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

<i>Saccharina japonica</i> 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 <i>S. japonica</i> 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 <i>S. japonica</i> still needs to be further elucidated. Here, we found that exogenous ABA significantly alleviated disease and decay in <i>S. japonica</i> 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 <i>S. japonica</i> 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 <i>S. japonica</i> germplasm resources and the cultivation of new stress-resistant varieties.