Influence of Nanoparticles on Morpho-Physiological, Growth and Yield Traits of Rice (Oryza sativa L.) Cultivars Under Early Seedling Cold Stress at Different Developmental Stages

Cold stress (CS) severely limits the growth and productivity of rice (Oryza sativa L.), particularly in temperate regions where abrupt temperature declines frequently occur during early developmental stages. In recent years, nanoparticle (NP) application has emerged as a promising approach for alleviating CS; however, systematic comparisons of different NPs across multiple growth stages remain unclear. This study evaluated the effectiveness and physiological mechanisms of four NPs (Fe2O3, ZnO, TiO2, and CeO2) in enhancing CS tolerance of rice seedlings using four cultivars with contrasting cold tolerance: two conventional cultivars (ZJZ-17, cold-sensitive; XZX-6, cold-tolerant) and two hybrid cultivars (LLY-7108, cold-tolerant; LLY-32, cold-sensitive). Seedlings were subjected to CS (14 °C day/10 °C night) for 5 days at three developmental stages (14, 21, and 28 days after emergence), followed by a 7-day recovery period under optimal conditions. CS markedly reduced plant height (34.8%), fresh weight (57.2%), dry weight (50.0%), and chlorophyll a and b contents (48%) following recovery. Foliar application of NPs significantly mitigated the adverse effects of CS, with Fe2O3 and ZnO showing the highest effectiveness. Fe2O3 treatment increased plant height, fresh weight, and dry weight by 25.6%, 43.5%, and 40.6%, respectively, relative to cold-stressed plants, while chlorophyll a and b contents increased by 41.6% and 42.2%. NPs application alleviated oxidative damage by reducing reactive oxygen species (up to 67.4%), malondialdehyde (up to 51.2%), and proline accumulation (up to 60.4%). Enhanced antioxidant defense was evidenced by increased activities of superoxide dismutase (66.6%), peroxidase (59.6%), and catalase (34.3%) under Fe2O3 treatment. Yield-related traits also showed significant recovery, with Fe2O3 increasing tiller number, spikelets per panicle, and grain yield per plant. The hybrid cultivar LLY-7108 consistently exhibited greater CS tolerance than conventional cultivars, while the cold-sensitive cultivar ZJZ-17 showed the greatest susceptibility. CS imposed at later growth stages (28-day-old seedlings) caused less damage and allowed greater recovery than early-stage stress (14-day-old seedlings). Overall, NP-mediated enhancement of photosynthesis and antioxidant capacity significantly improves CS tolerance and yield performance in rice, with Fe2O3 NPs emerging as a promising strategy for mitigating CS. These findings provide practical insights for rice cultivation in regions prone to chilling events and contribute to the development of nanoparticle-based approaches for rice production under climate stress.