Abstract
In 1998, premature heading occurred in the farmer's field of the extremely early rice cultivar 'Tosapika' bred in Kochi Prefecture. According to the survey of interview with the farmers at the middle part of Kochi Prefecture, the occurrence of the premature heading was observed in early May, although the frequency varied among fields. In these fields, 100-160 g air-dried seeds were sown per box and grown under the vinyl house without heating during hardening period, and 22-34-day-old seedlings were transplanted with a rice transplanter from March 31 to April 16. The temperature before and after transplanting might be one of the factors related to premature heading, because the temperature during the seedling-raising period and after transplanting was unusually high. The prematurely headed plants were morphologically characterized by smaller number (about 4) of the leaves on the main culm, shorter lengths of culm and panicle, as compared with the normally headed ones. Brown rice yield varied from 206 to 541 gm-2 among the fields, depending on the occurrence frequency of premature heading in the field. In some of the plants prematurely headed in 2001, panicle internode did not elongate sufficiently and the panicles with bract leaves at the neck nodes appeared from the leaf sheaths. In some plants, young panicles ceased their growth even though the flag leaves expanded normally. In the field with premature heading, the effective cumulative temperature (ECT, base temp. 10°C) from seeding to premature heading were 459-543°C days, and that from seeding to transplanting of seedlings (3.4-4.4 leaf age) was 253-351°C days. On the other hand, the ECT from seeding to heading in the normally headed plants were over 800°C days irrespective of cropping years, seedling types and transplanting times. Thus the heading period of normal panicles was longer than that of the prematurely heading panicles.
