Simulation of Defoliation Effects on Relay Strip Intercropping Soybean: Elucidating Foliar Shedding and Leaf-to-Nodule Growth Plasticity.

Extensive foliar shedding in monoculture soybeans post-anthesis negatively impacts yield, whereas relay strip intercropping prolongs leaf area duration, enhancing productivity. However, little is known about the causes of leaf shedding in monoculture and its impact on physiological functions and plasticity of source and sink organs, we conducted a 4-year field experiment and leaf-removal simulations in relay intercropped soybeans. Results revealed that monoculture soybeans experienced severe self-shading and defoliation, while relay intercropping maintained better light conditions, supporting higher leaf area, nodule numbers, and carbon allocation. Increasing leaf removal initially increased leaf area but eventually reduced it. Extensive leaf-removal reduced Rubisco and sucrose phosphate synthase (SPS) activity, as well as sucrose, malate, ATP, and energy charge (EC) in nodules, revealing a trade-off between leaf growth and nodule development. Moderate leaf-removal (L30), however, balanced compensation and consumption, increasing total non-structural carbohydrates (TNC) in roots and N and ureide in leaves and pods. Network analysis showed that L30 improved the synergies of functional traits in leaves and nodules, ultimately benefiting overall plant growth and nutrient accumulation in pods. This study elucidates a mechanism of foliar shedding and highlights how relay strip intercropping optimizes source-sink coordination to enhance photosynthesis and nitrogen fixation.