Alleviating lanthanum stress in tomato plants using MnO nanoparticles and triacontanol: Impacts on growth, photosynthesis, and antioxidant defense

Abstract

The present study evaluated the synthesis, characterization, and ameliorative potential of manganese oxide nanoparticles (MnO NPs) against lanthanum (La)-induced stress in tomato plants. Biosynthesized MnO NPs exhibited a characteristic UV-Vis absorption peak at 276 nm and a cubic crystalline structure with an average crystallite size of 13 nm, as determined by XRD. TEM images confirmed pseudo-spherical morphology and homogenous distribution. Under La stress, tomato plants showed a significant reduction in shoot length (54.90%), root length (62.39%), shoot dry weight (49.71%), and root dry weight (37.17%). Application of MnO NPs and triacontanol (TRIA) mitigated these effects, with combined treatments enhanced shoot and root lengths by 155.81% and 216.66%, respectively, and dry weights by 116.58% (shoot) and 173.06% (root). La stressed plants demonstrated decreased accumulation of La in roots and shoots by about 36.64% and 32.21%, respectively, upon synergistic application of MnO NPs and TRIA. La stress decreased photosynthetic pigments, including chlorophyll a (53.56%), chlorophyll b (51.28%), total chlorophyll (53.10%), and carotenoids (26.36%). Combined MnO NPs and TRIA treatment significantly increased these pigments by 110.23%, 263.15%, 142.27%, and 266.66%, respectively. Photosynthetic efficiency parameters, such as net photosynthetic rate, stomatal conductance, and transpiration rate, also improved by up to 74.44%, 119.00%, and 89.44%, respectively, under combined treatments. Relative water content (RWC) decreased by 49.83% under La stress but increased by 84.75% following combined treatments. Osmolytes like proline and glycine betaine were elevated by 20.13% and 38.47%, respectively. Reactive oxygen species (ROS)-related markers, including H₂O₂, malondialdehyde, and electrolyte leakage, were significantly reduced by 58.14%, 28.46%, and 39.81%, respectively, with MnO NPs and TRIA. Antioxidant enzyme activities were enhanced, with combined treatments elevating SOD (27.02%), CAT (15.38%), APX (90.37%), and GR (90.38%). Moreover, activities of DHAR and MDHAR, previously suppressed by La, increased by 91.64% and 81.75%, respectively. The findings highlight the synergistic role of MnO NPs and TRIA in alleviating La toxicity by enhancing growth, photosynthetic efficiency, antioxidant defense, and reducing ROS, offering a sustainable approach for crop improvement under metal stress conditions.