Abstract: We examined the effects of alginate oligosaccharides (AOS) with 900 mg·L^-1 concentration under 0.3% NaCl salt stress on shoot growth, construction of root morphology, activities of antioxidases, antioxidant content, O₂^-_· production rate, osmotic adjustment substance and malondialdehyde (MDA) content in rice seedlings of salt sensitive cultivar IR29 and salt resistant cultivar FL478. The results showed that salt stress had negative impacts on the growth of seedlings of IR29 and FL478, and positive impacts on the O₂^-_· production rate and MDA contents in leaves. Under the stress conditions, foliar spraying AOS significantly promoted the growth of rice seedlings and increased stem diameter, leaf area of IR29 by 10.53% and 8.54%, respectively. Foliar spraying AOS significantly increased stem diameter, leaf area and shoot fresh weight of FL478 by 9.09%, 7.50% and 6.06%, respectively. Foliar spraying AOS significantly increased root length, root dry weight, root surface area, root volume and root tip number of IR29 by 18.98%, 23.33%, 18.29%, 14.15% and 15.15%, respectively; significantly increased root length and root dry weight of FL479 by 8.36% and 10.73%, respectively. Foliar spraying AOS significantly increased the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX) and phenylalanine ammonialyase (PAL) in leaves of IR29 by 10.79%, 21.31%, 22.21%, 23.66% and 15.39%, respectively; significantly increased the activities of ascorbate peroxidase (APX) of FL478 by 26.15%. Foliar spraying AOS significantly increased the content of ascorbic acid (ASA), glutathione (GSH), soluble protein and free proline (Pro) of IR29 by 33.88%, 13.51%, 5.54% and 20.52%; significantly increased the content of ascorbic acid (ASA), glutathione (GSH) and soluble protein of FL478 by 25.16%, 14.86% and 2.69%. Foliar spraying AOS significantly decreased MDA content of IR29 and FL478 by 8.14% and 17.17%, respectively. O₂^-_· production rate in the leaves of IR29 and FL478 reduced by 34.68% and 29.24%, respectively. Our results indicated that exogenous AOS spraying could enhance the resistance of rice seedlings to salt stress by improving the antioxidant and osmotic regulation capacity and removing a large amount of accumulated ROS to reduce membrane lipid peroxidation level.

Key words: alginate oligosaccharides, salt stress, rice seedling, stress resistance, physiological characteristics.