Influence of TiO₂ Nanoparticles on the Photosynthetic Apparatus and Antioxidant System of Brassica napus L. Subjected to Drought

Drought is a critical abiotic stressor that severely limits global crop growth and productivity, including that of rapeseed (Brassica napus L.). Titanium dioxide nanoparticles (TiO₂ NPs) have emerged as promising nano-biostimulants for mitigating environmental stress in plant systems. This study evaluated the physiological, biophysical, and biochemical potential of TiO₂ NPs to alleviate the adverse impacts of drought stress on B. napus. Under controlled climate chamber conditions, rapeseed plants were subjected to water deficit following a pre-treatment soil drenching of TiO₂ NPs at concentrations of 0, 50, and 100 L^-1. Endogenous hydrogen peroxide (H₂O₂) accumulation in leaf tissues was quantified electrochemically using a novel nanostructured copper oxide (CuO) electrode, while photosystem II (PSII) integrity was monitored via in vivo chlorophyll a fluorescence kinetics.

The results demonstrated that drought stress induced acute photoinhibition in untreated plants (0 mg L^-1 TiO₂), as evidenced by a severe 17.89% drop in the maximum quantum yield of PSII photochemistry (Fv/Fm) upon light exposure. Conversely, soil pre-treatment with 50 mg L^-1 TiO₂ NPs effectively stabilized the primary quinone acceptor (Q_A) pool, limiting the light-induced Fv/Fm decline to just 5.12%. This photoprotective effect was closely associated with a significant reduction in endogenous H₂O₂ accumulation, indicating enhanced reactive oxygen species (ROS) scavenging capacity. Furthermore, the 50 mg L^-1 treatment successfully preserved photosynthetic pigment homeostasis (chlorophyll a, chlorophyll b, and carotenoids) and promoted root development under drought conditions.

In contrast, the beneficial biostimulatory effects were strictly concentration-dependent; the higher dosage (100 mg L^-1) exhibited diminished protective efficacy and induced visible signs of phytotoxicity, including pigment down-regulation. These findings indicate that low-concentration soil applications of TiO₂ NPs (50 mg L^-1) represent a viable agroecological strategy to enhance the climate resilience of B. napus. However, precise calibration of the nanoparticle dosage remains a critical prerequisite for practical field applications.