Effects of light intensity on growth and leaf functional trait plasticity of Paris polyphylla var. yunnanensis

To elucidate the light adaptation mechanism and optimal cultivation light regime for Paris polyphylla var. yunnanensis, a two-year continuous field positioning trial was conducted with five light gradients (20%, 40%, 60%, 80%, and 100% natural light, PAR = 300-2200 μmol·m^-2·s^-1). We systematically investigated the regulatory effects of light intensity on leaf phenotypic traits, anatomical and physiological functions, photosynthetic performance, vegetative growth and rhizome biomass accumulation. Significant differences in plant growth, leaf morphology and all functional trait indices were detected among the treatments (p < 0.05). The plants cultivated under 40%-60% natural light (PAR = 700-1400 μmol·m^-2·s^-1) exhibited balanced leaf anatomy, maximum photosynthetic efficiency and the highest rhizome fresh biomass. Compared with CK (100% natural light), plants under 40%-60% natural light showed 164.8% greater plant height, 35.5% thicker stem diameter, 50.0% larger rhizome diameter and 124.4% higher rhizome fresh weight. In contrast, 20% natural light induced excessive elongation, slender stems and underdeveloped mechanical tissues, with rhizome fresh weight, length and diameter decreasing by 52.2%, 61.4%, and 62.1%, respectively, relative to CK. Treatments of 80%-100% natural light triggered severe high-light stress, causing leaf scorch, chlorosis and suppressed stomatal development, with most assimilates allocated to antioxidant defense and thermal dissipation rather than rhizome carbon partitioning. In conclusion, P. polyphylla var. yunnanensis exhibits a clear light adaptation threshold with distinct gradient-specific responses: 80%-100% natural light induces photoinhibition and suppresses stomatal development; 20% natural light causes excessive elongation, slender stems and reduced rhizome biomass; while 40%-60% natural light optimizes leaf morphology, photosynthetic physiology and biomass allocation, resulting in the highest rhizome fresh weight (63.54-64.4  g) and overall growth performance. The 40%-60% natural light regime (PAR 700-1400 μmol·m^-2·s^-1) coordinates leaf morphogenesis, photosynthetic carbon fixation and dry matter accumulation, and is therefore identified as the optimal light condition for high-yield and high-quality medicinal production. This study provides quantitative light regulation parameters (PAR thresholds and corresponding growth responses) for understory bionic cultivation and precise light environment management in facility cultivation of this medicinal herb.