A wide range of phenotypic variation was observed in flowering time in the association panel across different photo-thermal conditions (Table 1). All cultivars can flower under the SD or natural-day condition regardless of the sowing season. However, some cultivars in the LD condition failed to flower at the harvest season. The soybean flowering time followed a normal distribution except for flowering time in natural day-length conditions, which was slightly skewed to the early flowering (Table 1, Additional file 3: Figure S1). The duration from emergence (VE) to the beginning bloom (R1) was shorter in the SD than that in the LD condition given the same sowing season. However, the time from emergence (VE) to the beginning bloom (R1) was accelerated in the HT compared with that in the LT under the same day-length.Table 1Descriptive statistics of soybean flowering time in different photo-thermal treatmentsYearEnvironmenta Min.Max.Mean ± SECV(%)SkewnessKurtosis(d)(d)(d)2009b SD + LT22.435.628.4 ± 0.711.850.470.16SD + HT21.531.325.9 ± 0.610.600.17−0.89LD + LT30.0>114.8c >70.9 ± 5.0c 33.87−0.10−0.85LD + HT24.0>80.2c >47.5 ± 3.6c 35.570.62−0.47SP28.5133.652.4 ± 4.945.681.774.59SU25.264.837.9 ± 2.835.11−0.651.942010SD + LT24.035.129.0 ± 0.38.690.10−0.27SD + HT22.131.826.6 ± 0.27.740.18−0.47LD + LT26.7>165.7c >98.0 ± 3.9c 35.67−0.03−0.61LD + HT26.6>103.4c >61.5 ± 2.3c 31.550.41−0.28SP25.3137.553.9 ± 2.949.261.341.37SU23.481.538.8 ± 1.229.181.151.56201414SP19.5132.950.9 ± 3.258.391.120.2514SU18.684.539.2 ± 1.639.510.950.25201515SP19.7124.847.6 ± 2.957.910.99−0.1315SU19.176.036.3 ± 1.436.790.830.08 aSD, 12 h; LD, 16 h; LT, low temperature (spring sowing); HT, high temperature (summer sowing); SP, Spring sowing season with natural day-length in pot experiment; SU, Summer sowing season with natural day-length in pot experiment; 14SP, Spring sowing in 2014 field experiment; 14SU, Summer sowing season in 2014 field experiment; 15SP, Spring sowing season in 2015 field experiment; 15SU, Summer sowing season in 2015 field experiment bA total of 91 cultivars were tested in the experiment in 2010, 2014 and 2015, and a subset of 25 cultivars from different maturity groups were used in the experiments in 2009, the cultivars were listed in the Additional file 1 cSome late cultivars failed to flower before the end of experiment. The flowering time of the latest-flowered cultivar in the same treatment was used as that of the un-flowered cultivars when calculating the means