Lignin macromolecular structural stability drives sapwood-heartwood color homogeneity in Populus deltoides

Radial xylem maturation in woody plants typically involves programmed cell death (PCD) and accumulation of colored secondary metabolites, leading to distinct color convergent between sapwood and heartwood. In fast-growing P. deltoides, however, the xylem exhibits remarkable color convergence along the radial gradient. Through the integration of anatomical observations, chemical analyses, and advanced spectroscopy (FTIR, Raman, 2D-HSQC NMR), color convergence was analyzed from two coordinated regulatory features: (1) a non-condensed lignin structure characterized with an elevated syringyl/guaiacyl (S/G) ratio and retention of β-O-4 linkages; and (2) a "metabolically silent" PCD during inner xylem maturation, in which parenchyma cells undergo death without any detectable accumulation of colored deposits in vessels or lumina. This suggested that pigment biosynthesis pathways were not activated. This coordinated regulatory pattern is consistent with a putative carbon allocation strategy that may prioritize growth over defense, potentially directing resources to structural polymers (S-rich non-condensed lignin) rather than defensive pigments. Our findings reveal a developmental adaptation in fast-growing trees that optimizes secondary growth efficiency, and provide a new perspective on the regulation of secondary metabolism and cell wall assembly during heartwood formation.