Studies on Premature Heading and Its Prevent Methods for Early Season Culture and Planting of Extremely Early Rice Cultivars in Warm South-west Japan

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The premature heading of ‘Tosapika', an extremely early rice cultivar bred in Kochi Prefecture occurred in many farmer's fields in the first year of planting (1988). This study was conducted to clarify the factors related to premature heading of the cultivar ‘Tosapika', and to establish a method of cultivation that would prevent premature heading.1.The effective cumulative temperature (base temp. 10℃) after sowing to flag leaf expansion was lower in ‘Tosapika' due to the fewer number of final leaves on the main culm in comparison with the cultivars bred in Hokkaido. The panicle differentiation and development was scarcely influenced by daylength in the seedling of ‘Tosapika' and the panicle formation stage was observed at 301-348℃ days from sowing. These results were suggested that the thermo- and photo-sensitivities and basic vegetative growth of ‘Tosapika' were comparatively small.2.Yield in the premature-heading group (PMHG) was 9-15% lower than that in the control group (CG). In the PMHG the number of spikelets per unit area was lower than in the CG, 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 tillers, was considered to be the reason for the lower yield in the PMHG. The apparent quality of the brown rice was low in the PMHG because of the presence of a lot of green rice kernels.3.As the methods of cultivation that would retard premature heading of ‘Tosapika', it was suggested that nursery management such as not topdressing nitrogen after 200℃days from sowing (after 3.5 leaf age) and higher sowing rates for the lowering dry weight/plant height ratio were necessary to retard young panicle differentiation in ‘Tosapika' seedlings as well as an increased basal nitrogen application for the nursery box and leaf cutting treatment during nursery period.