Chitosan has been widely used in agriculture to reduce harmful effects on plants during unfavorable conditions and
promote plant growth. Nanotechnology is one of the most potent tools in modern agriculture for improving crop productivity. The study was conducted to (i) determine the salt tolerance of rice cultivars including ST24, ST25, and OM18, (ii) determine the effects of nanochitosan in reducing the harmful effects of salinity on rice cultivars, and (iii) analyze the influences of various nanochitosan concentrations on the photosynthetic activity and antioxidant enzymes of the three rice cultivars at the seedling stage. The results of screening the salt tolerance in a medium of NaCl (2‰, 4‰, 6‰, 8‰, 10‰) showed that salinity stress affected the development of the three rice cultivars, ST24, ST25, and OM18. The salt concentrations and rate reductions for plant height and root length were positively correlated. The results showed that all forms of chitosan nanoparticle treatment enhanced the photosynthesis of rice cultivars by increasing the chlorophyll a, chlorophyll b, and proline content. The addition of 0.5% nanochitosan in the 6‰ NaCl treatment increased the activity of antioxidant enzymes compared with the control treatment. The catalase activity reached the highest value in rice cultivar OM18, with 0.5% nanochitosan and an enzyme content of 0.049 U/min/g. The peroxidase activity was highest in rice cultivar ST24, with 0.5% nanochitosan (0.098 U/min/g). The ascorbate peroxidase activity was the highest in rice cultivar ST25, with 0.5% nanochitosan (0.084 U/min/g). Treatment with nanochitosan 0.5% enhanced the gene expression level of all tested enzymes. This suggests that the chitosan nanoparticle treatment promoted the self-protection mechanism by raising the antioxidant enzyme activity to reduce the amount of H2O2 and O2 – that accumulated in the leaf cells.