Effects of premature heading on growth, yield and brown rice quality in extremely early rice cultivar 'Tosapika'

To clarify the factors responsible for the changes in yield and brown rice quality due to the premature heading in rice, we grew two kinds of seedling of extremely early rice cultivars, 'Tosapika' and 'Kitaake' , under two temperature conditions different in the effective cumulative temperature (base temp. 10 deg C ). Then, the seedlings with differentiated young panicles and those without differentiated panicles were transplanted to a paddy field. Similar results were obtained in the two cultivars. In the plants developed from the seedlings with differentiated young panicles, heading from the main culms was observed 30 - 32 days after transplanting (DAT), and that from the tillers 19-20 days later (denoted as premature-heading group). On the other hand, in the plants developed from the seedlings without differentiated panicles, the main culms headed at 57 - 58 DAT and the tillers two days later (control group). The heading duration of tillers in the premature-heading group (30 - 32 days) was longer than that in the control (13 days). In the premature-heading group, the final leaf number on the main culm was about 4 leaves lower than that in the control group (7.6 - 8.0 leaves) , but the numbers of tillers and panicles were higher. Yield in the premature-heading group was 9 - 15% lower than that in the control group. In the premature-heading group, although the number of panicles per unit area was higher, the number of spikelets per unit area was lower than in the control group, due to the markedly lower number of spikelets per panicle. The decrease in the number of spikelets, together with the lower percentage of filled grains on the pullers, was considered to be the reason for the lower yield in the premature-heading group. Furthermore, the apparent quality of the brown rice was low in the prematureheading group because of the presence of a lot of green rice kernels.