Spirulina-Derived Carbon Dots Promote Context-Dependent Effects on Rice Metabolism, Yield, and Grain Quality Under Non-Stress and Heat Stress Conditions

Carbon dots (CDs) have emerged as promising nanobioinputs capable of modulating plant metabolism and stress responses. However, their effectiveness under field conditions remains poorly understood. This study evaluated the effects of Spirulina-derived carbon dots (S-CDs) on rice metabolic, agronomic, and grain-quality responses under non-stress and heat stress conditions. Two independent field experiments were conducted under ambient and heat stress conditions, the latter imposed using temporary greenhouse structures during the reproductive stage. S-CDs were applied by foliar spraying (0.2 mg mL−1) at key developmental stages. Their effects were assessed through metabolic, agronomic, and grain quality analyses. Under non-stress conditions, daily average temperatures ranged from 20 to 27 °C, while daily maxima ranged from 24 to 38 °C. Heat stress increased daily average temperature by 3.9 °C and daily maximum temperature by 12.5 °C, with temperature peaks frequently exceeding 45 °C for several hours. Under non-stress conditions, S-CDs induced modest changes in antioxidant, carbon, and nitrogen metabolism, but without consistent improvements in grain yield or yield components. Under heat stress, however, S-CD application reduced spikelet sterility and increased grain yield despite limited changes in the metabolic variables evaluated. Grain quality and nutritional composition also responded differently depending on the environmental condition, indicating context-dependent effects. S-CDs showed limited agronomic relevance under favorable field conditions, but contributed to yield stability under heat stress. These findings support the potential of S-CDs as complementary nanobiostimulants to improve rice resilience under climate-related stress conditions.