Abstract: Rhododendron micranthum Turcz. is a shrub esteemed for its ornamental and medicinal attributes within the Changbai Mountain range of China. We selected 3-year saplings and subjected them to four distinct light conditions: full light (CK), 70% light (L1), 50% light (L2), and 30% light (L3) to investigate variations in morphology, photosynthetic responses, stomatal ultrastructure as well as the mechanisms through which these saplings adapt to differing lighting environments. The results indicate that L2 leaves exhibit significantly greater length, width, and petiole development compared to other treatments across varying intensities. Over time, chlorophyll content and PSII levels in L2-treated saplings surpass those observed in other treatments; Proline (PRO), malondialdehyde (MDA), and soluble protein (SP) contents are markedly lower under L2 treatment. Catalase (CAT) and superoxide dismutase (SOD) demonstrate significant correlations across various light conditions but respond differently among treatments, indicating distinct species sensitivities to light intensity while both contribute to environmental stress resistance mechanisms. Findings reveal that R. micranthum saplings at 50% light intensity benefit from enhanced protection via antioxidant enzymes, and shading reduces osmotic adjustment substances yet increases chlorophyll content. Stomatal length/width along with conductance rates and net photosynthesis rates for L2 exceed those of CK, suggesting an improved photosynthetic structure conducive to efficient photosynthesis under this condition. Thus, moderate shading represents optimal growth at 50% illumination, a critical factor promoting sapling development. This research elucidates the ideal environment for R. micranthum adaptation to varying light conditions supporting future conservation initiatives.

Key words: Light intensity, Eco-physiological characteristics, Stomatal ultrastructure, Rhododendron micranthum Turcz.