Many field observations have suggested that roots play an important role in the tolerance to the sterility-type cool-weather damage at booting stage in rice plants. However, how the roots affect fertilization is uncertain. This paper reports the relationship between root mass and fertilization under cool weather conditions. Three cultivars with different cool-temperature tolerance were used : Sasanishiki (lowland cultivar, moderately susceptible), Misatohatamochi (upland cultivar, moderately resistant) and Hitomebore (lowland cultivar, very resistant). Plants were grown in pots with three levels of nitrogen fertilizer, and under lowland, upland or water culture conditions. The higher the nitrogen level, the more vigorous the growth was and the smaller the dry-weight ratio of root to shoot (leaf, stem and panicle), irrespective of the culture conditions. The number of spikelets was higher and the root weight per spikelet was lighter at the higher levels of nitrogen application. Fertility of the plants cooled at the young microspore stage was lower at the higher levels of nitrogen. The correlations of the fertility with both the dry weight ratio of root to shoot and the root weight per spikelet were high. These results suggest that the relative amount of root to shoot or spikelet might be an important factor affecting the fertility when the plants were exposed to cool weather at the young microspore stage.
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