Physiological Mechanisms of Nano-CeO2 and Nano-TiO2 as Seed-Priming Agents in Enhancing Drought Tolerance of Barley Seedlings

Nanotechnology holds great promise for alleviating drought stress in crops. This study elucidates and compares the distinct physiological mechanisms by which two nanomaterials, nano-cerium oxide (CeO2) and nano-titanium dioxide (TiO2), function as seed-priming agents to enhance drought tolerance in barley. A comprehensive analysis encompassing germination performance, hormonal dynamics, starch metabolism, osmotic adjustment, photosynthetic pigments, and the antioxidant system revealed that each nanomaterial operates through a unique pathway. Specifically, priming with 150 mg·L−1 nano-CeO2 (CP-150) primarily promoted root development and stress resilience. This effect was achieved by persistently reducing abscisic acid (ABA) levels, elevating gibberellin (GA3), enhancing amylase activity to mobilize seed reserves, and increasing soluble protein accumulation in roots. In contrast, priming with 500 mg·L−1 nano-TiO2 (TP-500) was more effective in enhancing shoot physiology and adaptive capacity by rapidly inducing auxin (IAA), robustly stimulating the antioxidant enzyme system, and increasing photosynthetic pigment content. The temporally and spatially complementary actions of these nanomaterials, with nano-CeO2 fostering root-based resilience and nano-TiO2 boosting shoot-level functions, synergistically support seed germination and seedling establishment under drought conditions. This study provides a mechanistic foundation for designing targeted nano-priming strategies to improve crop drought resistance.