Effects of Shade Treatments on Growth, Photosynthetic Performance, Leaf Microstructure, and Pigment Accumulation in Horsfieldia hainanensis Seedlings

In this study, three-year-old seed-derived seedlings of Horsfieldia hainanensis Merr. were subjected to 0%, 25%, 50%, and 75% shade treatments in a nursery shade-net experiment from October 2025 to June 2026. Each treatment included three replicates, with three seedlings per replicate. The results showed that the light environment significantly affected growth. Seedlings under 0% shade showed severe wilting and mortality; therefore, this treatment was treated as a mortality outcome and excluded from subsequent statistical comparisons, which were conducted among the 25%, 50%, and 75% shade treatments. Within this range, plant height, ground diameter, crown width, leaf traits, and above- and belowground biomass increased with shade level, with the highest values observed under 75% shade. The 75% shade treatment was associated with higher maximum net photosynthetic rate (Pmax), apparent quantum yield (AQY), initial carboxylation efficiency (α), and CO2-saturated net photosynthetic rate (Amax), together with lower light compensation point (LCP) and carbon dioxide compensation point (CDCP), indicating improved light- and CO2-response performance within the tested shade range. Leaf thickness, epidermal thickness, and palisade and spongy parenchyma thicknesses increased with shade level, whereas variation in stomatal traits was mainly reflected in stomatal density. Chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids increased under stronger shading, showing clear variation in pigment accumulation along the shade gradient. Growth, biomass, photosynthetic capacity, leaf anatomical traits, and pigment contents were positively correlated, whereas the light saturation point (LSP), LCP, carbon dioxide saturation point (CDSP), and CDCP were generally negatively associated with these traits. Among the surviving treatments tested, the 75% shade treatment produced the best seedling performance and may be suitable for nursery cultivation, although further validation at different sites and in different seasons is needed. These findings provide practical guidance for artificial propagation, seedling production, plantation establishment, and light-environment management of this species.