Condition-dependent growth and physiological responses of interspecific Ulva hybrids under combined temperature-related stresses.

Interspecific hybridization can generate novel physiological variation, but its consequences for stress tolerance in marine green macroalgae remain poorly understood. In this study, we investigated the growth and photosynthetic pigment responses of parental Ulva prolifera, parental Ulva linza, and their hybrid offspring under two types of combined environmental stress: temperature × salinity and temperature × desiccation. Parental strains, an F1 hybrid sporophyte, and two F1 meiotically derived gametophyte lines were exposed to factorial stress treatments for 12 days. Specific growth rate, final wet biomass, and chlorophyll a, chlorophyll b, and carotenoid contents were measured to evaluate growth performance and pigment maintenance. Hybrid offspring did not consistently outperform the parental strains across all treatments, indicating that hybridization did not produce a universal advantage. However, condition-dependent better performance was observed under specific stress combinations. Under temperature × salinity stress, the F1 hybrid sporophyte maintained relatively high biomass under some specific temperature-salinity combinations, whereas the F1-derived gametophyte lines retained higher biomass or pigment levels under certain high-temperature or high-salinity treatments. Under temperature × desiccation stress, the F1 hybrid sporophyte showed relatively stable pigment maintenance across temperature and water treatments, while the F1-derived gametophyte lines exhibited stronger strain-specific variation. These results suggest that interspecific hybridization between U. prolifera and U. linza can broaden physiological variation under fluctuating environmental conditions, but hybrid performance is strongly dependent on stress combinations, life-history stage, and genotype. This study provides new insight into the potential role of hybridization in shaping adaptive responses of green tide-associated Ulva taxa in dynamic coastal habitats.