Abstract: 【Objective】Under the maize/soybean relay strip intercropping system, shading from high crop maize leads to soybean seedling thinning, lodging and decreasing soybean yield. We studied the effect of soybean photosynthesis and lodging resistance in seeding stage under different shade degrees in the maize/soybean relay strip intercropping system, then provided a theoretical basis for the establishment of a reasonable community population structure. 【Method】 The non-shade tolerance and non-lodging resistant Nan032-4 and the strong shade tolerance and strong lodging resistance Nandou 12 were taken as experimental materials in 2014 and 2015. We designed three light conditions in the outdoor pod experiment, the natural light (W0), one layer of black sun-shade net (W1), and two layers of black sun-shade net (W2) with light transmittances of 100%, 32% and 11% respectively. Photosynthetic characteristics such as soybean photosynthetic rate, stomatal conductance, transpiration rate, intercellular CO₂ concentration,SPAD values, leaf area and biomass accumulation of each organ and lodging characters, including plant height, each internode length, each internode diameter, stem snapping resistance, lodging resistance index were measured and analyzed. 【Result】 The results showed that as the shade increased, photosynthetic rate, stomatal conductance, leaf area and SPAD values, the dry matter accumulation of each organ and root/shoot ratio of the two soybean varieties were all reduced significantly. As shade increased, seedling lodging became serious, with the stem snapping resistance and lodging resistance index also reduced significantly. However, the shade avoidance responses on different shade degrees were significantly different between these two different shade tolerant varieties. Compared with W0, photosynthetic rates of the shade tolerant cultivar Nandou12 and the non-shade tolerance Nan032-4 under W1 and W2 were decreased by 35.6%, 99.6% and by 40.9%, 83.6% respectively. Compared with Nandou12, Nan032-4 was influenced more seriously. The dry matter accumulation of Nandou12 under W0 and W1was higher than Nan032-4, but under W2 there was no significance between the two varieties (P＜0.05), this indicated that the photosynthetic rate of soybean remained at a higher level under moderate shade and the photosynthetic product of the shade tolerant cultivar was higher than the non-shade tolerance cultivar. The lodging rate over 2014 and 2015 of Nan032-4 under W1 and W2 was higher than that of Nandou12 by 55.9% and 2.57%. Under the treatment W1, Nan032-4 showed moderate lodging and Nandou12 showed mild lodging; Nan032-4 while Nandou12 showed serious lodging under the treatment W2. Under the same shade level, the snapping resistance of Nandou12 (39.8%) was higher than Nan 032-4 (38.4%), but the snapping resistance of Nandou12 under W1 and W2 for 2014 and 2015 was higher than that of Nan032-4 by 27.3% and 26.6%. Under the same shade level, stem snapping resistance and lodging resistance index of the shade-tolerant cultivar Nandou12 were much higher than that of Nan032-4 (P＜0.05).【Conclusion】With increasing shade level, photosynthetic accumulation of soybean was reduced and lodging was more serious. The two materials showed the shade sensitive under the excessive shade. But under moderate shade, the strong shade tolerant cultivar maintained its high photosynthetic and lodging resistance. In a word, under the maize/soybean relay strip intercropping system, we should appropriately reduce the shading stress on soybean in production, then finally realize soybean potential of shade tolerance and lodging resistance.

Key words: soybean, shade tolerance, shade degree, photosynthetic, lodging