Abstract: The purpose of this study was to clarify the role of photosynthetic capacity of soybean at the seedling stage in maize soybean intercropping. Our research studied the strongly shade tolerant soybean variety (Nandou 12) and weakly shade tolerant variety (Nan032-4), under soybean monoculture and maize soybean intercropping systems. The percentage of lodging,stem morphology, photosynthetic rate, carbon content and nitrogen metabolic activity of stems and leaves were measured. Compared with monoculture soybean, the stem length:diameter ratio was significantly increased by maize shading, but leaf photosynthetic rate and the lodging percentage were decreased. However, the degree of response differed between varieties. In the intercropping system, the stem morphology changes and increase in lodging were relatively small in the shade tolerant variety Nandou 12, and greater in the intolerant variety, Nan032-4. Lodging percentage was highly significantly correlated (r=0.946; P<0.01) with the stem length:diameter ratio, and negatively correlated with the photosynthetic rate (r=-0.886; P<0.05).Meanwhile, the photosynthetic rate was highly significantly positively correlated with the photosynthetically active radiation (r=0.900; P<0.01). The results show that shading by maize reduced the photosynthetically active radiation and photosynthetic rate, which in turn triggered plant height increase and stem diameter decrease responses, leading to increased plant lodging. Compared with soybean monoculture, the carbon:nitrogen ratio of intercropped soybean was significantly decreased by maize shading, but again Nandou 12 was significantly less affected than Nan032-4. Leaf photosynthetic rate was highly significantly and negatively correlated with the carbon:nitrogen ratios of leaf and stem (r=-0.871 and -0.930, respectively; P<0.05). The leaf and stem carbon:nitrogen ratios were highly significantly (r=0.985 and 0.968, respectively; P<0.01) and positively correlated with lodging percentage. These findings show that a high carbon:nitrogen ratio in soybean stem and leaf tissues provides a physiological basis for maintenance of higher lodging resistance and the variety with this trait (Nandou 12) was more suitable for intercrop planting than Nan032-4. Compared with soybean grown in monoculture, the content of carbon and nitrogen and photosynthetic activity and substrate export of stem and leaf tissue was significantly decreased when shaded by maize in the intercrop system. Soybean monoculture and maize soybean intercropping systems were used as in this study to explore the relationship between plant lodging, and carbon and nitrogen metabolism to provide a theoretical basis for development of appropriate fertilization management and cultivation practices to promote lodging resistance and identify varieties to lodging in intercropping systems.

Key words: intercropping soybean, lodging, carbon to nitrogen ratio, soluble sugar, soluble protein, starch