Cultivar-specific regulation of soybean stem elongation under red/far-red light: Insights from stepwise regression and cross-validated modeling

Light quality, particularly the red to far-red ratio (R/FR), critically regulates stem elongation in soybean, yet the integrated roles of hormonal balance, cell wall dynamics, and light signaling genes remain poorly understood. Using stepwise regression combined with Ordinary Least Squares, Generalized Linear Models, partial regression plot and Leave-One-Out Cross-Validation, we identified key determinants of stem elongation in two contrasting soybean cultivars: Henong 60 (HN60, dwarf and lodging-resistant) and Heinong 48 (HN48, tall and lodging-susceptible). Under controlled R/FR conditions, we characterized phenotypic traits, phytohormone profiles, cell wall structural components, non-structural carbohydrates, and gene expression. Supplemental red light under shade inhibited stem elongation, increased structural components and soluble sugars, while starch accumulation was cultivar‑specific (observed only in HN60 under certain conditions). Far-red light promoted elongation and generally reduced soluble sugar levels. Hormonal ratios, particularly IAA/JA, IAA/SLs, and GA 3 /JA, were strongly correlated with plant height. Stepwise regression identified IAA/JA as the primary regulator in HN60, while IAA/SLs, GA 3 /JA, and cellulose content co-regulated plant height in HN48. Red light up-regulated PHYB and HY5 but suppressed PIF4A/B , whereas far-red light enhanced PHYA and PIF4A/B . This study establishes a cultivar-specific regulatory framework for R/FR-mediated stem elongation and provides potential targets for soybean plant architecture improvement.