Precise classification and regional delineation of maturity groups in soybean cultivars across China

The maturity group (MG) system is a widely recognized and effective approach for assessing the photothermal sensitivity of soybean cultivars and determining their optimal adaptation zones. In China, soybean have been cultivated extensively for thousands of years, evolving into various ecotypes through natural and artificial selection. In this study, the relative maturity groups (RMGs) of a total of 766 soybean cultivars collected from the Northeast Spring Planting Sub-region (NE), Huang-Huai-Hai Summer Planting Region (HH), Northwest Spring Planting Sub-region (NW,), and South Multiple Cropping Region (SC) of China were evaluated using linear regression models at 36 sites nationwide. The results show that the RMGs of Chinese soybean cultivars range from MG m1.6 to MG 9.5. Among all the identified soybean cultivars, MG III cultivars account for the largest proportion of 22.19 %, followed by MG II (21.67 %) and MG I (18.54 %). Conversely, MG IX has the fewest number of cultivars, representing only 0.26 % of all cultivars. The MG ranges for spring- and summer-sowing cultivars are MG 000-V and MG II-IX, respectively. The adaptive soybean MG zones across China were mapped using the method of Kriging interpolation based on the soybean RMGs data from 816 sites of 29 soybean-producing provinces. Cultivars of MG 0 and earlier groups are primarily distributed in the NE, and MG III and MG IV are the major MGs in HH and NW. The cultivars in MG V and later groups mainly distribute in SC. This study realizes the unification and normalization of the MG system between China and other major soybean- producing countries worldwide. Such unification and standardization will facilitate global germplasm exchanges and assist soybean producers in making more informed decisions when selecting cultivars.