Effects of AMPEP (<i>Ascophyllum</i> marine plant extract powder) and SJEP (enzymatic extracts of <i>Saccharina japonica</i>) on growth and resistance of red macroalga <i>Pyropia yezoensis</i>

Seaweed extracts are widely used as biostimulants in terrestrial crops but remain underexplored for economic macroalgae. This study investigated the effects of <i>Ascophyllum</i> marine plant extract powder (AMPEP) and <i>Saccharina japonica</i> extract powder (SJEP) on the growth and stress resistance of <i>Pyropia yezoensis</i>, a commercially significant red macroalga along the Northeast Asia coast. Through gradient analysis, optimal concentrations were determined to be 0.05 g L<sup>−1</sup> for SJEP and 0.1 g L<sup>−1</sup> for AMPEP, based on their effects on daily growth rate (DGR) and photosynthetic performance. Both extracts significantly improved resistance to red rot disease, reducing lesion area while increasing DGR, photosynthetic efficiency, and pigment content compared to the control. Additionally, SJEP and AMPEP elevated superoxide dismutase (SOD) activity and suppressed malondialdehyde (MDA) accumulation. Under cold stress, the extracts mitigated declines in DGR, photosynthetic parameters, pigment levels, and SOD activity, while restraining MDA elevation. Notably, the expression of cold-responsive genes (PyABCC1 and PyABCI1) peaked in SJEP treatment and remained elevated in AMPEP treatment, surpassing the control by several-fold. Overall, SJEP outperformed AMPEP, likely due to its higher polysaccharide and polyphenols. These results support brown algal extracts—particularly enzymatic extracts—as practical biostimulants to enhance growth, disease resistance, and cold tolerance in <i>P. yezoensis</i> cultivation.