*2.1. Plant Materials and Phenotype Evaluation*

Two *japonica* rice cultivars, Koshihikari with high eating quality [15] and Baegilmi with early maturity [14], were used in this study. Days to heading (DH) and grain filling rates of the two cultivars were evaluated at the experimental field of the National Institute of Crop Science (NICS), Suwon, Korea (37◦27 N 126◦99 E) in 2014. The seeds of each cultivar were sown on 25 April and transplanted on 25 May under a randomized complete block design (RCBD) with three replications. Each plot comprised of eight 4.5 m rows, with 30 hills per row and three plants per hill. The hills within a row were spaced by 15 cm and the rows were spaced by 30 cm. DH was determined by counting the number of days from sowing to heading when the panicles emerged in 40% of the plants in a plot. To evaluate the rate of grain filling of Baegilmi and Koshihikari, changes in grain weight of the two cultivars were monitored by measuring 1000 grain weight every three to four days during 19–50 days after heading.

To map QTLs for DH, a RIL population (*n* = 142) was constructed from the cross between Koshihikari and Baegilmi by the single seed descent method. The RIL population (F6 and F7 generation in 2016 and 2017, respectively) and its parents were grown at the experimental field of NICS, Wanju, Korea (35◦84 N 127◦05 E) in 2016 and 2017. The seeds were sown on 9 May and 10 May in 2016 and 2017, respectively, and the seedlings were transplanted four weeks after sowing. Each RIL was transplanted in a 4.5 m row with the individual plants spaced by 15 cm (30 plants per each line) and the rows spaced by 30 cm. DH of each RIL was determined by counting the number of days from sowing to heading when the panicles emerged in 40% of the plants in a row.

Days to heading of 295 commercial rice cultivars released by NICS, Rural Development Administration, Korea, were evaluated under the optimum and early planting conditions at the

experimental field of NICS, Wanju, Korea. Sowing and transplanting dates for optimum planting were 9 May and 1 Jun, respectively in 2018, and those for early planting were 10 Apr and 9 May, respectively in 2019. Planting density and DH evaluation were as described above for the RIL population.
