Shift from proactive to defensive response in seashore paspalum under escalating light intensity: Modulation by carbon-nitrogen relations

As a widely cultivated perennial turfgrass, seashore paspalum ( Paspalum vaginatum Sw.) exhibits plasticity in biomass allocation and stress tolerance phenotypes, with growth-resistance trade-offs critically influenced by light intensity. In this study, four light intensity treatments of 100, 300, 500, and 700 μmol·m −2 ·s −1 (L100, L300, L500, and L700, respectively) were applied to investigate the mechanisms underlying turf responses to increasing light intensity. Under L500, starch, cellulose, hemicellulose, leaf structural traits, leaf dry weight, ascorbate peroxidase (APX) and peroxidase (POD) activities reached their highest levels, achieving a synergistic enhancement of growth and resistance. In contrast, soluble sugars, malondialdehyde (MDA), root structural traits, and root dry weight peaked under L700. L700 also significantly decreased leaf nitrogen (N) concentration by 52.2 % and increased the carbon-to-nitrogen (C/N) ratio by 51.5 % compared with L100. Principal component analysis identified growth components (turf quality, dry matter accumulation, and morphological structure) and resistance components (antioxidant capacity: APX, CAT, POD, SOD and MDA; leaf anatomical structure: vascular bundle, leaf and xylem thickness). Optimal light treatments for growth and resistance were L700 and L500, respectively. Correlation and structural equation modeling (SEM) revealed that N concentration was negatively correlated with starch, cellulose, SOD, and MDA, while C/N ratio was also negatively correlated with these traits. Further analysis identified three pathways through which light intensity mediates growth-resistance trade-offs: photosynthetic capacity (net photosynthetic rate, transpiration rate, stomatal conductance), carbohydrate metabolism (starch, soluble sugars, cellulose), and carbon–nitrogen relationship (N concentration and C/N ratio).Escalating light intensity enhanced resistance via photosynthesis, carbohydrate metabolism, and C/N regulation, whereas the carbon–nitrogen relationship could constrain growth. Overall, moderate light intensity (L500) maximized photosynthetic capacity and carbohydrate metabolism, achieving synergistic improvement of growth and resistance, while high light intensity (L700) disrupted C/N homeostasis, leading to reduced stress tolerance. These findings provide a theoretical basis for optimizing light management in seashore paspalum turf. • The response of turf to escalating light intensity shifts from proactive to defensive. • The principal components of growth and resistance are quantified by PCA. • SEM reveals carbon and nitrogen relation dominates growth-resistance regulation. • L500 was the optimal light intensity for synergistic growth and resistance.