd toxicity. Uniformly dispersed zinc-doped carbon quantum dots (Zn-CQDs; 2.67 ± 0.36 nm) were synthesized in this study. The synthesized dots demonstrated enzyme-mimetic activities in vitro and potent radical-scavenging capacity. Foliar application of Zn-CQDs markedly alleviated Cd-induced growth inhibition in lettuce (Lactuca sativa L.), increasing shoot fresh and dry biomass by 30.8% and 23.9%, respectively. Moreover, Zn-CQDs treatment reduced the Cd concentration in lettuce shoots by 44.7% while elevating the Zn concentration by 167.9%. Photosynthetic performance was also substantially restored, as evidenced by a 61.0% increase in total chlorophyll content. Concurrently, Zn-CQDs mitigated Cd-triggered oxidative damage, reducing H2O2 and MDA levels by 31.4% and 38.9%, respectively, and lowering the activities of the antioxidant enzymes POD, SOD, and CAT by 23.3%, 71.5%, and 44.3%, respectively. Transcriptomic profiling revealed that Zn-CQDs modulate stress-responsive signaling networks involving MAPK cascades, Ca2 +, and phytohormone signaling, as well as their associated transcription factors, thereby regulating downstream genes involved in ROS detoxification and metal transport. Metabolomic analysis further demonstrated that Zn-CQDs extensively reprogram Cd-responsive metabolic pathways, particularly those related to phenylpropanoid, lipid, amino acid, and phytohormone metabolism, accompanied by increased accumulation of jasmonic acid, flavonoids, and organic acid derivatives. The above-mentioned findings offer mechanistic insights into Zn-CQD-mediated alleviation of Cd stress and lay a theoretical basis for functional carbon nanomaterials' application to reduce heavy metal risks in agricultural systems.