Planting density regulates stem structural reinforcement through coordinated morphological, biochemical and anatomical responses in maize

Context Increasing planting density is widely used to maximize maize ( Zea mays L .) productivity; however, excessive crowding often weakens stem structure and increases lodging susceptibility, thereby reducing yield stability under intensive production systems. Objective This study aimed to determine the effects of increasing planting density on maize stem development, structural characteristics, and associated physiological and metabolic processes, with the goal of elucidating the mechanisms underlaying density-dependent changes in stem integrity and productivity. Methods A two-year field experiment was conducted to evaluate the effects of four planting densities (3, 6, 9, and 12 plants m⁻²) on morphological, anatomical, physiological, and biochemical characteristics of the third basal internode at different developmental stages in maize. Results Increasing planting density promoted internode elongation while restricting radial stem development, resulting in thinner and mechanically weaker stems. High-density treatments also altered stem anatomical organization by decreasing vascular bundle number, cortex thickness, and sclerenchyma thickness, indicating weakened structural support under crowded conditions. Under 12 plants m⁻², vascular bundle development and supporting tissue thickness were markedly reduced compared with lower planting densities. In addition, lignin, cellulose, and hemicellulose accumulation declined progressively with increasing density, indicating suppressed secondary cell wall formation and stem structural reinforcement. Soluble sugar accumulation declined markedly during reproductive development, indicating limited assimilate availability for stem structural development under intensified interplant competition. Activities of enzymes associated with lignin biosynthesis and oxidative defense were also reduced under high-density, further limiting lignin deposition and stem mechanical stability. Collectively, these coordinated morphological, anatomical, physiological and biochemical responses weakened stem structural stability under excessive planting density. In contrast, moderate density (6 plants m⁻²) maintained a more favorable balance between stem structural development and grain yield, whereas excessive density negatively affected yield-related traits. Conclusion The study demonstrated that excessive planting density compromises stem structural reinforcement through coordinated changes in stem morphology, anatomy, carbohydrate accumulation, and lignin biosynthesis-related enzyme activities, thereby increasing lodging risk in maize. Implications Optimizing moderate planting density improved stem structural integrity, enhanced yield stability, and reduced the potential risk of lodging, highlighting its importance for sustainable maize production.