Physiological Responses and Adaptive Strategies of Codonopsis pilosula to Gradient Light Stress in a Forest Understory

Light availability is a primary factor limiting the growth of understory plants, yet how they systemically acclimate to a gradient of shade conditions remains poorly understood. Here, we exposed Codonopsis pilosula (Franch.) Nannf. to four canopy densities (0, 50, 65, 80%) simulating a gradient of light stress, and measured growth, chlorophyll, soluble sugars, flavonoids, and the activities of superoxide dismutase (SOD) and peroxidase (POD). C. pilosula exhibited a non-linear physiological response pattern. Under moderate shade (50% canopy density), root development and biomass accumulation peaked; SOD and POD activities showed a positive correlation with biomass (r = 0.89, P < 0.01). However, no significant treatment effects were detected on the activities of these enzymes, and the biological significance of this correlation remains unclear. Under severe shade (80% canopy density), chlorophyll, soluble sugars, and flavonoids accumulated to their highest levels, while photosynthetic capacity declined, revealing a tolerance strategy with resource allocation shifted from growth to defense. At 65% canopy density, root development was significantly suppressed and overall performance was lowest, suggesting a potential physiological threshold where compensatory mechanisms fail before stress tolerance is fully activated. In summary, C. pilosula adopts a biphasic adaptive strategy: a compensatory growth strategy under moderate light limitation, and a “defense-priority” tolerance under severe limitation. These findings provide physiological insights into shade acclimation and resource allocation under gradient light stress.