Effects of the growth stage at transplanting of paddy rice seedlings on heading date and floral sterility caused by low temperatures - In the cool summer of 2003 in Miyagi prefecture

Paddy rice seedlings transplanted at different growth stages in Miyagi prefecture in 2003 were examined for heading date and the percentage of ripening on all stems in each hill in relation to cool summer damage. In that year, the low temperature and insufficient sunshine continued from June for a long period, and floral sterility due to the low temperature occurred frequently. The heading date was earliest in the plants transplanted at the middle-seedling stage (MS plants) followed by those transplanted at the young-seedling stage (YS plants), and those transplanted at the nursling-seedling stage (NS plants), in this order. The heading period was longer than that in normal years, and was 11, 13 and 9 days in the MS, YS and NS plants, respectively. The difference in the 50% heading dates between the MS and YS plants was 4.7 days, and that between the YS and NS plants was 4.2 days. Irrespective of the growth stage of the seedlings at transplanting, the later the heading date within the period from August 10 to August 20, the higher the percentage of ripening on the stem. The percentage of ripening on the stems headed after August 20 was nearly the same. Weight of brown rice per unit area was lower in the MS plants and this was attributed to the sterility caused by the low temperature. We conclude that the damage caused by floral sterility from low temperatures can be mitigated by these seedling types.