The Huang-Huai-Hai region is a major production area for summer soybeans (Glycine max L.) in China. In this region, frequent wind, rain, and hail during the summer often cause damage to leave at the seedling stage, resulting in a reduction in leaf area. By using artificial defoliation, this study investigated the changes in photosynthetic characteristics, pod distribution, and yield formation of soybeans following leaf removal at the seedling stage, so as to provide support for the integrated application of stress-resistant and high-yield soybean cultivation techniques.

Using 'Zhonghuang 301' as the experimental material, a two-year field experiment was conducted. Different defoliation treatments were established at the soybean V3 stage: CK (no defoliation), C1 (removal of 1 trifoliolate leaf), C2 (removal of 2 trifoliolate leaves), and C3 (removal of 3 trifoliolate leaves). These treatments simulated the reduction in leaf area caused by adverse weather conditions in seedlings to systematically analyze the effects of seedling defoliation on soybean yield and physiological characteristics.

Defoliation at the seedling stage reduced soybean yield, and the degree of yield loss was positively correlated with defoliation intensity. The C1, C2, and C3 treatments resulted in average yield reductions of 5.4%, 10.2%, and 19.3% over two years, respectively. This was primarily attributed to an increase in the height of the lowest pod (which increased by 7.09%, 24.5%, and 42% compared with CK, respectively) and a decrease in the number of pods in the lower layer (0-30 cm) of the plant (which decreased by 13.6%, 33.8%, and 59.6%, respectively). The Leaf Area Index (LAI) decreased with increasing defoliation, leading to reduced aboveground dry matter accumulation; compared with CK, the accumulation under C1, C2, and C3 treatments at the R1 and R3 stages decreased by an average of 8.5%, 16.5%, and 37.1%, respectively. At the R8 stage, the grain dry weight for C1, C2, and C3 decreased by 12.1%, 24.3%, and 32.7% compared with CK, respectively. The photosynthetic characteristics analysis showed that there were no significant differences in photosynthetic indices between the C1/C2 treatments and the CK treatment at any growth stage. However, the C3 treatment significantly inhibited the net photosynthetic rate, relative chlorophyll content, and Photosystem Ⅱ activity at the R1 stage, although its photosynthetic capacity recovered by the R3 stage. Nitrogen metabolism studies indicated that defoliation treatments reduced total aboveground nitrogen accumulation. Although the proportion of grain nitrogen reached 93.6% in the late reproductive stage, insufficient nitrogen supply from source organs limited sink capacity. At maturity stage, the difference in protein content between the treatment with the highest and the lowest values was only 1.27%. Principal Component Analysis (PCA) indicated that yield was positively correlated with LAI, photosynthetic rate, nitrogen accumulation, and lower-layer pod number, but negatively correlated with the height of the lowest pod.

Defoliation at the seedling stage exerts a significant negative impact on summer soybean yield, with the magnitude of yield loss increasing with defoliation intensity. Comprehensive analysis indicates that leaf area index (LAI), photosynthetic performance, nitrogen accumulation, and basal pod number serve as key sensitive indicators characterizing the degree of leaf injury. Elucidating the response patterns of these parameters offers a scientific basis for developing stress-resilient cultivation strategies for soybean.