Responses of Hydrangea macrophylla In Vitro Plantlets to Different Light Intensities

Light intensity strongly influences the morphological development and photoprotective responses of in vitro plantlets, yet the optimal conditions for hydrangea remain undefined. This study investigated the effects of five light intensity gradients (TrA: 80–120 lux, TrB: 380–480 lux, TrC: 1500–1800 lux, TrD: 3800–4000 lux, TrE: 6000–6400 lux) on Hydrangea macrophylla ‘Qingtian’ plantlets. Plantlets exhibited optimal growth at TrB, showing maximal biomass, leaf expansion, chlorophyll content, and root activity, accompanied by low antioxidant enzyme activities and soluble sugar levels. Nutrient accumulation was greater under low light than under high light conditions. Transcriptome analysis of treatments (TrB and TrE) with marked phenotypic differences revealed 7119 differentially expressed genes (DEGs). Of these, 4582 genes were up-regulated and 2537 were down-regulated. The up-regulated genes were significantly enriched in pathways related to cell walls, the microtubule cytoskeleton, and developmental processes, which are involved in the plant growth and development process, such as photosynthesis, nutrient ion transport and regulation, as well as plant hormone responses and transport; whereas the down-regulated genes were significantly enriched in pathways related to carbohydrate metabolism, oxidoreductase activity, and glutathione metabolism, suggesting that high light stress impairs growth by disrupting carbon and antioxidant processes. These results demonstrated that 380–480 lux is the optimal light intensity for ‘Qingtian’ Hydrangea macrophylla in vitro plantlets. This study provides a foundation for optimizing culture conditions and offers new insights into the molecular regulation of light-responsive genes.