*4.1. Plant Materials and Growth Conditions*

Seeds of soybean cultivar Jindou 23 (wildtype) were mutagenized with ethyl methane sulfonate (EMS). The phenotypes of the second mutant generation (M2 progeny) plants were observed and the *yl* mutant with yellow foliage was identified from the M2 progeny. The *yl* mutant has been continuously self-pollinated and selected based on the yellow foliar phenotype. For phenotype characterization of the *yl* mutant, the simultaneous field trial consisted of three replicates per genotype, planted in 0.50 m row spacing and 0.10 m spacing in the rows. To analyze the inheritance pattern of the *yl* mutant and fine-mapping of the *YL* gene, three F2 populations were generated from reciprocal crosses between the *yl* mutant and two soybean cultivars, Williams 82 and Zhonghuang 13 (*yl* × Williams 82, *yl* × Zhonghuang 13 and Zhonghuang 13 × *yl*). All soybean materials were planted in the experimental field at the Institute of Genetics and Developmental Biology, Chinese Academy of Sciences (Beijing, China), during the natural growing seasons (May to October). The common meteorological conditions during soybean growing seasons in Beijing were as follows: (1) mean daily temperatures: about 24 ◦C. (2) overall precipitation: 400 ~ 600 mm.
