Satureja hortensis-Derived Carbon Dots Alleviate Drought Stress in Maize via Concentration-Dependent Modulation of Oxidative Balance and Water Status

Drought severely limits crop productivity, and carbon-based nanomaterials are emerging tools to mitigate stress damage; yet carbon dots (CDs) from phenolic-rich medicinal plants remain underexplored, especially their concentration-dependent effects on maize. Carbon dots were hydrothermally synthesized from Satureja hortensis (S-CDs) and characterized by UV–Vis, Raman, FTIR, XRD, and HR-TEM, confirming predominantly amorphous, blue-emitting nanoparticles with an sp2 carbon core and an oxygen-rich surface. The S-CDs retained high phenolic content (58.19 ± 2.98 mg GAE/g) and strong antioxidant capacity (CUPRAC: 685.26 ± 17.87 µmol Trolox/g; DPPH IC50: 0.0451 mg/mL). They were foliar-applied at seven concentrations (200–10 000 ppm) to maize seedlings after PEG-6000 treatment (10%, w/v, 48 h). Drought reduced net photosynthesis (Pn), stomatal conductance (gs), transpiration (Tr) and relative water content (RWC), suppressed superoxide dismutase (SOD) and guaiacol peroxidase (GPOD) activities, and raised H2O2 and lipid peroxidation (TBARS). The S-CD response was concentration-dependent and biphasic: low doses (200–250 ppm) intensified oxidative stress, whereas higher doses progressively alleviated it. Strongest protection occurred at 10 000 ppm, restoring RWC, SOD and GPOD to near-control levels and markedly reducing lipid peroxidation; photosynthesis improved partially (maximal at 1000 ppm), remaining below control, with persistently high intercellular CO2 indicating non-stomatal (mesophyll) limitation. Proline was a stress-severity marker, peaking at the least-effective dose and correlating negatively with RWC (r = −0.96, P < 0.05). Overall, S. hortensis-derived CDs act as concentration-dependent nano-biostimulants protecting maize by restoring oxidative balance and water status, highlighting phenolic-rich medicinal plants as a sustainable CD precursor class for crop stress management.