A new large-grain cultivar, Akita 63, of japonica-type rice exhibited high yields with high physiological nitrogen (N)-use efficiency for grain production. Akita 63 and three reference cultivars, Yukigesyou, Toyonishiki and/or Alitakomachi were grown in a field with different levels of N supply for three years. The grain yield of Akita 63 was 22-58% greater than that of the reference cultivars. The highest yield was 9.83 t ha-1 of brown (hulled) rice (approximately 12.3 t ha-1 of rough (unhulled) rice). The dry weight of the aboveground part and the number of spikelets at harvest, and the total leaf area (LAI) at the full-heading stage for a given amount of N accumulated in the aboveground part (plant N) did not differ between Akita 63 and the reference cultivars. The grain yield or panicle dry weight for a given amount of plant N at harvest and for a given unit of LAI were, however, greater in Akita 63 (superior in physiological N-use efficiency) than in the reference cultivars because of a higher proportion of dry matter partitioning into panicles in Akita 63 than in the reference cultivars. As the grain size of Akita 63 was about 35% larger than that of the reference cultivars, the sink capacity (the number of spikelets per unit land area x grain size) for a given amount of plant N was much larger in Akita 63. Reaccumulation of starch in the culms and leaf sheaths was very limited in Akita 63, but remarkable in the reference cultivars in the late stage of grain filling. When pot-grown Akita 63 and the reference cultivar, Toyonishiki, at different stages of ripening were fed with 13CO 2 and 13C partitioning into constituent organs was examined at harvest, the proportion of 13C partitioned into panicles was much higher in Akita 63 throughout the ripening period. These results indicate that Akita 63 is a new type of high yielding cultivar of japonica-type rice, superior in physiological N-use efficiency.
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