Responses of growth, photosynthesis, leaf microstructural properties, and bioactive compounds of Dendrobium fimbriatum to different light intensities

Light availability is a key ecological factor determining the growth and physiological acclimation of epiphytic orchids. To evaluate how Dendrobium fimbriatum responds to heterogeneous light environments, plants were cultivated under three relative light intensities (40%, 20%, and 8% of full sunlight). Growth traits, diurnal gas exchange, light-response parameters, chlorophyll content, leaf microstructural characteristics, and bioactive compounds were measured after five months of treatment.

Plants grown under 20% light showed the best overall vegetative performance. Compared with 40% light, 20% light increased basal stem diameter and single-leaf area by 41.7% and 150.4%, respectively, and increased daily photosynthetic average (DPA) and maximum net photosynthetic rate (P_max) by 17.3% and 66.6%, respectively. Compared with 8% light, DPA and P_max under 20% light increased by 211.1% and 186.9%, respectively. In contrast, plants under 40% light exhibited the highest stomatal density and dark respiration rate (R_d), with R_d being 57.5% and 158.8% higher than that under 20% and 8% light, respectively. Plants under 8% light showed the highest total chlorophyll content and the lowest chlorophyll a/b ratio, which was 9.3% lower than that under 40% light and 13.6% lower than that under 20% light. Polysaccharide and alcohol-soluble extract contents were highest under 40% light, exceeding those under 8% light by 38.7% and 25.4%, respectively.

D. fimbriatum manages environmental heterogeneity through highly coordinated trait adjustments rather than isolated responses. The 20% light optimized the balance between carbon gain and metabolic maintenance, maximizing vegetative performance. At 40% light, structural investment and secondary metabolite accumulation were enhanced, whereas 8% light promoted chlorophyll accumulation and shade-acclimation traits at the expense of maximal growth. These quantified trade-offs provide an empirical foundation for understanding the ecological resilience of epiphytic orchids and optimizing cultivation protocols.