Differences in Salinity Stress Responses Across Developmental Stages and Tissue Regions in Saccharina japonica

Saccharina japonica is an economically important stenohaline brown seaweed whose growth and yield are significantly affected by frequent salinity fluctuations in coastal aquaculture areas. The differences in salt tolerance and response characteristics among developmental stages and among tissue regions of adult-stage thalli remain unclear, and the dynamic temporal patterns of responses across stages and tissues have not been systematically elucidated. In this study, we compared the physiological responses of juvenile and adult-stage thalli under varying salinity conditions and further analyzed the responses of the basal, middle, and tip regions of adult-stage thalli to define stage- and tissue-specific patterns of salt tolerance. The results indicate that low-salinity stress caused more severe injury than high-salinity stress, as reflected by sustained decreases in Fv/Fm, increased accumulation of MDA, and aggravated tissue decay with green-rot symptoms. Juvenile sporophytes exhibited higher salt tolerance than adult-stage thalli, and within the latter, tolerance differed markedly among tissue regions, with the basal region showing greater tolerance than the middle and tip regions. The basal region maintained higher photosynthetic activity, lower lipid peroxidation levels, and more stable antioxidant and osmotic regulatory responses under stress, whereas the tip region experienced early photosynthetic inactivation and irreversible damage. qRT-PCR results showed that antioxidant- and osmotic-regulation-related genes, including SjGSH, SjGST, SjPro, SjSOD, and SjPOD, were differentially expressed under salinity treatments at 24 h and 72 h, and their expression dynamics were generally consistent with the changes in physiological indicators. Overall, this study demonstrates that the response of S. japonica to salinity stress exhibits clear developmental stage-dependent differences and tissue-specific characteristics. In adult-stage thalli, the tip region may serve as a sensitive monitoring region for low-salinity damage, the middle region may serve as a transitional region for evaluating the progression of stress-induced damage, and the basal region may be an important region for maintaining thallus growth and physiological homeostasis. This study also provides experimental evidence for low-salinity stress risk assessment, the management of key growth stages, the monitoring of sensitive tissues, and the evaluation of salt tolerance traits during S. japonica aquaculture.