Moderate shading improves photosynthetic growth traits and phenanthrene production in Oreorchis patens

Oreorchis patens is an endangered shade-adapted medicinal orchid whose pseudobulbs contain phenanthrenes with antitumor activity. Insufficient science-based light management has limited its artificial cultivation. We examined the effects of five sunlight transmittance (SLT) levels (3%, 23%, 46%, 74%, 90%) on photosynthetic traits, plant growth, and pseudobulb phenanthrene accumulation. Moderate shading at 46% SLT optimized net photosynthesis per leaf, pseudobulb biomass, root development, and leaf expansion. This treatment also yielded the highest per-unit-area phenanthrene yield, 93.5% higher than under 23% SLT. High light induced leaf chlorosis and premature senescence, whereas deep shade resulted in negative net photosynthesis; both extremes reduced survival rate. Our findings suggest that 46% SLT optimizes photosynthetic efficiency, biomass accumulation, and phenanthrene production, supporting light management for its cultivation.