Evaluation for shade tolerance of black soybean germplasms and their root structure response to shade stress at seedling stage

Maize-soybean relay strip intercropping systems is an important planting mode in Southwest China. However, shade-tolerant germplasms of black soybean are scarce and the mechanism of shade tolerance of black soybean is still unclear, which significantly affects further promotion of the mode. The objective of this study was to establish a mathematical model to evaluate shade-tolerance ability of black soybean seedlings, screen shade-tolerance appraisal indicators and discuss the diverse responses of the root structures of different black soybean germplasms seedlings to shade stress. The study was also intended to lay the foundation for breeding varieties and for explaining the mechanism of shade tolerance of black soybean. Thus in the study, shade tolerance of 23 black soybean germplasms were evaluated in a pot culture experiment with normal light and shade treatments. The morphological characteristics (plant height, stem diameter, hypocotyl length, leaf dry weight, stem dry weight, main stem count, total root length, root surface area, total root volume, etc.) and physiological parameters were measured at the 3^rd trifoliolate leaf stage (V3). An evaluation mathematical model was established using multivariate statistical analysis. Additionally, based on the evaluation results, correlation analysis was conducted to determine differential responses of roots to shade stress. The 23 black soybean germplasms were divided into three groups, including tolerant, moderately tolerant and sensitive germplasms. A total of 5 evaluation indexes closely related to shade tolerance were screened out. The study showed that the response of root structure to shade stress varied with shade-tolerance ability of black soybean germplasms. Root parameters such as total root length, root surface area, total root volume and root dry weight all had a similar order, which was shade tolerant germplasms > moderate shade tolerant germplasms > sensitive germplasms. The results indicated that shade tolerant germplasms of black soybean had a stronger root system.