Undaria pinnatifida is a major cultivated seaweed in South Korea, yet recent winter warming has threatened productivity and seedling survival. This study evaluated the effects of a seaweed-derived biostimulant, Sargassum horneri extract (SHE), on the heat tolerance and antioxidant responses of U. pinnatifida. Male and female gametophytes (0.05 g each) were pretreated with SHE at 0, 0.05, and 0.5 mL L⁻¹ for 10 days at 15 °C, crossed into five combinations, and cultured under 15 °C and 25 °C. Specific growth rate (SGR), total protein, pigments, oxidative stress indicators (ROS, H₂O₂, LPO), and antioxidant enzyme activities (SOD, APX, CAT, GR) were analyzed. High temperature significantly reduced SGR and protein and increased oxidative stress (ROS, LPO, H₂O₂) and antioxidant enzyme activities (SOD, APX, CAT).
SHE treatments enhanced SGR and protein accumulation, particularly in female-treated groups at 25 °C, and alleviated pigment loss and oxidative stress (Figure 1). ROS, LPO, and H₂O₂ were significantly reduced by SHE, most notably in high-concentration female treatments (Figure 2). At 25 °C, SOD, APX, and CAT activities were lower in all SHE-treated groups than in the control (Figure 3), while GR remained unchanged, suggesting that SHE mitigates oxidative damage mainly through non-enzymatic suppression of ROS generation rather than by upregulating enzymatic antioxidant activity. Sex-specific responses were evident: male-treated groups showed stronger effects on growth, whereas female-treated groups exhibited greater protein accumulation and stronger reductions in ROS and LPO. These findings demonstrate that SHE pretreatment of gametophytes enhances the heat tolerance of U. pinnatifida sporophytes, providing a promising strategy to improve the resilience and sustainability of seaweed aquaculture under climate warming.