Biogenic Carbon Quantum Dots as Spectral-Converting Nano-Biostimulants: Coupling Photosynthetic Efficiency with Sustainable Productivity in Ocimum basilicum

This study elucidates a sustainable nanobiotechnological intervention using Carbon Quantum Dots (CQDs) synthesized from the red macroalga Gracilaria corticata via a facile one-step hydrothermal carbonization method. The synthesized CQDs, optimized using response surface methodology, exhibited a mean particle size of 2.65 ± 0.6 nm and distinct optical properties, characterized by strong UV absorption (∼283 nm) and blue-violet fluorescence (∼425 nm). This Stokes shift enables the CQDs to function as light-harvesting nanoantennae, converting photosynthetically inactive UV radiation into useful visible light. In a controlled hydroponic trial with basil ( Ocimum basilicum ), the biological impact of these nanomaterials was evaluated across a concentration gradient (0–75 mg.L -1 ) via foliar and root application pathways. Foliar application at 50 mg L -1 emerged as the optimal biostimulation strategy, eliciting a profound 44.65% increase in net photosynthetic rate (A) and a ∼30% gain in total biomass compared to the controls. Physiological profiling revealed that this enhancement was driven by a synergistic mechanism involving accelerated electron transport, upregulated chlorophyll biosynthesis, and optimized stomatal conductance, without compromising water use efficiency. Conversely, high-concentration root applications (>50 mg.L -1 ) induced non-stomatal limitations to photosynthesis, highlighting the critical importance of the application interface. These findings establish red algae-derived CQDs as potent, multifunctional agents capable of simultaneously expanding the light-harvesting capacity of crops and modulating metabolic flows, offering a scalable, green solution for high-value industrial crop production.