Leaf Area Regulates the Growth Rates and Seed Yield of Soybean (Glycine max L. Merr.) in Intercropping System

Biotic or abiotic stresses reduce leaf area of soybean plants in the intercropping system, especially during critical reproductive growth phase (from pod-initiation to physiological-maturity) of soybean, which finally influences yield and yield components. However, total yield loss due to reduction in soybean leaf area under maize/soybean intercropping system is still unclear. In a three-year field study, an experiment consisted of four treatments: no removal of trifoliate (CK, 100% leaf area), removal of three-trifoliate (SI, 85% leaf area), removal of six-trifoliate (SII, 70% leaf area), and removal of nine-trifoliate (SIII, 55% leaf area) from the top of the soybean canopy under maize/soybean intercropping. These defoliation treatments were applied at the pod initiation (R-3) stage by removing the different number of fully developed trifoliate from the top of the soybean canopy in maize/soybean intercropping system. Results revealed that defoliation significantly decreased total dry matter accumulation and partitioning to vegetative and reproductive organs. Compared with CK (no defoliation), treatments SI, SII, and SIII reduced crop growth rate (by 25%, 46%, and 75%), reproductive growth rate (by 21%, 44%, and 63%), pod-initiation (by 11%, 23%, and 32%), while increased pod-abscission (by 11%, 20%, and 37%) and photosynthetic-rate (by 8%, 19%, and 28%), respectively at physiological-maturity. These negative responses reduced pods plant(-1) by 16%, 32%, and 49% and seeds plant(-1) by 20%, 34%, and 46% in SI, SII, and SIII, respectively, compared to non-defoliated. Overall, in SI, SII, and SIII, soybean produced 80%, 67%, and 55% of CK yield. Results implied that any change in leaf area of intercropped-soybean, especially during reproductive phase, will directly affect the availability of photoassimilates and nutrients for developing pods and seeds. Thus, more attention should be paid to improve leaf area of intercropped soybean for the high productivity of intercropping systems via appropriate variety selection or planting arrangement. Furthermore, breeders can evolve new soybean varieties, particularly for the intercropping systems, which can cope with the shading effects of tall crops in intercropping systems. Future studies are needed to understand the internal signaling and the molecular mechanism controlling in soybean in intercropping system.