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

在高知县培育的极早稻品种“Tosapika”在种植第一年（1988年）就在许多农民的田地里发生了早穗病。本研究旨在阐明‘Tosapika’品种过早抽穗的相关因素，并建立防止过早抽穗的栽培方法。与北海道栽培品种相比，‘Tosapika’主茎末叶数较少，播种至旗叶膨大后的有效积温（基础温度。10℃）较低。在播种后301 ~ 348℃时，“托萨皮卡”幼苗的穗分化和发育几乎不受日照长短的影响，穗形成阶段。这些结果表明，‘Tosapika’的热敏性、光敏性和基本营养生长相对较小。早熟抽穗组（PMHG）的产量比对照组（CG）低9-15%。PMHG的单位面积颖花数低于CG，这是由于每穗颖花数明显低于CG。小穗数的减少和分蘖实粒率的降低被认为是PMHG产量较低的原因。在PMHG中，糙米的表观质量较低，因为含有大量的绿米粒。在延缓‘土刺卡’过早抽穗的栽培方法方面，建议在播种后200℃（3.5叶龄后）不追肥、提高播种量以降低干重/株高比等苗圃管理措施，以及苗期增加苗圃箱底施氮量和切叶处理，以延缓‘土刺卡’幼苗幼穗分化。

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.