Response of Colored Poplar ‘Xuanyang2’ to Shading and Purification by Tissue Culture

‘Xuanyang2’ ( populus×euramericana cv. “ Neva ” ‘Xuanyang2’) is a newly released chimera cultivar whose variegated foliage is highly ornamental. Deciphering how its leaf coloration responds to low light is essential for maximizing landscape value. We subjected plants to full sun or 30%, 50% or 80% shade and combined gas-exchange, chloroplast ultrastructure, RNA-seq and tissue-culture analyses. Chlorophyll peaked under 50% shade, where P n and F v / F m exceeded full-sun values by 28.4% and 5.1%, respectively; 80% shade elevated MDA by 41.97% and triggered re-greening of yellow sectors. TEM revealed that chloroplast envelopes in yellow tissue were hypersensitive to weak light and possessed sparsely stacked grana lamellae. Transcriptome profiling identified 7 542 differentially expressed genes; gene–gene network modeling pinpointed four master transcription-factor families—C2H2, MBF1, C2C2-CO-like and HMG—as low-light regulators. Weighted gene co-expression analysis, integrated with physiological data, yielded four modules tightly correlated with P n , SS, L * and b * , with bZIP, C2H2 and MYB serving as central hubs for chloroplast development and pigment biosynthesis. Serial subculture progressively eliminated the chimera, yielding genetically stable green shoots and confirming that variegation is epigenetic rather than genetic. These results illuminate the molecular basis of shade tolerance in colored-leaf poplars and provide a foundation for breeding elite, low-light adapted cultivars.