Abstract: 【Objective】 Shade tolerance is an important factor that affects the yield of intercropped soybean, thus limiting the extension of intercropping patterns of soybean. It is necessary to establish an efficient, versatile and reliable shade tolerance identification system to be used in identifying the shade tolerant sources from germplasm populations, which can be used for shade tolerance breeding. 【Method】 Samples of 60 soybean varieties with different shade-tolerance levels were chosen from the germplasm storage in Guangxi Academy of Agricultural Sciences were used in this study. Firstly, the suitable shading treatment for shade tolerance evaluation was chosen among the three shading levels (i.e. 15%, 30% and 60%) according to a reasonable lodging rate of the tested varieties. Secondly, the suitable shade tolerance indicators were chosen from the 17 traits, including plant height, mean internode length, petiole length, shoot fresh weight and so forth. Then, the suitable measuring time was chosen among the three dates, i.e. on the 40th, 50th and 60th day after sowing according to the error coefficient of variation and phenotypic coefficient of variation of the selected shade tolerance indicator. Finally, the shade tolerance evaluation procedure was assembled to include the suitable shading level, shade tolerance indicator and measuring time. Using the designed procedure, the shade tolerance of 453 soybean varieties from southern China was evaluated, from which the tolerant sources were screened out. 【Result】 The 30% shading intension condition was better than other shading degrees. More specifically, there showed lower lodging rate of accessions (22%), higher phenotypic coefficient of variation (25%), and better distinction among accessions. The shade tolerance indicator which composed of the plant height and the average internode length is better than other indicators after comprehensive tradeoff, with advantages as follows: i) It is more accurate due to its smaller error coefficient of variation (9.36%) and higher heritability (95.43%); ii) It is more stable, as its correlation (0.92) between two environments is the largest; iii) It has a better distinction degree, because its phenotypic coefficient of variation (31.25%) and genotypic coefficient of variation (30.52%) are larger; iv) It is more consistent to the field shade tolerance situation with the highest correlation (0.73) between the indicator and visual shade tolerance level. Compared to other measuring times, the shade tolerance indicator on the 50th day after sowing is better, for example its correlation (0.87) between two environments is the highest and error coefficient of variation (7.75%) is the smallest. Therefore, the average of relative plant height and average length of internode was defined on the 50th day after sowing under 30% shading intension condition as the shade tolerance index (STI). The smaller STI, the more shade tolerance of the accession. The STI of 453 soybean varieties from southern China ranged from 1.11 to 2.61, with an average of 1.55 and there showed significant differences among the accessions. In addition, the heritability (91.13%) of these soybean varieties indicated that soybean shade tolerance had high accuracy from phenotypic selection. The STI in eco-region Ⅲ, Ⅳ ,Ⅴ and Ⅵ showed abundant variations with the range of 1.19-2.08, 1.17-2.61, 1.27-2.37, and 1.11-2.54, respectively, which provide materials for the improvement of shade tolerance. Furthermore, the released cultivars and landraces also have abundant variations with the range of 1.11-2.61 and 1.17-2.54, respectively. However, eight of the eleven accessions with high shade tolerance in two environments were released cultivars, which implies the achievement in shade tolerance breeding of soybeans. 【Conclusion】 The shade tolerance indicator which composed of plant height and average internode length on the 50th day after sowing was concluded to be accurate, stable, sensitive and authentic. Using this evaluation system, a number of shade tolerant soybean varieties were screened out from the southern China germplasm population.

Key words: soybean, shade tolerance, variation, germplasm