Abstract
<jats:p>Background: Sorbus sibirica "Dong Hong" is a promising ornamental cultivar, but its optimal light conditions remain unclear. Aims: This study evaluated the effects of shading on seedling growth, physiology, root morphology, and leaf surface micromorphology. Methods: One-year-old seedlings were grown under full sunlight (CK) and 30%, 50%, or 70% shade for 100 days. Growth, biomass, root traits, chlorophyll, invertase, soluble protein, stomatal characteristics, and epicuticular wax morphology were determined. Results: Shading significantly affected all measured traits. The 30% shade treatment produced the greatest seedling height, which increased by 118.18% compared with CK, and the highest chlorophyll content, which increased by 120.21%. Soluble protein content was slightly increased, whereas invertase activity decreased under moderate shading. Although total biomass decreased by 18.63%, root development remained relatively stable under 30% shade, with slight increases in total root length and average root diameter. Stomatal density was highest under this treatment, and the epicuticular wax structure remained relatively regular. In contrast, 70% shade markedly inhibited biomass accumulation and root development. Conclusions: Moderate shading, particularly 30%, provided the most favorable light environment for Sorbus sibirica "Dong Hong" seedlings and is recommended for summer nursery cultivation in Northeast China.</jats:p>