2.1 Crop husbandry and experimental design
Field experiments were performed over four years of 2015-2018 at Lvfeng Experimental Station of Chongqing Academy of Agricultural Sciences (29.86°N, 106.0°E), southwest of China. The region’s climate was classified as Cfa according to Köppen, with mild winter and fully humid, hot, and rainy summer, with an average annual precipitation of 1000 – 1350 mm and an average annual temperature between 16 – 18 °C. The soil of the region was red earth soil with 4.6 – 5.5 pH, 0.4 – 1.2% organic matter, 0.05 – 0.06% total nitrogen, 0.01 – 0.02% total phosphorus, and 1.2 – 1.8% total potassium at a depth of 0 – 20 cm.
The experimental design was a randomized complete block with three replicates. Each block consisted of six rows, 5 m long, spaced 0.8 m apart. There were 162 maize inbred lines used in each of four experimental years, of which, 40 temperate (Te), 45 tropical (Tr), and 77 Te×Tr lines were included. Germplasms information was provided in the Supplementary Table 1. All the inbred lines were sown at two sowing dates in 2015 and 2016 (first sowing date, Mar. 21 in two years; second sowing dates were Apr. 28 in 2015 and May. 24 in 2016) and one sowing date in 2017 (Apr. 6) and 2018 (Apr.13). Plants that were sown at the first sowing date in 2015 and 2016 were not or slightly subjected to high temperature only during late growth period, that were regarded as control. Plants sown at the second sowing date were subjected to high temperature mainly during flowering that were regarded as high temperature treatment at flowering (HDF). And, plants sown in 2017 and 2018 were exposed to high temperature during the early grain filling period, being regarded as high temperature treatment during early grain filling (HEGF). The temperature data during the entire growing period of maize was shown in Supplementary Table 2.
At sowing, three seeds per hill were hand sown and thinned to one plant at the 3-leaf stage. Crops were fertilized with 60 kg N ha-1, 105 kg P2O5ha-1, and 120 kg K2O ha-1 at sowing, and additional extra 70 kg N ha-1 was applied at the 6- and 12-leaf stages, respectively. Crop management consistent with local practices were performed to control weeds, insects, and diseases during the entire growing period.