Light intensity-mediated acclimation in Dendrobium denneanum: coordinated responses of growth, photosynthesis, leaf microstructure, and medicinal constituents

Introduction This study aimed to characterize light-acclimation responses of the endangered medicinal orchid Dendrobium denneanum and identify suitable light conditions for cultivation and ex situ conservation. Methods We examined growth, gas exchange, chlorophyll content, leaf microstructure, and medicinal constituent accumulation under three relative light intensities: 40%, 20%, and 8% of full natural light. Results The 20% treatment produced the greatest increments in plant height, crown width, basal stem diameter, leaf area, and leaf number, with particularly clear advantages in crown width, basal stem diameter, and leaf area. It also resulted in the highest daily average net photosynthetic rate, maximum net photosynthetic rate, and light saturation point, indicating that this light level was most favorable for vegetative growth and photosynthetic carbon assimilation under the tested cultivated conditions. Plants under 40% light showed lower photosynthetic performance than those under 20% light but higher performance than those under 8% light. This treatment was associated with the highest apparent quantum yield, thicker leaves and mesophyll, and greater stomatal density, width, and area, suggesting structural adjustment to relatively higher irradiance. Under 8% light, chlorophyll a, chlorophyll b, and total chlorophyll contents were highest, indicating shade acclimation, whereas growth increments, daily photosynthesis, maximum photosynthetic capacity, and light saturation point were reduced. Medicinal constituents showed contrasting responses to light intensity. Polysaccharide content increased with increasing light intensity and was highest under 40% light, whereas alcohol-soluble extract content decreased with increasing light intensity and was highest under 8% light. Discussion Therefore, the suitable light condition for D. denneanum depends on the cultivation objective: 20% relative light intensity favors vegetative growth and photosynthetic performance, 40% favors polysaccharide accumulation, and 8% favors alcohol-soluble extract accumulation. These results provide objective-specific guidance for sustainable cultivation and ex situ conservation of D. denneanum .