Interactive Effects of Shading Intensity and Container Type on Growth, Photosynthesis, and the Carbon–Antioxidant Trade-Off in Dalbergia odorifera Seedlings

Dalbergia odorifera T. Chen is a rare and endangered tree species endemic to China. However, efficient cultivation protocols for high-quality seedling production remain to be optimized. This study investigated the interactive effects of shading intensity and container type on seedling growth, photosynthesis, antioxidant systems, and resource allocation. A two-factor completely randomized design was used with five shading levels (0%, 20%, 40%, 60%, and 80%) and three container types (rigid plastic pots, non-woven fabric containers, and black plastic bags). Shading suppressed growth by limiting carbon assimilation, with total dry weight reduced by 93–94% under heavy shade, triggering a carbon–antioxidant trade-off. Moderate shading (40–60%) increased antioxidant enzyme activities, but 80% shading caused antioxidant dysfunction and membrane lipid peroxidation, with malondialdehyde (MDA) content increasing by 343%. Non-woven fabric containers improved root development and maintained higher photosynthetic efficiency under shade, whereas black plastic bags exacerbated stress. Container type significantly interacted with shading intensity: non-woven fabric containers performed best under 0–20% shading, while rigid plastic pots showed advantages under 60–80% shading. Principal component analysis identified full sunlight combined with non-woven fabric containers as the optimal cultivation model. These findings suggest that container type modulates light responses, potentially through changes in the rhizosphere environment, and offer practical guidance for precision cultivation of D. odorifera and other valuable tree species.