Abstract: 【Objective】 From the perspective of endogenous gibberellins(GAs) metabolism, the reasons of soybean seedling vine lodging under the maize-soybean relay strip intercropping system were studied. 【Method】Under maize-soybean relay strip intercropping system and soybean sole cropping system, shade-tolerant and lodging resistant Nandou12 and non-shade tolerant and non- lodging resistant Nan032-4 were taken as experimental materials. The stalk morphology, anatomical structure, the level of gibberellin A₄, and the genes that are concerned with gibberellins expression profiles were analyzed with the paraffin section, Gas chromatography-mass spectrometry analyses, real-time qPCR with SYBR^® Green Ⅱ and other. methods. 【Result】Compared with soybean monoculture, due to the effects of maize shading at the seeding stage, both Nandou12 and Nan032-4 appeared obvious shade avoidance responses in relay strip intercropping system, including reduction of node number, significant increase of plant height and internode length, vining and lodging of the seedlings. However, the shade avoidance phenotypes were quite different between these two materials. Compared with the shade-tolerant cultivar Nandou12, the shade-sensitive Nan032-4 was influenced more seriously, for which 93.94% of the plants showed a lodging phenotype, much higher than that of Nandou12 (29.93%) (P＜0.05). The height of Nan032-4 was increased by 32.64 cm, different from that of Nandou12 (22.95 cm). The node number of stem reduced, but there was no significance between the two varieties. The upper nodes of Nan032-4 were longer than Nanou12 remarkably, that is to say, in the relay intercropping system, the increase of plant height of these two varieties resulted from internodes elongation, not from the increase of the node number, and the excessive elongation of middle and upper nodes could lead stems to vine and lodge easily. Vertical section of soybean stem revealed that the cell length of stems showed different degrees in elongation between the two varieties, especially for Nan032-4, not Nandou12, showed an apparent increase in cell length of stem pith, xylem and phloem. Therefore, these results suggest that the increase of plant height of these two varieties was caused by cell elongation, but not cell division. Endogenous gibberellins metabolism analyses showed the GA₄ content in the stems of both varieties was decreased significantly in relay strip intercropping system, but was much lower in Nandou12 compared to Nan032-4. The transcript levels of genes encoding key enzymes in gibberellins metabolic pathways were also investigated in the present study. The results show that the expression of genes encoding GA20-oxidase, GA3-oxidase, and GA2-oxidase were not only higher in Nan032-4 than that in Nadou12, but also up-regulated by intercropping in both varieties. However, GA synthetic genes of GmGA20ox5 and GmGA3ox6, as well as the GA deactivating gene GmGA2ox4 displayed a lower expression pattern in different plant modes and varieties. These results suggest that the content of endogenous gibberellins negatively regulate the expression of key genes in GA related pathways with a feedforward and feedback manner. Low level of GA₄ in the stem tips of Nandou12 might repress the stem elongation excessively, and contribute to a shade avoidance syndrome.【Conclusion】Synthesis of soybean endogenous GA₄ is affected by the genotypes and the light environment, thus influencing the growth and development of soybean stem, which finally results in the typical differences in shade lodging phenotype among varieties.

Key words: soybean, relay intercropping, lodging at seedling stage, stem, endogenous gibberellins