Cropping patterns and plant population density alter nitrogen partitioning among photosynthetic components, leaf photosynthetic capacity and photosynthetic nitrogen use efficiency in field-grown soybean

Soybean is essential for industrial applications, with its yield and production distribution significantly influencing global agricultural sectors. In maize-soybean strip intercropping (SI) systems, optimizing soybean yield requires a comprehensive understanding of photosynthetic physiology under conditions of limited light availability. This three-year study examined nitrogen (N) partitioning among photosynthetic components and photosynthetic N use efficiency (PNUE) in SI compared to soybean monocropping (Mono) system. Effects of different plant population densities (PPD) (8.3 x104 plants ha-1, 9.5 x104 plants ha-1 and 11.1 x104 plants ha-1) on photosynthetic N allocation, PNUE and their interrelationships in inner and border rows were also analyzed. Results indicated that, compared to Mono, SI increased chlorophyll and N content, allocating more N to the light-harvesting system while reducing N allocation to carboxylation, electron transfer systems, and the overall photosynthetic system. This shift in N allocation led to reduced photosynthetic capacity and PNUE. Higher PPD in SI further reduced the proportion of N allocation to carboxylation, electron transfer and total photosynthetic system, thereby reducing PNUE. In inner rows, N was more efficiently allocated to the photosynthetic system, particularly to the carboxylation and electron transfer systems, supporting a relatively higher photosynthetic capacity, PNUE and yield than border rows. A significant trade-off was observed between cell wall N and total photosynthetic system N in inner rows, while a quadratic relationship was noted in border rows. In conclusion, soybean leaves optimized photosynthetic capacity and PNUE by modulating N partitioning among photosynthetic components. Under SI system with a PPD of 9.5 x 104 plants ha-1, soybean leaves demonstrated balanced photosynthetic N allocation, achieving the highest yield. These findings offer a theoretical basis for refining leaf N allocation strategies to maximize yield benefits in SI systems.