Survival Wisdom of Tradescantia spathacea Sw: Trade-Off Between Anthocyanin-Mediated Photoprotection and Photosynthetic Capacity Under High-Light Stress

Light heterogeneity shapes plant survival, yet how shade-tolerant species balance photoprotection and photosynthesis under changing light remains unclear. Tradescantia spathacea Sw, with green adaxial and purple-red abaxial leaf surfaces, exhibits both shade tolerance and high-light acclimation, but the functional specialization of its leaf sides and the underlying trade-off mechanisms are poorly understood. This study exposed T. spathacea to 100% (FL) and 30% (LL) sunlight for 2 years and systematically assessed light adaptation mechanisms through morphological observation, physiological and biochemical detection, and transcriptomic analyses. Results showed that under FL conditions, leaf morphology underwent adaptive remodeling. Anthocyanins accumulated specifically on the abaxial side, thermal dissipation and antioxidant systems were fully activated, while photosynthetic capacity was markedly inhibited, reflecting a survival-priority strategy. Under LL conditions, leaves became broader and flatter, chlorophyll content increased, and the expression of photosynthesis-related genes was upregulated. This enhanced light capture and carbon assimilation efficiency, leading to significantly greater biomass accumulation, which reflected a growth-oriented resource allocation strategy. Transcriptomic and physiological data consistently showed that FL upregulated anthocyanin biosynthetic genes, while LL activated photosystem and photosynthetic electron transport genes. T. spathacea achieves a flexible photoadaptation strategy across different light environments through leaf surface functional specialization and an anthocyanin-mediated trade-off between photoprotection and photosynthesis.