Effects of Nitrogen Application and Planting Density on Morphological Traits, Dry Matter Production and Yield of Large Grain Type Rice Variety Bekoaoba and Strategies for Super High-Yielding Rice in the Tohoku Region of Japan

To achieve super high yield of more than 10 t ha-1 in the Tohoku region (colder area of Japan), we conducted a two-year field experiment using a large grain type high-yielding rice variety Bekoaoba. Although high nitrogen application (HN) increased the top dry weight at the full heading stage only slightly, it increased the sink size (single grain weight x the number of spikelets per area), leaf area index and nitrogen content at the full heading stage and accordingly the dry matter production during the ripening period. As a result, the gross hulled rice yield was higher under HN than under standard nitrogen application (SN). Under HN, early topdressing increased not only the number of differentiated spikelets but also the number of degenerated spikelets. As a result, the sink size and gross hulled rice yield of the plants were not changed by the early topdressing. Under HN, the higher planting density increased the number of panicles per area but decreased the number of spikelets per panicle. As a result, the sink size and gross hulled rice yield were not changed with the planting density. In conclusion, HN produced the gross hulled rice yield of more than 9 t ha-1. However, the gross hulled rice yield could not be increased more than 10 t ha-1 by regulation of the timing of topdressing and /or the planting density. We discussed the strategies for super high-yielding rice in the Tohoku region.