Exploring the Mechanisms of Hydrangea macrophylla Adapting to Low Light-Induced Ornamental Whitening Through Physiological, Transcriptional, and Metabolomic Analyses

Background/Objectives: To explore the mechanisms of Hydrangea macrophylla adapting to low light-induced ornamental whitening, this study established treatments involving normal light (CK, 200 μmol·m−2·s−1), moderate low light (L1, 100 μmol·m−2·s−1), and severe low light (L2, 20 μmol·m−2·s−1). Methods: Meanwhile, physiological indicators, including growth, photosynthesis, and antioxidant activity, were assessed, alongside transcriptomic and metabolomic analyses. Results: Results indicate that L1 increased the proportion of leaf whitening area while maintaining plant growth (crown width, biomass), photosynthetic efficiency comparable to CK, and superior to L2. Concurrently, L1 activated a coordinated antioxidant defence system, namely by increasing the activity of key enzymes (e.g., SOD, GR) and the accumulation of protective metabolites (e.g., soluble proteins, total phenolics and total flavonoids), thereby minimising oxidative damage (low MDA). Multi-omics analyses revealed that L1 specifically activated these networks associated with carbon assimilation, energy metabolism, secondary metabolite synthesis, and hormone signalling, indicating a systemic molecular mechanism towards enhanced defence. Conclusions: In summary, moderate low light triggers a synergistic molecular network involving enhanced antioxidant defences and secondary metabolism, enabling H. macrophylla to maintain overall physiological homeostasis and healthy growth while exhibiting ornamental whitening phenotypes, thereby revealing a unique aesthetic adaptation mechanism to environmental stress.