Intercropping enhances spatial photosynthetic efficiency through stomatal and anatomical adaptations in rapeseed–vetch systems

Contexts Enhancement of leaf photosynthetic capacity and complementary canopy light distribution arising from species diversity are fundamental for biomass accumulation in forage intercropping systems. However, the mechanisms by which intercropping alters the spatial distribution of leaf photosynthesis and biomass formation through changes in the light environment remain poorly understood. Objectives This study aimed to elucidate how light distribution and the plasticity of leaf morphological, physiological, and anatomical traits influence spatially distributed leaf photosynthesis and plant biomass in a rapeseed–hairy vetch intercropping system. Methods We used a five-year field experiment comprising sole–cropped rapeseed (SR), sole–cropped hairy vetch (SH), and rapeseed–hairy vetch intercropping (RH). Across different growing seasons, we measured canopy light distribution, leaf gas exchange, chlorophyll fluorescence, stomatal and mesophyll conductance ( g s and g m ), leaf anatomical and biochemical traits, and biomass to evaluate how intercropping regulates spatial photosynthesis and biomass formation. Results Intercropping strengthened the positive relationship between light intensity and net photosynthetic rate ( A ) in both species and improved the utilization of low-light conditions. In intercropped systems, g m played a more critical role than g s in determining A for both rapeseed and hairy vetch. Principal component analysis revealed that intercropping increased g m by enlarging the chloroplast surface area exposed to intercellular airspace ( S c / S ) and by reducing both cell wall thickness and lignin content. Structural equation modeling showed that intercropping directly enhanced g s and g m in both upper and lower leaves of rapeseed, thereby increasing A . In contrast, photosynthesis in the upper leaves of hairy vetch was not strongly associated with any measured parameters. Notably, A of rapeseed’s lower leaves positively contributed to biomass accumulation, whereas leaf area had no significant effect. Conversely, in hairy vetch, intercropping reduced leaf area but increased A , with leaf area showing a stronger influence on biomass than A . Conclusion Intercropping improved the spatial efficiency of weak-light photosynthesis in both species. Biomass accumulation in the dominant crop (rapeseed) was primarily driven by the photosynthetic activity of lower canopy leaves, while in the subordinate crop (hairy vetch), leaf area was the main determinant of biomass. Significance This study provides novel insights into the multi-scale mechanisms—from cellular to whole-plant level—by which intercropping modulates spatial photosynthetic performance and biomass formation. Our findings offer a theoretical foundation for improving leaf photosynthetic efficiency and biomass productivity in forage intercropping systems.