Astaxanthin application enhances salinity tolerance in rice seedlings by abating oxidative stress effects and enhancing Na+/K+ homeostatic balance

Since many cultivated plants, including rice, are susceptible to stress and salt stress, resulting in a tremendous reduction in yield, threatens food security worldwide. Strategies such as using biostimulants to ameliorate salt stress can reduce stress effects and sustain production. The effects of soaking Koshihikari (salt-sensitive) seeds in astaxanthin (AS) under salt stress were determined in the present study. In particular, the seeds of the rice cultivar were subjected to control, salt stress (50 mM NaCl), AS (50 µM), and AS + salt stress treatments for two weeks in hydroponic culture. Thereafter, the plants were harvested, and their growth, physiological, and molecular parameters were analyzed. The results showed that the growth of plants under salt stress was significantly reduced; however, the growth was restored to levels comparable to those of non-stressed plants treated with AS. Salt stress significantly increased the concentrations of malondialdehyde, hydrogen peroxide, and the electrolyte leakage ratio in untreated plants and significantly decreased their concentration in AS-treated plants under the same conditions, with corresponding increases in leaf catalase, peroxidase, and ascorbate peroxidase activities. Leaf Na⁺ concentration markedly increased under salt stress in non-treated plants, and AS treatment reduced the concentration. However, the difference was not statistically significant, which resulted in a significant decrease in the Na⁺/K⁺ ratio in AS-treated plants compared to that in non-treated plants. Salt stress and AS treatment did not alter the concentration of photosynthetic pigments but enhanced the expression of OsBHY, OsNHX1, OsSOS1, and OsHKT1;5 genes. Overall, soaking seeds in AS induced salt stress tolerance in the Koshihikari rice cultivar by reducing oxidative stress damage and enhancing shoot Na⁺/K⁺ balance. Therefore, seed-soaking methods using AS could serve as a good strategy for improving the cultivation of salt-sensitive rice cultivars in saline soils.