Crop heat units (CHU) and ≥10 °C active accumulated temperature (≥10 °C AAT) are important indexes to quantify the effects of temperature on soybean development. The relative maturity group (RMG) is widely used in the classification of different soybean varieties. However, CHU and ≥10 °C AAT (AAT) were applied in Canada and northeastern China, respectively, and the relationships among CHU, AAT and RMG are poorly documented. The objective of this study is to analyze the conversion function among CHU, AAT and RMG based on two datasets. The first dataset was obtained to analyze the relationship between RMG and AAT in 395 varieties in Northeast China. The second dataset was obtained to calculate the relationship between CHU and AAT at 95 weather stations based on 30-year climatic data (1990–2019). The results showed that both relationships were significantly and positively correlated, and the R-square of these relationships were 0.90 and 0.98, respectively. The distribution of CHU or AAT in the Northeast is proposed. These results can be extensively used for predicting the CHU or AAT of soybean cultivars given the known RMG, thus determining the adaptation zone as well as the growth stage of agricultural practices and responses to heat accumulation. The conclusion of the current study is expected to be widely adopted by soybean regionalization and germplasm exchanges throughout the world.
Soybean is planted in a wide span of the world, and flowering and maturity time is an important trait determining soybean yield formation and adaptation. Maturity loci E1, E2, E3 and E4 were frequently reported as the most influential genetic loci for soybean flowering and maturity. To understand the allelic variation and assess the phenological traits of cultivars with different E allelic combinations in natural environments, 251 cultivars of maturity group (MG) I–V were field tested in 42 locations across four sub-regions in the Huang-Huai-Hai and Northwest region of China and genotyped with KASP markers for E1–E4 loci. The results indicated that mutant alleles were only found in the E1 and E2 locus, all of the cultivars carried functional alleles in the E3 and E4 loci in this area, with the frequency of mutant allele to be higher in early maturity groups (MGs) than late MGs. Among nine E allelic combinations in this area, one photoperiodic insensitive mutation in E2 loci (E1/e2-ns/E3-Ha/E4 and E1/e2-ns/E3-Mi/E4) made up the largest proportion (25.10 and 18.33%), while two photoperiodic insensitive mutations in both E1 and E2 loci (e1-as/e2-ns/E3-Ha/E4) (1.20%) occupied the lowest proportion in this panel. The major combinations of E locus for MGI, MGII and MG III in this area were E1/E2-dl/E3-Mi/E4, E1/e2-ns/E3-Mi/E4 and E1/e2-ns/E3-Ha/E4, respectively. Cultivars carrying e1-as/e2-ns/E3-Ha/E4 genotype flowered earliest (34 days) on average, 7.6 days earlier than the latest-flowering E haplotype (E1/e2-ns/E3-Ha/E4). This study provided an opportunity to detect the E allelic combinations in the Huang-Huai-Hai River Valley and the Northwest China, which would facilitate the improvement of soybean adaptation in the future.
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