Sowing Date Regulates Japonica Rice Yield by Modulating Grain Weight and the Proportion of Grains in Secondary-to-Primary Branches

Late sowing has been common since the popularization of mechanical transplantation, and it has led to reduced grain yield due to low grain weight. However, the uneven contribution of the amount and weight of grains in different positions to grain yield remains unclear. A 2 yr field experiment was conducted with two super rice varieties (Ningjing 7th: N7 and Nanjing 9108th: N9108) and three sowing dates (S1: 30 April; S2: 30 May; and S3: 30 June) in Hefei City. In this study, delaying sowing shortened the growth duration, reduced temperature, and further reduced the grain yield mainly by suppressing the total grain weight. Compared with S1, the grain weight of grains in secondary branches (SGs) in S2 and S3 was reduced by 11.1% ± 2.9% and 18.5% ± 1.4%, respectively, due to a lower reproductive-stage temperature. However, no significant difference was found in the grains in primary branches (PGs). Moreover, a shorter growth duration enhanced the ratio of tillers in a lower position (LT) per unit area, which contained more SGs per panicle, and finally led to a higher ratio of SG amount per unit area (SG%). The SG% increased by 13.4% ± 1.4% and 21.1% ± 1.9% in S2 and S3 compared with S1, respectively. In conclusion, delaying the sowing date mainly reduced the grain weight of SGs and enhanced the SG%, leading to a lower apparent grain weight and further decreasing grain yield.