Effects of Abscisic Acid on the Physiological and Biochemical Responses of Saccharina japonica Under High-Temperature Stress

Saccharina japonica is one of the most productive aquatic plants in the world, widely used in food, feed, medicine, and other industries. Predominantly inhabiting temperate marine environments in mid- to high-latitude regions of the Northern Hemisphere, the growth of S. japonica is significantly limited by high-temperature stress. Abscisic acid (ABA) plays an important role in plant growth and development and stress responses. However, the role of ABA on high-temperature stress tolerance in S. japonica still needs to be further elucidated. Here, we found that exogenous ABA significantly alleviated disease and decay in S. japonica under high-temperature stress while also increasing the relative growth rate, chlorophyll fluorescence parameters, photosynthetic pigment, and osmotic substance content. Meanwhile, exogenous ABA enhanced the activity of protective enzymes and up-regulated the transcript levels of antioxidant-related genes, thereby reducing oxidative damage. Most importantly, we observed a significant increase in ABA content and the transcript levels of key genes involved in ABA synthesis in S. japonica under high-temperature stress, which were further amplified by the addition of exogenous ABA. In conclusion, this study provides evidence that ABA can moderate the detrimental effects of high-temperature stress and provides a theoretical basis for the screening of S. japonica germplasm resources and the cultivation of new stress-resistant varieties.