Selenium and nitrogen-doped carbon quantum dots: Alleviation on arsenic and/or lead toxicity through water balance, nutrient accumulation, photosynthetic performance and antioxidant metabolism in Lactuca sativa

Assessing plant responses to heavy metal stress, a major environmental pollutant, is essential for improving crop resilience and ensuring environmental sustainability. Carbon quantum dot (CQD), especially when functionalized with elements or non-metals, presents a biocompatible and eco-friendly approach for detecting and alleviating stress. However, there is limited information on selenium/nitrogen co-doped CQDs (Se/N-CQDs) mimicking antioxidant activity in plants exposed to arsenic (As) and lead (Pb) toxicity. This study investigated the protective effects of Se/N-CQD on lettuce plants subjected to As and Pb stress, focusing on growth, elemental composition, gas exchange, PSII efficiency, reactive oxygen species accumulation, and antioxidant systems. Exposure to As and/or Pb negatively affected growth, water status, chlorophyll fluorescence, gas exchange (carbon assimilation rate, stomatal limitation, and carboxylation efficiency), PSII reaction center activity, and excitation energy capture efficiency. These effects were further intensified by imbalances in essential nutrients such as K, Mg, P, B, Cu, Zn, and Fe, along with increased accumulation of As and Pb. Under stress conditions, key enzymatic and non-enzymatic components, particularly those involved in the ascorbate–glutathione (AsA–GSH) pathway responsible for regulating H2O2 levels, were disrupted. However, Se/N-CQDs alleviated the negative effects of heavy metal stress. It improved chlorophyll fluorescence, growth, water status, and stomatal regulation in stressed plants. Additionally, Se/N-CQDs enhanced the efficiency of photon energy utilization in PSII. Following exposure to As or Pb in the presence of Se/N-CQDs, superoxide dismutase (SOD) activity increased significantly. In As-treated plants, Se/N-CQDs stimulated peroxidase (POX), glutathione S-transferase (GST), glutathione peroxidase (GPX), and both enzymatic and non-enzymatic components of the AsA–GSH cycle, improving ascorbate-dependent reduction and glutathione redox balance. In contrast, increased catalase (CAT) activity played a key role in H2O2 scavenging under Pb or combined As + Pb stress. Visualization of H2O2 accumulation in guard cells using a cell-permeable dye showed reduced fluorescence intensity (green signals) after Se/N-CQD exposure, indicating decreased oxidative stress. In conclusion, Se- and N-doped CQDs enhance heavy metal tolerance in Lactuca sativa by modulating photosynthetic efficiency, antioxidant capacity, and redox regulation, highlighting their potential as an effective nanobiotechnological approach for sustainable crop production under metal stress conditions.