Rice Materials and Cultivation Conditions

Over 20 rice lines, varieties, or cultivars were collected from Northeastern China to be screened for cultivating adaptations (Supplementary Table S1). “Nipponbare” was used as a control line. Rice seeds were germinated on water-submerged filter papers at 28°C in darkness for 4 days (Supplementary Figure S1A). Germinated seeds were transferred individually to a pot (10 × 10 cm on top and 7 × 7 cm on bottom with a height of 7.5 cm) with organic soil (called “såjord/seed compost” containing at least perlite, cow manure, lime, and natural humus; Hasselfors Garden, Örebro, Sweden) and cultivated in a greenhouse (Supplementary Figure S1B). Cultivating conditions of the greenhouse were 16 h light and 8 h dark at 30°C/15°C with a constant humidity of 80% and light intensity of 300 μmol per m^–2 s^–1. After 10 days’ cultivation in the greenhouse, seedlings at a stage of ca three leaves were transplanted to open fields. For screening of adaption, the cultivation of different lines was not covered. For a production test, “Heijing 5” was covered with a simple plastic polytunnel in the fields when the temperature was below 10°C (Supplementary Figure S1). Field cultivations were performed for ca 4.5 months with normal farming practices before harvesting. Paddy irrigation was mainly dependent on rainfall, and, when necessary, groundwater was used for the water supply. During cultivation, air and paddy soil temperatures were recorded. “Heijing 5” plants were also cultivated in a greenhouse and a field without a cover as controls. Plants were harvested and stored dry at room temperature for different analyses.